基于双发光位点能量传递构建高效窄带钙钛矿构型K₂BaPO₄F:Eu²⁺荧光粉 增强出版

Efficiently Narrow-band Perovskite-type Phosphor K 2 BaPO 4 F:Eu 2+ based on Dual-sites-triggered Energy Transfer Behavior 基于双发光位点能量传递构建高效窄带钙钛矿构型K 2 BaPO 4 F:Eu 2+ 荧光粉 1 2 first-author ZHANG Rong (1999-), doctoral candidate. She received her master’s degree in Hebei University of Technology under the supervision of associate professor Jian-Feng Sun in 2021. She obtained her bachelor degree from Shaanxi University of Science & Technology in 2018. Her research interests mainly focus on inorganic luminescent materials for solid-state lighting. E-mail: zhangrong@hebut.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979198&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979200&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979199&type= 26.19311523 35.91738129 ZHANG Rong (1999-), doctoral candidate. She received her master’s degree in Hebei University of Technology under the supervision of associate professor Jian-Feng Sun in 2021. She obtained her bachelor degree from Shaanxi University of Science & Technology in 2018. Her research interests mainly focus on inorganic luminescent materials for solid-state lighting. E-mail: zhangrong@hebut.edu.cn zhangrong@hebut.edu.cn 1 2 corresp corresp E-mail： sunjianfeng10@hebut.edu.cn E-mail： sunjianfeng10@hebut.edu.cn sunjianfeng10@hebut.edu.cn sunjianfeng10@hebut.edu.cn SUN Jian-Feng (1983-), obtained his bachelor degree from Beijing Technology and Business University, and received his PhD degree in chemistry from Tsinghua University in 2014 under the supervision of Prof. Dezhong Shen, academician of the Chinese Academy of Engineering. From 2014, he has been working as an associate professor at Hebei University of Technology. His current research interest focuses on designing novel rare earth phosphors for solid-state lighting, including the structural discovery and structure-property relation studies. E-mail: sunjianfeng10@hebut.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979201&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979203&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979202&type= 27.19616699 35.81179047 SUN Jian-Feng (1983-), obtained his bachelor degree from Beijing Technology and Business University, and received his PhD degree in chemistry from Tsinghua University in 2014 under the supervision of Prof. Dezhong Shen, academician of the Chinese Academy of Engineering. From 2014, he has been working as an associate professor at Hebei University of Technology. His current research interest focuses on designing novel rare earth phosphors for solid-state lighting, including the structural discovery and structure-property relation studies. E-mail: sunjianfeng10@hebut.edu.cn sunjianfeng10@hebut.edu.cn 生态环境和信息特种功能材料教育部重点实验室（河北工业大学）， 天津　300130 生态环境和信息特种功能材料教育部重点实验室（河北工业大学）， 天津　300130 Key Laboratory of Special Functional Materials for Ecological Environment and Information （Hebei University of Technology）， Ministry of Education， Tianjin 300130， China Key Laboratory of Special Functional Materials for Ecological Environment and Information （Hebei University of Technology）， Ministry of Education， Tianjin 300130， China 材料科学与工程学院， 河北工业大学，天津　300130 材料科学与工程学院， 河北工业大学，天津　300130 School of Materials Science and Engineering， Hebei University of Technology， Tianjin 300130， China School of Materials Science and Engineering， Hebei University of Technology， Tianjin 300130， China Along with the development of versatile trichromatic phosphors， exploring novel narrow-band blue-emitting phosphors has been an ongoing focus in liquid crystal display backlighting to satisfy the high-quality display demands. Herein， an intriguing narrow-band blue-emitting phosphor K 2 BaPO 4 F：Eu 2+ belonging to the perovskite-type structural prototype is synthesized via the solid-state reaction. Structural analysis indicates that the ［FK 4 Ba 2 ］ octahedra are interlinked with each other by sharing Ba and K atoms to build a three-dimensional network together with ［PO 4 ］ tetrahedra in the cavities. Rietveld refinement analysis shows that the Eu 2+ ions reside in both Ba 2+ and K + sites to form the ［Eu（1）O 8 F 2 ］ and ［Eu（2）O 6 F 2 ］ polyhedra， which totally induce an intense blue emission peaking at 432 nm with a narrow full width at half-maximum of 43 nm， theoretically verified by the dipole-dipole interaction in the Eu（1）→Eu（2） energy transfer process. The electronic structure of K 2 BaPO 4 F matrix is studied by the first-principle calculation on the basis of the density functional theory， demonstrating that it has a large indirect band of 5.035 eV. The K 2 BaPO 4 F：Eu 2+ phosphor exhibits a suitable thermal stability （I 493K /I 293K = 64%） and possesses the satisfactory internal/external quantum efficiency of 72.8%/46.4%， which although is inferior to that of the representative BaMgAl 10 O 17 ：Eu 2+ ， K 1.6 Al 11 O 17+δ ：Eu 2+ ， and Na 3 Sc 2 （PO 4 ） 3 ：Eu 2+ phosphors， yet surpasses some other recently reported blue-emitting phosphors. Consequently， the present work not only demonstrates the potential of K 2 BaPO 4 F：Eu 2+ phosphor as a new narrow-band blue-emitting phosphor candidate， but provides the possibility for exploring the novel phosphors inspired by the mineral-type structural prototypes. 随着近紫外芯片激发三基色荧光粉技术的发展，开发用于液晶显示背光源以满足高品质显示需求的新型窄带蓝色荧光粉已经成为材料研究者日益关注的焦点。本文采用高温固相法合成了具有钙钛矿结构原型的K 2 BaPO 4 F：Eu 2+ 窄带蓝色荧光粉。K 2 BaPO 4 F基质由［FK 4 Ba 2 ］八面体以共享Ba、K原子的方式相连形成三维网状阴离子框架，进而与孔道中的［PO 4 ］四面体连接形成K 2 BaPO 4 F钙钛矿结构框架。Rietveld精修分析表明，Eu 2+ 同时占据Ba 2+ 和K + 位点形成［Eu（1）O 8 F 2 ］和［Eu（2）O 6 F 2 ］配位多面体，两种多面体发光中心存在由偶极-偶极效应引起的Eu（1）→Eu（2）能量传递过程，使荧光粉具有发射峰值波长为432 nm、半峰宽为43 nm的高亮度窄带蓝光发射。基于密度泛函理论的第一性原理计算表明，K 2 BaPO 4 F基质为间接带隙化合物，理论带隙值为5.035 eV。K 2 BaPO 4 F：Eu 2+ 荧光粉展现了合适的荧光热稳定性（I 493K /I 293K = 64%）和较高的内、外量子效率（分别为72.8%、46.4%），其性能指标虽然劣于BaMgAl 10 O 17 ：Eu 2+ 、K 1.6 Al 11 O 17+δ ：Eu 2+ 、Na 3 Sc 2 （PO 4 ） 3 ：Eu 2+ 等典型蓝粉，但优于其他一些已报道的蓝色荧光粉。上述研究工作不仅展现了K 2 BaPO 4 F：Eu 2+ 作为一种新型窄带蓝色荧光粉的应用潜力，而且也为基于矿物结构原型策略探索新型荧光粉提供了可能性。 钙钛矿型结构 K 2 BaPO 4 F：Eu 2+ 能量传递 光致发光 perovskite-type structure K 2 BaPO 4 F：Eu 2+ energy transfer photoluminescence 1　Introduction 1 Phosphor-converted light-emitting diodes (pc-LEDs) have been recognized as a key enabler for the emerging backlighting technology used in modern display devices such as liquid crystal displays (LCDs) owing to their superiorities of rapid response, tunable color, and high visual quality [ 1 1 ] . For the pc-LED backlighting technology used in LCDs, color gamut is a particularly significant performance index that can determine the color reproducibility of LCD devices, which is determined by the color space constructed by the red, green, and blue (RGB) emissions of LEDs passing through the corresponding RGB color filters [ 2 2 ] . The RGB phosphors with a narrow emission bandwidth (full width at half-maximum (FWHM)≤60 nm) can ensure that most of their emissions pass through the RGB color filters instead of being excluded, which contributes to the improvement of color gamut [ 3 3 ] . Therefore, in order to obtain the wide color gamut, exploring narrow-band RGB phosphors to reduce the emissions excluded by RGB color filters is the focus of developing LED backlighting technology for LCDs. Until recently, a series of UCr 4 C 4 -type nitrides and silicates based on the design strategy of natural mineral structures, e.g., Sr[Mg 3 SiN 4 ]:Eu 2+ (λ max = 615 nm, FWHM = 43 nm) [ 4 4 ] , (Sr,Ca)[LiAl 3 N 4 ]:Eu 2+ (λ max = 654 nm, FWHM = 52 nm for Sr[LiAl 3 N 4 ]:Eu 2+ ; λ max = 668 nm, FWHM = 60 nm for Ca[LiAl 3 N 4 ]:Eu 2+ ) [ 5 5 - 6 6 5-6 ] , Rb(Li,Na)[Li 3 SiO 4 ] 2 :Eu 2+ (λ max = 530 nm, FWHM = 42 nm for RbLi[Li 3 SiO 4 ] 2 :Eu 2+ ; λ max = 523 nm, FWHM = 41 nm for RbNa[Li 3 SiO 4 ] 2 :Eu 2+ ) [ 7 7 - 8 8 7-8 ] , NaLi 3 SiO 4 ∶Eu 2+ (λ max = 469 nm, FWHM = 32 nm) [ 9 9 ] , RbNa 3 [Li 3 SiO 4 ] 4 :Eu 2+ (λ max = 471 nm, FWHM = 22 nm) [ 10 10 ] , have been developed as narrow-band RGB phosphors. Their common structural features, viz., highly structural rigidity induced by small polyhedral units and symmetrical coordination environment of activators as well as uniform coordinating elements, not only hinder the occurrence of the structural relaxation of