Rare earth oxide is widely used in X-ray imaging and cathode-ray imaging technology. In experiment

it is found that co-doped ions

such as Li

+

Mg

2+

Al

3+

with luminescence center Eu

3+

could increase the light yield of rare earth oxide. However the mechanism for this effect is still under debates. It is well known that the co-doped ions may have the flux effect

which will have an influence on the size of the particle. But other mechanism may coexist to enhance the light output of the material. Based on molecular dynamics and density functional simulation (ADF2005)

the defect forming mechanism associated with co-doped ions and its influence on light emission yield of rare earth oxide luminescent material were studied. The results show that the co-doped ions (Li

+

Mg

2+

Al

3+

) tend to form interstitial pairs around

C

2

site in rare earth oxide (Y

2

O

3

Gd

2

O

3

)

and caused the increase of Y(Gd)O bond length of host lattice. This factor leads to the decrease of phonon energy and non-radiation transition rate for the rare earth oxide

thus causesd the increase of host's quantum efficiency and light emission yield of luminescence center Eu

3+

doped into the host material. With the extension of Y(Gd)O bonds

the deformed host lattice also causes the band gap of the material narrower and the small red shift of HL peaks in the excitation spectrum. However

limited by the simulation power of our computer system and the methods we adopted

our results here can only give qualitative explain to the experimental results. In case of X-ray excitation

smaller band gap will leads to more electron-hole pairs producing in the excitation process and larger absorption efficiency

which means the light emission yield will be increased even greater compared with the undoped samples. The result of simulation also shows that the optimal molar concentration ratio for co-doped ions and rare earth element is 5.77%

which is close to the experimental result. On conclusion

the co-doping effect of Li

Mg

Al in rare earth oxide produce the deform in the host lattice

the deform will cause the increase of quantum efficiency of the material. This might be one of factors that may cause the increase in light emission yield.