Eu 2+ in its excited state to resist the thermal quenching behavior, but endow these phosphors with the narrow emission bandwidth. Learning from hieratite and cryolite-type prototype structures has also contributed to discovering narrow red-emitting Mn 4+ -activated fluoride phosphors, for instance, K 2 (Ti,Si)F 6 :Mn 4+ (λ max = 630 nm, FWHM=30 nm) [ 11 11 - 12 12 11-12 ] , K 2 NaAlF 6 :Mn 4+ (λ max = 631 nm, FWHM = 30 nm) [ 13 13 ] , Na 3 GaF 6 :Mn 4+ (λ max = 626 nm, FWHM = 28 nm) [ 14 14 ] , all of which are constructed by three-dimensional network with regular arrangement of sharing-vertex [MF 6 ] octahedra, thereby offering the octahedral local coordination environment of Mn 4+ in favor of its efficiently red emission (K 2 (Ti,Si)F 6 ∶Mn 4+ , internal quantum efficiency (IQE)=92-98%; K 2 NaAlF 6 :Mn 4+ , IQE = 89%; Na 3 GaF 6 ∶Mn 4+ , IQE = 69%) [ 11 11 , 13 13 - 15 15 13-15 ] . Some of the abovementioned fluoride phosphors further exhibit the ideal thermal stability owing to the abnormal thermal quenching behavior deriving from the unit cell expansion of fluoride hosts and the vibration modes enhancement from Mn 4+ under high temperature, which eventually benefit to utilize them as the narrow red component [ 11 11 ] . Distinguished from aforementioned UCr 4 C 4 -type and hieratite/cryolite-type narrow-band phosphors, Ce 3+ -doped garnet-type phosphors typified by the yttroalumite (Y 3 Al 5 O 12 ) structural prototype tend to emit wide-band fluorescence with the varied colors depending on a wide range of inorganic elements in its adaptable structure framework, such as (Y,Lu) 3 Al 5 O 12 :Ce 3+ (λ max = 505-532 nm) [ 16 16 - 17 17 16-17 ] , MgY 2 Al 4 SiO 12 :Ce 3+ (λ max = 566 nm) [ 18 18 ] , Lu 3 (Al,Mg) 2 (Al,Si) 3 O 12 :Ce 3+ (λ max = 542-571 nm) [ 19 19 ] , and CaLu 2 Mg 2 (Si,Ge) 3 O 12 :Ce 3+ (λ max = 605 nm) [ 20 20 ] . Definitely, this design principle based on different natural mineral-inspired structures is timesaving and effective, which is not only conducive to discovering new chemical phases for seeking novel luminescent materials, but can be adopted to regulate photoluminescence properties associated with structures. Based on such a structural engineering strategy, the fascinating perovskite-type compounds have aroused scientists’ interest due to the compositional diversity with a variety of elements in the general ABX 3 stoichiometry [ 21 21 ] . The burgeoning lead-based organohalide perovskites (e.g., CH 3 NH 3 PbX 3 , X = Cl, Br, I) or all-inorganic halide perovskites (e.g., CsPbX 3 , X = Cl, Br, I) have recently emerged as the promising photoelectric materials applied in solar cells or quantum-dot LEDs [ 22 22 - 23 23 22-23 ] . Meantime continuous research is ongoing to develop novel rare-earth/transition-metal ions-activated perovskite-type phosphors with favorable luminescence performance packaged in display and solid-state lighting devices. For instance, both double perovskite Cs 2 (Li,Na)BiCl 6 :Mn 2+ and quadruple perovskite Cs 4 CdBi 2 Cl 12 :Mn 2+ phosphors exhibit the broad orange-red emission peaking at 590-612 nm via the spin-forbidden 4 T 1 (G)- 6 A 1 (S) transition of Mn 2+ attributing to the energy transfer from Bi 3+ to Mn 2+ , demonstrating their potential applications in the field of solid-state lighting [ 24 24 - 26 26 24-26 ] . In the sight of structural evolution for the substitution 4[MgF 6 ] = 3[MgF 6 ]+[CaF 6 ] in the CsMgF 3 structure, our group further developed a novel perovskite-type blue-emitting Cs 4 Mg 3 CaF 12 :Eu 2+ phosphor (λ max = 474 nm), which possessed a satisfactory thermal stability (I 473 K /I 298 K = 80%) and a higher IQE of 64%, being competently served as the blue component to fabricate the near-ultraviolet (near-UV)-pumped WLEDs [ 27 27 ] . Nonetheless, the above two phosphors both exhibit the wide emission band with relatively large FWHM, limiting their further applications in the field of LCD backlighting. Until recently, a series of perovskite-type phosphate fluorides M 2 CaPO 4 F (M = K, Rb), which belong to the Pnma space group, have also been discovered by combing mineral-type structural prototype with isovalent substitution [ 28 28 ] . Both of them are constructed by one-dimensional perovskite-type [FM 2 Ca] ∞ chains consisted of face-sharing [FM 4 Ca 2 ] octahedra, which connect with [PO 4 ] tetrahedra to build the structural framework. The introduction of Eu 2+ activator not only endows the phosphors (K 2 CaPO 4 F:Eu 2+ and (Rb,K) 2 CaPO 4 F:Eu 2+ ) with a wide range of multicolor spectral modulation owing to the site-selective occupation of Eu 2+ among different cation sites manipulated by the different Eu 2+ -doping concentrations, synthesis temperatures or alkaline-metal substitution, but also enables the broad emission band of Rb 2 CaPO 4 F:Eu 2+ phosphor to fill the “cyan gap” towards full-visible-spectrum lighting [ 29 29 - 32 32 29-32 ] . Unlike the isostructural phosphate fluorides M 2 CaPO 4 F (M = K, Rb), another perovskite-type K 2 BaPO 4 F (KBPOF) compound adopts the three-dimensional structure with interlinked F-centered [FK 4 Ba 2 ] octahedra (B-site) connected by [PO 4 ] tetrahedra (A-site), which is analogous to the cubic ABX 3 perovskite-type structure [ 33 33 ] . By introducing Eu 2+ ion into KBPOF host lattice, we herein synthesize the perovskite-type blue-emitting phosphor KBPOF:Eu 2+ induced by the energy transfer process between two luminescence centers, which exhibits the narrow-band emission characteristic (FWHM = 42 nm) and is comparable to some currently emerged UCr 4 C 4 -type narrow-band blue phosphors such as NaLi 3 SiO 4 :Eu 2+ (λ max = 469 nm, FWHM = 32 nm) [ 9 9 ] , RbNa 3 [Li 3 SiO 4 ] 4 :Eu 2+ (λ max = 471 nm, FWHM = 22 nm) [ 10 10 ] , and (Rb,Cs)Na 2 K[Li 3 SiO 4 ] 4 :Eu 2+ (λ max = 480-485 nm, FWHM = 26 nm) [ 34 34 ] . Although our developed KBPOF:Eu 2+ phosphor does not exhibit the zero-thermal-quenching behavior as previously reported by Leng et al. [ 35 35 ] and is inferior to the representative blue phosphors such as K 1.6 Al 11 O 17+δ :Eu 2+[52] and Na 3 Sc 2 (PO 4 ) 3 :Eu 2+[53] , it still possesses the suitable thermal stability (I 493 K /I 298 K = 64%) and remarkable IQE up to 72.8%, significantly surpassing some other recently reported blue phosphors, e.g.,Ba 9 Lu 2 Si 6 O 24 :Eu 2+ (I 473K /I 298K =18%, IQE = 68%) [ 36 36 ] , KBaYSi 2 O 7 :Eu 2+ (I 473K /I 298K =10%, IQE = 35%) [ 37 37 ] , and Ba 2 Y 5 B 5 O 17 :Bi 3+ (I 473K /I 298K ＜ 50%, IQE = 46%) [ 38 38 ] . Our present study also aims to provide the further understanding of the intrinsic thermal quenching effect of KBPOF:Eu 2+ by systematically analyzing the temperature-dependent luminescence behaviors of different Eu 2+ luminescence centers, which is not only yet unexplored by Leng et al. , but is conducive to clarifying the existing issue on the thermal quenching behavior of the perovskite-type KBPOF:Eu 2+ phosphor. In addition, other investigations such as the electronic band structure of KBPOF and microscopic coordination environments of two Eu 2+ luminescence centers as well as the mechanisms on the energy transfer process and concentration quenching behavior are also systematically performed. The present study based on the mineral-inspired structures can provide the further understanding for developing unique narrow-band blue-emitting phosphors applicable in LEDs as the components of LCD backlighting devices. 2　Experimental Section 1 2.1　Materials and synthesis 2 The KBPOF: x Eu 2+ ( x = 0-0.10) samples were prepared by a conventional solid-state reaction. Firstly, all starting materials of KF (analytical reagent (A.R.); Tianjin Sailboats Chemical Reagent Co. Ltd., Tianjin, China), K 2 CO 3 (A.R.; Tianjin Bodi Chemical Co. Ltd., Tianjin, China), BaCO 3 (A.R.; Tianjin Sailboats Chemical Reagent Co. Ltd., Tianjin, China), NH 4 H 2 PO 4 (A.R.; Tianjin Bodi Chemical Co. Ltd., Tianjin, China) were weighted based on the stoichiometric ratio (KF:K 2 CO 3 :BaCO 3 :NH 4 H 2 PO 4 = 2:1:2:2). Secondly, all raw materials were uniformly mixed in agate mortars, and the mixtures were further transferred into the corundum crucibles. All corundum crucibles were wrapped by carbon powder, which was designed to the CO reducing atmosphere for the samples. Finally, all samples were placed into the muffle furnace and sintered at 873 K for 5 h. After gradually natural cooling, all samples were collected for further measurement and analysis. 2.2　Measurements and characterization 2 The X-ray diffraction (XRD) patterns data of the samples were collected by a Bruker D8 Discover X-ray diffractometer with Cu Kα radiation of λ = 1.5418 Å ranging from 5 to 80°. The XRD data for Rietveld refinement were recorded by the same Bruker D8 Discover X-ray diffractometer in the range of 2θ from 5 to 130° in steps of 0.02° with a counting time of 4 s per step. The detailed crystal structure was analyzed by the general structure analysis system (GSAS) software [ 39 39 ] . A field emission scanning electron microscope (SEM) (FESEM, FEI Quanta 450 FEG) equipped with the energy-dispersive X-ray spectroscopy (EDS) was used to study the morphology of the as-prepared samples and the distribution of individual elements. The theoretical calculations including band structure and density of states were performed by the Cambridge Serial Total Energy Package (CASTEP) mode from Materials Studio software, which was conducted with the density functional theory (DFT) based on the first-principle calculation [ 40 40 ] . In the calculation process of exchange-correlation potential, the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed [ 41 41 ] . The K-3p 6 4s 1 , Ba-5p 6 5d 10 6s 2 , P-3s 2 3p 3 , O-2s 2 2p 4 , F-2s 2 2p 5 were considered as the valence electrons. The plane-wave energy cut-off was determined to be 940 eV. The Monkhorst-Pack scheme for the host was chosen as 2×2×3 in the Brillouin zone. Diffuse reflection spectra of the samples were measured by a dual beam ultraviolet-visible spectrophotometer (TU-1901，PERSEE) coupled with the accessory of an integrating sphere and referred with the powder material BaSO 4 . Photoluminescence excitation (PLE) spectra and the photoluminescence (PL) spectra of the phosphors were carried on a fluorescence spectrometer (FS5, Edinburgh Instruments), which is equipped with 150 W xenon arc lamp. The slit widths of the fluorescence spectrometer in the excitation and emission spectra measurements were set to be 0.5 nm. Temperature-dependent PL spectra of the representative phosphor were also investigated by the FS5 fluorescence spectrometer, along with a self-regulating temperature-control accessory. The fluorescence lifetimes and IQE were measured using a steady and transient state fluorescence spectrometer (JOBIN YVON FL-1057) by monitoring the given emission form the samples under the 370 nm pulsed laser excitation. The external quantum efficiency (EQE) was performed on an absolute PL quantum yield measurement system (C9920-02, HAMAMATSU). All measurements were conducted under room temperature. 3　Results and Discussion 1 3.1　Crystal structure and morphology 2 Fig.1 Fig.1 (a) exhibits the powder XRD patterns of KBPOF: x Eu 2+ ( x = 0-0.10) samples. It is observed that all diffraction peaks of KBPOF: x Eu 2+ match well with those of the calculated data of KBPOF crystal, demonstrating that Eu 2+ ions at the current level of doping concentration are incorporated into the host lattice without any impurity phase [ 33 33 ] . As shown in Fig.1 Fig.1 (b), one can see that the main peak locating at around 2θ = 30° shifts to the direction of higher angle with the increasing concentration of Eu 2+ ions, suggesting that the doping Eu 2+ ions substituting for the larger cations in KBPOF host cause the reduction of d value according to Bragg’s law of 2dsinθ = nλ [ 42 42 ] . Thus, the 2θ angles get gradually enlarged with the incremental concentration of Eu 2+ ions. Furthermore, the crystallographic occupancies of the doping Eu 2+ ions can be qualitatively identified by the respective radius difference of the cations in KBPOF:Eu 2+ phosphor, which is expressed as follows [ 32 32 ] : <math display="block" id="M1"><msub><mrow><mi mathvariant="normal">D</mi></mrow><mrow><mi mathvariant="normal">r</mi></mrow></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><msub><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mi mathvariant="normal">m</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">N</mi></mrow></mfenced><mo>-</mo><msub><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mi mathvariant="normal">d</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">N</mi></mrow></mfenced></mrow><mrow><msub><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mi mathvariant="normal">m</mi></mrow></msub><mfenced separators="|"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">N</mi></mrow></mfenced></mrow></mfrac><mo>×</mo><mn mathvariant="normal">100</mn><mi mathvariant="normal">%</mi></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979208&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979207&type= 49.69933319 8.80533314 herein, R m (CN) and R d (CN) represent the ionic radii of the potentially substituted cations (K + and Ba 2+ ) in the host and dopant ion (Eu 2+ ), respectively. CN means the coordinated number of cation in KBPOF:Eu 2+ , and D r denotes the calculated percentage. Given that the radii of 8-coordinated K + /Eu 2+ and 10-coodinated Ba 2+ /Eu 2+ cations are 1.51/1.25 and 1.52/1.35 Å, the corresponding values of D r based on Eq.(1) Eq.(1) are calculated to be 17.2% and 11.2%, respectively [ 43 43 ] . If the value of calculated percentage is less than 30%, the doped ions tend to occupy the corresponding crystallographic cation sites in the host lattice [ 32 32 ] . Therefore, it is reasonable to infer that the doped Eu 2+ ions are likely to occupy the 8-/10-coordinated K + /Ba 2+ sites in the KBOPF host. In order to verify this inference, Rietveld refinement was conducted to obtain the detailed crystallographic information of the representative KBPOF:0.03Eu 2+ phosphor, as depicted in Fig.1 Fig.1 (c), Tab. 1 Tab. 1 , and Tab.2 Tab.2 . By taking the crystal structure of KBPOF as the starting model for the structural refinement of KBPOF:0.03Eu 2+ , the refinement result demonstrates that all simulated XRD peaks correspond well with those of experimental data. As shown in Tab.1 Tab.1 , the KBPOF:Eu 2+ phosphor crystallizes in a tetragonal crystal system with space group of I 4/ mcm (140), and the unit cell parameters ( a = b = 7.014(3) Å, c = 11.271(6) Å, and V = 554.56(7) Å 3 ) are slightly smaller than the reported crystallographic parameters of KBPOF crystal ( a = b = 7.026(4) Å, c = 11.279(4) Å, and V = 556.86(7) Å 3 ) due to the shrinkage of crystal lattice resulting from the substitution of larger cations by the smaller Eu 2+ ions [ 33 33 ] . During the process of Rietveld refinement, the sums of occupations, viz., occ(Ba) + occ(Eu) and occ(K) + occ(Eu), are restricted to be 1. The final refinement result demonstrates that Ba 2+ and K + cation sites are both occupied by the doped Eu 2+ ions with the ratios of 0.9812/0.0188 (Ba/Eu) and 0.9888/0.0112 (K/Eu), respectively, which are supported by the convincingly residual factors R p = 6.28%, R wp = 4.53%, and χ 2 = 2.367. Taking into account both of Rietveld refinement analysis and similar cationic radii, it is reasonably confirmed that Eu 2+ ions will occupy Ba 2+ and K + sites in the KBPOF:Eu 2+ host lattice. Formula KBPOF：0.03Eu 2+ Crystal system Tetragonal Profile range （°） 5-130 Radiation type； λ （Å） X-ray； 1.5418 Temperature （K） 298 Space group （Z） I 4 /mcm （140） （4） a （Å） 7.014（3） c （Å） 11.271（6） Unit cell volume V （Å 3 ） 554.56（7） Profile R -factor， R p （%） 6.28% Weighted profile R -factor， R wp （%） 4.53% χ 2 2.367 Atom Occupancy x/a y/b z/c U Ba1 0.9812 1/2 1/2 3/4 0.0276（2） K1 0.9888 0.17656（8） 0.32344（8） 1/2 0.0201（8） P1 1 1/2 0 1/4 0.0215 （6） F1 1 1/2 1/2 1/2 0.0173（8） O1 1 0.37404（19） 0.12596（19） 0.32932（18） 0.0211（9） Eu（1） 0.0188 1/2 1/2 3/4 0.0276（2） Eu（2） 0.0112 0.17656（8） 0.32344（8） 1/2 0.0201（8） Fig.2 Fig.2 (a) depicts the crystal structure of KBPOF and classic perovskite CaTiO 3 compound for comparison. For the crystal structure of KBPOF, the F-centered [FK 4 Ba 2 ] octahedra interconnect by sharing K and Ba atoms to makes up its three-dimensional perovskite-type structure with the [PO 4 ] tetrahedra residing in the space of adjacent [FK 4 Ba 2 ] octahedra, which is in analogy with the crystal structure of CaTiO 3 . As depicted in Fig.2 Fig.2 (b), they share the similar structural features, viz., the position site of [TiO 6 ] octahedron is occupied by [FK 4 Ba 2 ] octahedron in the cubic perovskite structure, and the position of Ca 2+ cation is similar to that of [PO 4 ] tetrahedron. The [KO 6 F 2 ] and [BaO 8 F 2 ] polyhedra presented in Fig.2 Fig.2 (c) further demonstrate the detailed local coordination environments of corresponding cations with different bond lengths ( d K-O1 = 2.749(0) Å, d K-O2 = 2.889(8) Å, d K-F = 2.589(2) Å; d Ba-O = 2.913(9) Å, d Ba-F = 2.819(8) Å). Thus, two different Eu 2+ luminescence centers which occupy the K + and Ba 2+ cation sites of the [KO 8 F 2 ] and [BaO 6 F 2 ] polyhedra tend to exhibit the distinct photoluminescence behaviors, being generally responsible for the intense narrow-band blue-emitting luminescence in our perovskite-type KBPOF:Eu 2+ phosphor as discussed below. Since the morphology and crystallinity of the phosphor have the significant influence on its fluorescence performance and the luminous efficiency of fabricated devices, the SEM measurement of the as-prepared KBPOF:0.03Eu 2+ phosphor was conducted and its photograph is displayed in Fig.3 Fig.3 (a). It is observed that the phosphor composes of distinctly irregular particles with slight agglomeration owing to the high-temperature solid-state synthesis process [ 44 44 ] . The sizes of ununiform particles basically distribute ranging from 1 to 7 μm with the average size of 3.15 μm, which is attributed to the long calcination time ( Fig.3 Fig.3 (b)). For further investigation on the distribution of individual element in the KBPOF:Eu 2+ phosphor, the selected particle was utilized for EDS mapping measurement as shown in Fig.3 Fig.3 (c). It is found that all of the elements are uniformly distributed and accumulated in the whole particle. As displayed in Fig.3 Fig.3 (d), The measured atomic ratio of K:Ba:P:O:F:Eu is determined to be 19.89:9.37:10.03:49.47:11.16:0.08, being close to the nominal ratio in the chemical formula KBPOF:0.03Eu 2+ and thus indicating the phase formation of our perovskite-type phosphor. 3.2　Band structure and optical band gap 2 Investigation on the band structure of KBPOF and density of electronic states (DOS) can provide further insight into the nature of intrinsic electronic structure. Fig.4 Fig.4 (a) and (b) exhibit the band structure as well as total and partial density of states (TDOS and PDOS) of KBPOF host by using the total-energy code CASTEP based on the first-principle calculation with DFT [ 40 40 ] . As shown in Fig.4 Fig.4 (a), the bottom of the conduction band locating at G point and the top of the valence band locating at P point in the Brillouin zone clearly demonstrates that the KBPOF host exhibits the indirect bandgap nature with the large value of 5.035 eV, being beneficial for realizing the efficient down-conversion photoluminescence in the corresponding phosphor since such a large bandgap can favorably accommodate the ground and excited states of the doping Eu 2+ ions in the KBPOF host lattice [ 27 27 ] . As depicted in Fig.4 Fig.4 (b), the energy distribution of PDOS projected onto K, Ba, P, O, and F contributions in the valence band is divided into the three portions, namely, the low-energy region (from -30 to -25 eV), the middle-energy region (from -25 to -10 eV), and the high-energy region (from -10 to 0 eV). One can see that the low-energy region is attributable to the contributions of K-4s and Ba-6s orbitals. The K-3p, Ba-5p, P-3s, P-3p, O-2s, and F-2s orbitals are responsible for the middle-energy region of valence band, in which the O-2s orbital hybridizes with the P-3s and P-3p orbitals owing to their superposition, thereby indicating the formation of [PO 4 ] tetrahedra in the crystal structure of KBPOF host. The high-energy region near the Fermi energy level is dominated by O-2p and F-2p orbitals with bits of P-3s, P-3p and O-2s orbitals. Among them, the overlap between O-2p and P-3p orbitals demonstrates that the O-2p orbital hybridizes with the P-3p orbital to form P-O bond in the [PO 4 ] tetrahedra. The energy of the bottom of conduction band ranging from 0 to10 eV is ascribed to the dominated contribution of Ba-5d orbital. The theoretical optical absorption spectrum was also calculated and exhibited in Fig.4 Fig.4 (c). It is observed that the absorption region mainly locates in the range of 70-220 nm. Given that the absorption band of Eu 2+ in the KBPOF:Eu 2+ phosphor is located at the near-UV region as depicted in the following Fig.6 Fig.6 , the calculated absorption region shown in Fig.4 Fig.4 (c) reflects the intrinsic absorption of the KBPOF host. Fig.5 Fig.5 (a) shows the diffuse reflection spectra of KBPOF host and KBPOF:0.03Eu 2+ phosphor. For the pure KBPOF host, the reflectance starts from 190 nm and it drops until up to around 210 nm. The reflectance then increases and gradually stretches in the range of 210-800 nm. Among them, the absorption region in the range of 190-400 nm is assigned to the absorption of KBPOF host. Nonetheless, it is observed that a distinct absorption region at 210-400 nm in the reflection curve of KBPOF:0.03Eu 2+ phosphor, which is attributed to the spin-allowed transition 4f 7 -4f 6 5d 1 of Eu 2+ ion [ 32 32 ] . The experimental value of band gap can be calculated by the diffuse reflection spectra of KBPOF host based on the extrapolation method, which is guided by the following equations [ 45 45 ] : <math display="block" id="M2"><msup><mrow><mfenced open="[" close="]" separators="|"><mrow><mi mathvariant="normal">F</mi><mfenced separators="|"><mrow><mi mathvariant="normal">R</mi></mrow></mfenced><mtext> </mtext><mi mathvariant="normal">h</mi><mi mathvariant="normal">υ</mi></mrow></mfenced></mrow><mrow><mn mathvariant="normal">1</mn><mo>/</mo><mn mathvariant="normal">2</mn></mrow></msup><mtext> </mtext><mo>=</mo><mtext> </mtext><mi mathvariant="normal">A</mi><mfenced separators="|"><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">υ</mi><mo>-</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">g</mi></mrow></msub></mrow></mfenced></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979229&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979228&type= 40.30133438 5.50333309 <math display="block" id="M3"><mi mathvariant="normal">F</mi><mo stretchy="false">(</mo><mi mathvariant="normal">R</mi><mo stretchy="false">)</mo><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><msup><mrow><mfenced separators="|"><mrow><mn mathvariant="normal">1</mn><mo>-</mo><mi mathvariant="normal">R</mi></mrow></mfenced></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msup></mrow><mrow><mn mathvariant="normal">2</mn><mi mathvariant="normal">R</mi></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979231&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979230&type= 26.07733345 8.29733276 where R and F(R) are the reflectance coefficient and absorption coefficient, respectively. A is the absorption constant, and hυ represents the photon energy. E g is the band gap. Fig.5 Fig.5 (b) exhibits the relationship of [F(R) hυ] 2 on the photon energy for the KBPOF host. It is found that the tangent intersects the X-axis at a point with the value of 5.19 eV, which is larger than the value of the theoretical band gap (5.035 eV) based on the first-principle calculation. This discrepancy is attributed to the underestimate of insulators and semiconductors that derives from the simplification of the form of self-consistent band structure performed by the GGA module during the calculated process with DFT [ 46 46 ] . Therefore, the value of experimental band gap is usually larger than that of the theoretical band gap calculated by the first principle based on DFT. 3.3　Photoluminescence properties 2 Fig.6 Fig.6 exhibits the PLE and PL spectra of KBPOF:0.03Eu 2+ phosphor under room temperature. Upon the excitation of 365 nm, the KBPOF:Eu 2+ phosphor displays the intense blue emission band peaking at 432 nm with narrow FWHM of 43 nm, which is attributed to the characteristic electric-dipole spin-allowed 4f 6 5d 1 -4f 7 transition of Eu 2+[32] . Tab.2 Tab.2 exhibits the narrow FWHM of KBPOF:Eu 2+ in comparison with that of some recently reported blue-emitting phosphors, such as Ca 8 La 2 (PO 4 ) 6 O 2 :Ce 3+ (FWHM = 100 nm), NaCaBO 3 :Ce 3+ (FWHM = 75 nm), and Cs 4 Mg 3 CaF 12 :Eu 2+ (FWHM = 119 nm), demonstrating its favorable color saturation of our developed narrow-band blue-emitting phosphor. Furthermore, one can see that the shape of the PL spectrum is slightly asymmetric, which implies this spectrum curve could be superimposed by multiple PL spectra curves deriving from the distinct luminescence centers. As shown in Fig.6 Fig.6 , by utilizing the Gaussian fitting method, the PL spectrum is reasonably decomposed into two sub-bands with peaks at 410 and 433 nm, corresponding to two distinct Eu 2+ luminescence centers which is consistent with the occupancy result obtained from the Rietveld refinement as discussed above. Normally, the fluorescence feature of Eu 2+ luminescence center is closely related to its local coordination environment of Eu 2+ ions in the phosphor. In the perovskite-type KBPOF host lattice, there are two kinds of large cations surrounded by the different local coordination environments, in which Ba 2+ coordinates with eight O and two F atoms to establish the [BaO 8 F 2 ] polyhedron, and K + coordinates with six O and two F atoms to form the [KO 6 F 2 ] polyhedron. In order to understand the relationship between the local coordination environment and spectral properties of KBPOF:Eu 2+ phosphor, the following equation based on the Van Uitert theory can be used as a reference [ 32 32 ] : Phosphor FWHM （nm） Excitation range （nm） Emission range （nm） Thermal stability IQE EQE Cs 4 Mg 3 CaF 12 ：Eu 2+［ 27 27 ］ 119 300-400 400-600 I 473K /I 298K =80% 64% - Ba 9 Lu 2 Si 6 O 24 ：Eu 2+［ 36 36 ］ 50 250-450 450-600 I 473K /I 298K =18% 68% 45% KBaYSi 2 O 7 ：Eu 2+［ 37 37 ］ 58 300-450 420-500 I 473K /I 298K =10% 35% - Ba 2 Y 5 B 5 O 17 ：Bi 3+［ 38 38 ］ 49 250-400 385-500 I 473K /I 298 ＜50% 46% - Ba 2 Lu 5 B 5 O 17 ：Ce 3+［ 47 47 ］ 112 250-390 400-550 I 423K /I 298K =56% 92% - NaCaBO 3 ：Ce 3+［ 48 48 ］ 75 250-400 400-500 I 453K /I 298K =40% 75% - Ca 8 La 2 （PO 4 ） 6 O 2 ：Ce 3+［ 49 49 ］ 100 250-400 400-600 - 59% - Sr 3 MgSi 2 O 8 ：Eu 2+［ 50 50 ］ 62 200-400 400-500 I 473K /I 298 ＜60% - - BaMgAl 10 O 17 ：Eu 2+［ 51 51 ］ 51 200-400 400-550 I 473K /I 298K =95% 89% 35% K 1.6 Al 11 O 17+δ ：Eu 2+［ 52 52 ］ 70 200-450 400-600 I 373K /I 298K =99% 97% 62% Na 3 Sc 2 （PO 4 ） 3 ：Eu 2+［ 53 53 ］ 50 250-430 400-550 I4 73K /I 298K =100% 74% - KBPOF：Eu 2+ 42 300-400 400-500 I 493K /I 298K = 64% 72.8% <math display="block" id="M4"><mi mathvariant="normal">E</mi><mtext> </mtext><mo>=</mo><mtext> </mtext><mi mathvariant="normal">Q</mi><mfenced open="[" close="]" separators="|"><mrow><mn mathvariant="normal">1</mn><mo>-</mo><msup><mrow><mfenced separators="|"><mrow><mfrac><mrow><mi mathvariant="normal">V</mi></mrow><mrow><mn mathvariant="normal">4</mn></mrow></mfrac></mrow></mfenced></mrow><mrow><mrow><mrow><mn mathvariant="normal">1</mn></mrow><mo>/</mo><mrow><mi mathvariant="normal">V</mi></mrow></mrow></mrow></msup><msup><mrow><mn mathvariant="normal">10</mn></mrow><mrow><mrow><mrow><mo>-</mo><mi mathvariant="normal">n</mi><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">a</mi></mrow></msub><mi mathvariant="normal">r</mi></mrow><mo>/</mo><mrow><mn mathvariant="normal">80</mn></mrow></mrow></mrow></msup></mrow></mfenced></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979235&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979234&type= 40.21666718 10.41399956 <math display="block" id="M5"><mi mathvariant="normal">E</mi><mtext> </mtext><mi mathvariant="normal">λ</mi><mtext> </mtext><mo>=</mo><mtext> </mtext><msup><mrow><mn mathvariant="normal">10</mn></mrow><mrow><mn mathvariant="normal">7</mn></mrow></msup></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979237&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979236&type= 14.56266594 3.04800010 herein, E is the peak position of the phosphor in the PL spectra (cm -1 ); Q refers to the lowest position of edge band of 5d energy level for crystal field splitting (34000 cm -1 for Eu 2+ generally), and V represents the valence state of doped Eu 2+ ions (V = 2); n and r are the coordinated number and the radius of the possible cationic occupancy replaced by doped ions (n = 8 for K + , n = 10 for Ba 2+ ; <math id="M6"><msub><mrow><mi mathvariant="normal">r</mi></mrow><mrow><msup><mrow><mi mathvariant="normal">K</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979239&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979238&type= 3.38666677 3.64066648 = 1.51 Å; <math id="M7"><msub><mrow><mi mathvariant="normal">r</mi></mrow><mrow><mi mathvariant="normal">B</mi><msup><mrow><mi mathvariant="normal">a</mi></mrow><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msup></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979241&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979240&type= 5.16466665 3.64066648 = 1.52 Å); E a stands for the electronic affinity of anion in the host. <math id="M8"><mi mathvariant="normal">λ</mi></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979243&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979242&type= 2.11666679 2.62466669 is the emission wavelength of the phosphor. According to Eqs. ( 4 4 ) and ( 5 5 ), it is concluded that the coordinated number n is positively correlated with the emission peak position E, while the emission wavelength λ is inversely proportional to the emission peak position E. Therefore, the Eu 2+ ion substituting for ten-coordinated Ba 2+ site forms the Eu(1) luminescence center with the emission wavelength of 410 nm, whereas that occupying eight-coordinated K site forms the Eu(2) luminescence center with the emission wavelength of 433 nm. It is also observed from Fig.6 Fig.6 that two PLE spectra under monitoring at 410 and 433 nm exhibit the distinct shape profiles and different excitation peak positions (329 and 351 nm), convincingly demonstrating that the existence of distinct Eu(1) and Eu(2) luminescence centers in the perovskite-type KBPOF:Eu 2+ phosphor. The corresponding PL spectra of KBPOF:Eu 2+ excited by 329 and 351 nm also display the different emission bands with different peak positions, thereby confirming the distinct luminescence features of Eu(1) and Eu(2) in the KBPOF:Eu 2+ phosphor. Additionally, it is observed that the PLE spectrum of Eu(2) center partly covers with the PL spectrum of Eu(1) center in the wavelength range of 380-400 nm ( Fig.6 Fig.6 ), thus indicating that the energy transfer process is likely to occur between Eu(1) and Eu(2) centers in our perovskite-type phosphor. 3.4　Energy transfer process from Eu（1） to Eu（2） 2 The investigation in the fluorescence decay dynamics of different Eu luminescence centers can provide further evidence for confirming their intrinsic energy transfer process. Fig.7 Fig.7 (a) and (b) present the fluorescence decay curves of KBPOF: x Eu 2+ phosphors ( x = 0.01-0.10) for Eu(1) and Eu(2) luminescence centers. The decay curves under monitoring at 410 nm and 433 nm accord with the double-exponential decay profiles, which can be well fitted by the following formula [ 27 27 ] : <math display="block" id="M9"><msub><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow></msub><mo>=</mo><mtext> </mtext><msub><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mn mathvariant="normal">0</mn></mrow></msub><mo>+</mo><msub><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow></msub><mi mathvariant="normal">e</mi><mi mathvariant="normal">x</mi><mi mathvariant="normal">p</mi><mfenced separators="|"><mrow><mrow><mrow><mo>-</mo><mi mathvariant="normal">t</mi></mrow><mo>/</mo><mrow><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow></msub></mrow></mrow></mrow></mfenced><mo>+</mo><msub><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msub><mi mathvariant="normal">e</mi><mi mathvariant="normal">x</mi><mi mathvariant="normal">p</mi><mfenced separators="|"><mrow><mrow><mrow><mo>-</mo><mi mathvariant="normal">t</mi></mrow><mo>/</mo><mrow><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msub></mrow></mrow></mrow></mfenced></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979245&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979244&type= 57.91199875 5.24933338 where I t and I 0 represent the emission intensity of corresponding phosphor at time t and 0, respectively. A 1 as well as A 2 refers to the constant. <math id="M10"><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979250&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979249&type= 2.87866688 3.72533321 and <math id="M11"><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979252&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979251&type= 2.87866688 3.72533321 denote the rapid and slow lifetimes of the double-exponential components, respectively. The average luminescence lifetimes ( <math id="M12"><msup><mrow><mi>τ</mi></mrow><mrow><mi>*</mi></mrow></msup></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979254&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979253&type= 2.87866688 2.96333337 ) can be calculated by the following equation [ 27 27 ] : <math display="block" id="M13"><msup><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mi mathvariant="normal">*</mi></mrow></msup><mo>=</mo><mtext> </mtext><mfrac><mrow><msubsup><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msubsup><msubsup><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msubsup><mo>+</mo><msubsup><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msubsup><msubsup><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msubsup></mrow><mrow><msub><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow></msub><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">1</mn></mrow></msub><mo>+</mo><msub><mrow><mi mathvariant="normal">A</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msub><msub><mrow><mi mathvariant="normal">τ</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msub></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979256&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979255&type= 25.99266624 8.29733276 based on Eq. (7) Eq. (7) , the average luminescence lifetimes for Eu(1) luminescence center monitored at 410 nm are calculated to be 1.84, 1.57, 1.32, 1.16, 1.08, and 0.79 μs, and those for Eu(2) luminescence center are calculated to be 0.89, 0.96, 1.02, 1.09, 1.14, and 1.16 μs with x = 0.01-0.10. Thus, it can be concluded that the luminescence lifetime of Eu(1) center sharply decreases with the incremental doping content of Eu 2+ , while that of Eu(2) center exhibits the increasing tendency, evidently confirming the existence of energy transfer process from Eu(1) to Eu(2) in our perovskite-type KBPOF:Eu 2+ phosphors. The whole energy transfer process between Eu(1) and Eu(2) can be intuitively understood by a schematic configuration coordination model, as shown in Fig.7 Fig.7 (c). As discussed above, the Gaussian emission band peaking at 410 nm originates from the Eu(1) center that occupies ten-coordinated Ba 2+ site to form the [Eu(1)O 8 F 2 ] polyhedron, while another Gaussian emission band peaking at 433 nm derives from the Eu(2) center which substitutes the eight-coordinated K site to form the [Eu(2)O 6 F 2 ] polyhedron. Under the excitation of 365 nm, the electrons of Eu(1) center transit from the 4f 7 ground state to the 4f 6 5d 1 excited state, and then return to the ground state along with the shorter emission wavelength of 410 nm. On the other hand, these excited electrons in the 4f 6 5d 1 excited state of Eu(1) can also transfer to the lower 4f 6 5d 1 excited state of Eu(2) through the point N. Then, the electrons return back to the 4f 7 ground state by overcoming the energy barrier, exhibiting the longer emission with the wavelength of 433 nm. Based on the above electron transition process, the energy transfer is reasonably expected to occur between Eu(1) donor and Eu(2) acceptor in the perovskite-type KBPOF:Eu 2+ phosphors. Fig.7 Fig.7 (d) exhibits the concentration-dependent PL spectra of the KBPOF: x Eu 2+ ( x = 0.01-0.10) phosphor upon the excitation of 365 nm. One can see that all of the spectral profiles are asymmetric along with the long trailing in the long-wavelength side. The FWHM value of PL spectrum increases from 43 to 48 nm with increasing of Eu 2+ doping concentration. The corresponding emission peak position also shows the blue-shift behavior ranging from 432 to 420 nm, which is assigned to the changing strength of crystal field splitting relying on the doping concentration of Eu 2+ ion. The crystal splitting effect can be qualitatively described by the following equation [ 32 32 ] : <math display="block" id="M14"><msub><mrow><mi mathvariant="normal">D</mi></mrow><mrow><mi mathvariant="normal">q</mi></mrow></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><mi mathvariant="normal">Z</mi><msup><mrow><mi mathvariant="normal">e</mi></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msup><msup><mrow><mi mathvariant="normal">r</mi></mrow><mrow><mn mathvariant="normal">4</mn></mrow></msup></mrow><mrow><mn mathvariant="normal">6</mn><msup><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mn mathvariant="normal">5</mn></mrow></msup></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979258&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979257&type= 16.51000023 7.87400007 where Z and e stand for the anion charge and electron charge, respectively; r is the radius of the d wave function; R represents the bond length of the cation and its coordinated ions; D q is energy level separation. When the smaller Eu 2+ ion substitutes the larger Ba 2+ and K + cations in the perovskite-type KBPOF host, the bond lengths of corresponding coordination polyhedra tend to get elongate. According to Eq. (8) Eq. (8) , it is concluded that the crystal field splitting D q is proportional to 1/R 5 . The longer bond length can cause the smaller D q of Eu 2+ 5d energy level. Therefore, the blue-shift behavior of emission peak position occurs in the KBPOF:Eu 2+ phosphors. In addition, as also shown in Fig.7 Fig.7 (d), the PL intensity increases when the Eu 2+ concentration changes from 0.01 to 0.03 and then it gradually decreases, which is ascribed to the concentration quenching effect of Eu 2+ in the KBPOF:Eu 2+ phosphor and can be determined by the critical distance ( <math id="M15"><msub><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mi mathvariant="normal">c</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979260&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979259&type= 3.55599999 3.80999994 ) of the neighboring Eu 2+ ions [ 51 51 ] : <math display="block" id="M16"><msub><mrow><mi mathvariant="normal">R</mi></mrow><mrow><mi mathvariant="normal">c</mi></mrow></msub><mo>≈</mo><mn mathvariant="normal">2</mn><msup><mrow><mfenced separators="|"><mrow><mfrac><mrow><mn mathvariant="normal">3</mn><mi mathvariant="normal">V</mi></mrow><mrow><mn mathvariant="normal">4</mn><mi mathvariant="normal">π</mi><msub><mrow><mi mathvariant="normal">X</mi></mrow><mrow><mi mathvariant="normal">c</mi></mrow></msub><mi mathvariant="normal">N</mi></mrow></mfrac></mrow></mfenced></mrow><mrow><mrow><mrow><mn mathvariant="normal">1</mn></mrow><mo>/</mo><mrow><mn mathvariant="normal">3</mn></mrow></mrow></mrow></msup></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979262&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979261&type= 28.19400024 9.73666668 here, V and N represent the cell volume of the host (V = 556.86(7) Å 3 ) and the potential cation number replaced by the dopant Eu 2+ (N = 4), respectively [ 33 33 ] ; X c is the critical concentration of Eu 2+ dopant in the KBPOF:Eu 2+ phosphor (X c = 0.03). Based on Eq. (9) Eq. (9) , the value of critical distance (R c ) is calculated to be 20.69 Å. It is widely recognized that the electric multipole interaction between the adjacent Eu 2+ ions is responsible for the energy transfer process and resulting concentration quenching effect when the critical distance R c is greater than 5 Å. Thus, the following equations are further given in order to determine the specific type of electric multipole interaction in the KBPOF:Eu 2+ phosphor [ 54 54 ] : <math display="block" id="M17"><mfrac><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mi mathvariant="normal">x</mi></mrow></mfrac><mtext> </mtext><mo>=</mo><mtext> </mtext><mi mathvariant="normal">k</mi><msup><mrow><mfenced open="[" close="]" separators="|"><mrow><mn mathvariant="normal">1</mn><mo>+</mo><mi mathvariant="normal">β</mi><msup><mrow><mfenced separators="|"><mrow><mi mathvariant="normal">x</mi></mrow></mfenced></mrow><mrow><mrow><mrow><mi mathvariant="normal">θ</mi></mrow><mo>/</mo><mrow><mn mathvariant="normal">3</mn></mrow></mrow></mrow></msup></mrow></mfenced></mrow><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979264&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979263&type= 31.83466530 7.28133297 <math display="block" id="M18"><mi mathvariant="normal">l</mi><mi mathvariant="normal">g</mi><mfrac><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mi mathvariant="normal">x</mi></mrow></mfrac><mtext> </mtext><mo>=</mo><mtext> </mtext><msup><mrow><mi mathvariant="normal">K</mi></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msup><mo>-</mo><mfrac><mrow><mi mathvariant="normal">θ</mi></mrow><mrow><mn mathvariant="normal">3</mn></mrow></mfrac><mi mathvariant="normal">l</mi><mi mathvariant="normal">g</mi><mi mathvariant="normal">x</mi><mtext>      </mtext><mfenced separators="|"><mrow><msup><mrow><mi mathvariant="normal">K</mi></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msup><mo>=</mo><mi mathvariant="normal">l</mi><mi mathvariant="normal">g</mi><mi mathvariant="normal">k</mi><mo>-</mo><mfrac><mrow><mi mathvariant="normal">θ</mi></mrow><mrow><mn mathvariant="normal">3</mn></mrow></mfrac><mi mathvariant="normal">l</mi><mi mathvariant="normal">g</mi><mi mathvariant="normal">β</mi></mrow></mfenced></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979266&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979265&type= 57.99666977 8.46666718 where I stands for the PL intensity of the corresponding Eu 2+ concentration ( x ) which starts from its critical concentration ( x = 0.03); k and β are the constants with the same excitation condition for the KBPOF:Eu 2+ phosphor. θ is a symbol to denote the type of the electric multipolar interaction, particularly, θ = 6, 8, or 10 corresponding to the dipole-dipole, dipole-quadrupole, or quadrupole-quadrupole interaction [ 54 54 ] . The dependent relationship between log( x Eu 2+ ) and log(I/ x Eu 2+ ) obtained from Eq. (11) Eq. (11) is given in Fig.7 Fig.7 (e). Therefore, the θ value is calculated to be 5.88, indicating that the electric dipole-dipole interaction involves in the energy transfer process and dominates the concentration quenching effect in our phosphor system. 3.5　Thermal stability and quantum efficiency 2 The thermal stability is commonly utilized to evaluate the application potential of phosphor which is used in the LED device applicable for LCD backlighting field. In 2021, Leng et al. firstly reported the zero-thermal-quenching property of the perovskite-type KBPOF:Eu 2+ phosphor in the increasing temperature range of 298-393 K, which was ascribed to the large band gap, high lattice and high symmetry of the perovskite-type KBPOF host [ 35 35 ] . In order to check this inference, the temperature-dependent PL spectra of the representative KBPOF:0.03Eu 2+ phosphor are displayed in Fig.8 Fig.8 (a) and (b). It is observed that the PL intensity obviously decreases with the temperature increasing from 293 to 493 K, attributing to the temperature quenching effect. It is also evidenced that the respective PL intensity of two distinct Eu 2+ luminescence centers decreases with the increasing temperature, further disagreeing with the aforementioned conclusion referring to the remarkable zero-thermal-quenching feature of KBPOF:Eu 2+ as previously reported by Leng et al [ 35 35 ] . The whole temperature quenching process for different Eu 2+ luminescence centers is reasonably elucidated according to the schematic configuration coordination model ( Fig.7 Fig.7 (c)). In general, the thermal quenching process can be interpreted as the phonon-electron mutual effect, which is usually strengthened with increasing of the temperature. At room temperature, the electrons in the respective lowest energy level of Eu(1) and Eu(2) 4f 6 5d 1 excited state (C point and F point) will return to the 4f 7 5d 0 ground state (D point and G point), resulting in the blue luminescence in the perovskite-type KBPOF:Eu 2+ phosphor. Nonetheless, as the temperature increases, the electrons will surmount the respective energy level difference and transport to the higher energy 4f 6 5d 1 excited state through thermal phonon assisting (this process is denoted as C-H curve and F-I curve), which means that the electrons are more likely to cross the respective intersection point (H point and I point) between the 4f 6 5d 1 and 4f 7 5d 0 states and eventually return to the 4f 7 5d 0 ground state nonradiatively. As a consequence, the temperature quenching behavior is reasonably observed in the KBPOF:Eu 2+ phosphor with increasing temperature. The PL intensity of phosphor remains 77%/64% of the initial intensity when the temperature increases up to 393 K/493 K, which is inferior to the representative blue-emitting phosphors such as commercial BAM:Eu 2+ (I 473 K /I 298 K = 95%) as well as K 1.6 Al 11 O 17+δ :Eu 2+[52] and Na 3 Sc 2 (PO 4 ) 3 :Eu 2+[53] phosphors with zero thermal quenching properties, but significantly superior to some other previously reported phosphors as listed in Tab.2 Tab.2 . As presented in Fig.8 Fig.8 (a), it is observed that the emission peak position slightly shifts from 432 to 419 nm with increasing of the temperature, which is consistent with the variation trend of their Commission Internationale de l’Eclairage (CIE) chromaticity coordinates with the increasing temperature of 293-493 K, varying from (0.159, 0.025) to (0.160, 0.019). The CIE chromaticity shift (ΔE) of the blue-emitting KBPOF:Eu 2+ phosphor with the increasing temperature can be calculated by the following equation [ 55 55 ] : <math display="block" id="M19"><mi mathvariant="normal">Δ</mi><mi mathvariant="normal">E</mi><mtext> </mtext><mo>=</mo><mroot><mrow><mo stretchy="false">(</mo><msubsup><mrow><mi mathvariant="normal">u</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><mo>-</mo><msubsup><mrow><mi mathvariant="normal">u</mi></mrow><mrow><mn mathvariant="normal">0</mn></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><msup><mrow><mo stretchy="false">)</mo></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msup><mo>+</mo><mo stretchy="false">(</mo><msubsup><mrow><mi mathvariant="normal">v</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><mo>-</mo><msubsup><mrow><mi mathvariant="normal">v</mi></mrow><mrow><mn mathvariant="normal">0</mn></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><msup><mrow><mo stretchy="false">)</mo></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msup><mo>+</mo><mo stretchy="false">(</mo><msubsup><mrow><mi mathvariant="normal">w</mi></mrow><mrow><mi mathvariant="normal">t</mi></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><mo>-</mo><msubsup><mrow><mi mathvariant="normal">w</mi></mrow><mrow><mn mathvariant="normal">0</mn></mrow><mrow><mi mathvariant="normal">'</mi></mrow></msubsup><msup><mrow><mo stretchy="false">)</mo></mrow><mrow><mn mathvariant="normal">2</mn></mrow></msup></mrow><mrow/></mroot></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979268&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979267&type= 64.60066223 5.24933338 where u’ = 4 x /(3-2 x +12 y ), v’=9 y /(3-2 x +12 y ), as well as w’ = 1-u’-v’; 0 and t refer to the initial temperature (293 K) and increased temperature (493 K), respectively. Among them, u’ and v’ correspond to the color coordinates in the u’v’ uniform color space. x and y are the color coordinates in the CIE 1931 color space. Based on Eq. (12) Eq. (12) , the chromaticity shift (ΔE) of KBPOF:Eu 2+ phosphor is calculated to be 20.6 <math id="M20"><mo>×</mo></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979275&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979274&type= 1.77800000 2.62466669 10 -3 , which demonstrates the better color stability of KBPOF:Eu 2+ in comparison with that of BAM:Eu 2+ (21.3 <math id="M21"><mo>×</mo></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979275&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979274&type= 1.77800000 2.62466669 10 -3 ). Considering the activation energy (E) as the key parameter which reflects the strength of electron-phonon interaction in the phosphor system, it can be calculated by the following Arrhenius equation [ 27 27 ] : <math display="block" id="M22"><msub><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mi mathvariant="normal">T</mi></mrow></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><msub><mrow><mi mathvariant="normal">I</mi></mrow><mrow><mn mathvariant="normal">0</mn></mrow></msub></mrow><mrow><mn mathvariant="normal">1</mn><mo>+</mo><mi mathvariant="normal">A</mi><msup><mrow><mi mathvariant="normal">e</mi></mrow><mrow><mfenced separators="|"><mrow><mrow><mrow><mo>-</mo><mi mathvariant="normal">E</mi></mrow><mo>/</mo><mrow><msub><mrow><mi mathvariant="normal">K</mi></mrow><mrow><mi mathvariant="normal">b</mi></mrow></msub><mi mathvariant="normal">T</mi></mrow></mrow></mrow></mfenced></mrow></msup></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979277&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979276&type= 26.41600037 8.80533314 where I 0 and I T stand for the PL intensity of the initial temperature and the heating temperature t. K b is Boltzmann constant (8.62×10 -5 eV·K -1 ), A denotes a constant. According to Eq. (13) Eq. (13) , the dependent relationship of ln[I 0 /I T -1] versus 1/K b T is depicted in Fig.8 Fig.8 (d). It can be found that the value of E is determined to be 0.151 eV. Theoretically, the activation energy (E) is closely related to the band gap between the lower energy level of Eu 2+ 5d excited state and the bottom of the conduction band. The larger the value of activated energy, the better the thermal stability of the phosphor. Therefore, the thermal stability of KBPOF:Eu 2+ is superior to some other representative blue-emitting phosphors as listed in Tab. 2 Tab. 2 . Given that the IQE value is another crucial parameter to evaluate the photoelectric performance of phosphor applicable for LEDs, it can be obtained by the formula on the basis of Fig.9 Fig.9 [ 32 32 ] : <math display="block" id="M23"><msub><mrow><mi mathvariant="normal">η</mi></mrow><mrow><mi mathvariant="normal">I</mi><mi mathvariant="normal">Q</mi><mi mathvariant="normal">E</mi></mrow></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><mo>∫</mo><msub><mrow><mi mathvariant="normal">L</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow></msub></mrow><mrow><mo>∫</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">R</mi></mrow></msub><mo>-</mo><mo>∫</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow></msub></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979279&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979278&type= 24.72266769 8.97466660 where E R and E S are the spectrum of excitation light with and without the aim sample in the integrating sphere; L S stands for the emission spectrum of the KBPOF:Eu 2+ phosphor. Furthermore, the absorption efficiency (AE) and EQE values of the representative phosphor are calculated using the following equations [ 56 56 ] : <math display="block" id="M24"><msub><mrow><mi mathvariant="normal">η</mi></mrow><mrow><mi mathvariant="normal">A</mi><mi mathvariant="normal">E</mi></mrow></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><mo>∫</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">R</mi></mrow></msub><mo>-</mo><mo>∫</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow></msub></mrow><mrow><mo>∫</mo><msub><mrow><mi mathvariant="normal">E</mi></mrow><mrow><mi mathvariant="normal">R</mi></mrow></msub></mrow></mfrac></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979284&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979283&type= 24.12999916 8.97466660 <math display="block" id="M25"><msub><mrow><mi mathvariant="normal">η</mi></mrow><mrow><mi mathvariant="normal">E</mi><mi mathvariant="normal">Q</mi><mi mathvariant="normal">E</mi></mrow></msub><mo>=</mo><mtext> </mtext><msub><mrow><mi mathvariant="normal">η</mi></mrow><mrow><mi mathvariant="normal">I</mi><mi mathvariant="normal">Q</mi><mi mathvariant="normal">E</mi></mrow></msub><mo>×</mo><mi mathvariant="normal">A</mi><mi mathvariant="normal">E</mi></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979286&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=38979285&type= 25.48466682 4.48733330 Based on the Eqs.( 14 14 )-( 16 16 ), the calculated IQE, AE, and EQE values are determined to be 72.8%, 63.7%, and 46.4%. which are also higher than some recently reported blue-emitting phosphors as exhibited in Tab.2 Tab.2 , thus demonstrating that the perovskite-type KBPOF:Eu 2+ is a potential candidate as a efficiently narrow-band blue-emitting phosphor for the fabrication of LED devices applicable in the field of LCD backlighting. 4　Conclusions 1 In summary, a novel narrow-band blue-emitting phosphor KBPOF:Eu 2+ is successfully synthesized by the conventional solid-state reaction. The crystal structure of KBPOF is formed by the vertex-sharing [FK 4 Ba 2 ] octahedron interconnected with each other together with [PO 4 ] tetrahedra. Under the excitation of near-UV light, KBPOF:Eu 2+ displays a blue emission band peaking at 432 nm with narrow FWHM of 43 nm. Spectral analysis further confirms the existence of two distinct luminescence centers, viz., Eu(1) and Eu(2), which occupy ten-coordinated Ba 2+ site and eight-coordinated K + site in the perovskite-type KBPOF:Eu 2+ phosphor. The energy transfer mechanism between Eu(1) and Eu(2) and PL properties of concentration-dependent and temperature-dependent spectra are also investigated in detail. The KBPOF:Eu 2+ phosphor not only exhibits a suitable thermal stability (I 493 K /I 293 K = 64%), but also displays the high IQE of 72.8% and EQE of 46.4%, surpassing some other reported blue-emitting phosphors. The present work not only demonstrates the potential of our developed perovskite-type KBPOF:Eu 2+ phosphor, but also provides an avenue for exploring the novel phosphors inspired by the mineral-type structural prototypes. References 郑鹏 ， 丁国真 ， 解荣军 . Ce 3+ 和Eu 2+ 掺杂荧光材料的光猝灭机理研究进展 ［J］. 发光学报 ， 2021 ， 42 （ 10 ）： 1447 - 1457 . doi: 10.37188/cjl.20210173 http://dx.doi.org/10.37188/cjl.20210173 Ce3+和Eu2+掺杂荧光材料的光猝灭机理研究进展 ZHENG P ， DING G ， XIE R J . Research progress on optical quenching of Ce 3+ - and Eu 2+ -doped luminescent materials ［J］. Chin. J. Lumin. ， 2021 ， 42 （ 10 ）： 1447 - 1457 . （in Chinese） . doi: 10.37188/cjl.20210173 http://dx.doi.org/10.37188/cjl.20210173 Research progress on optical quenching of Ce3+- and Eu2+-doped luminescent materials LIAO H ， ZHAO M ， ZHOU Y ， et al . Polyhedron transformation toward stable narrow-band green phosphors for wide-color-gamut liquid crystal display ［J］. Adv. Funct. 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HF-free hydrothermal route for synthesis of highly efficient narrow-band red emitting phosphor K 2 Si 1- x F 6 ： x Mn 4+ for warm white light-emitting diodes ［J］. Chem. Mater. ， 2016 ， 28 （ 5 ）： 1495 - 1502 . HF-free hydrothermal route for synthesis of highly efficient narrow-band red emitting phosphor K2Si1-xF6：xMn4+ for warm white light-emitting diodes YI X ， LI R ， ZHU H ， et al . K 2 NaAlF 6 ：Mn 4+ red phosphor： room-temperature synthesis and electronic/vibronic structures ［J］. J. Mater. Chem. C ， 2018 ， 6 （ 8 ）： 2069 - 2076 . doi: 10.1039/c7tc05784a http://dx.doi.org/10.1039/c7tc05784a K2NaAlF6：Mn4+ red phosphor： room-temperature synthesis and electronic/vibronic structures DENG T ， SONG E ， SUN J ， et al . The design and preparation of the thermally stable， Mn 4+ ion activated， narrow band， red emitting fluoride Na 3 GaF 6 ：Mn 4+ for warm WLED applications ［J］. J. Mater. Chem. 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