The Tb

3+

-doped calcium

strontium and barium tungstates have been prepared. The emission and excitation spectra were studied. The X-ray powder diffraction data reveals that these compounds are all tetragonal crystals

space group

I4

1

/a(C

4h

6

). The lattice parameters are: a=0.5238 and c=1.1364nm for CaWO

4

:0.01Tb

a=0.5409 and c=1.1941nm for SrWO

4

:0.01Tb

and a=0

5611

c=1.2712nm for BaWO

4

:0.01Tb. The alka-liearth metal atoms are coordinated with eight oxygen atoms.In these compounds

the Tb

3+

ion substitutes the site of alkali-earth metal ion and does not occupy a site with a centre of symmetry

and shows relatively strong emissions of

5

D

3

→

7

F

6

7

F

6

7

F

4

and

5

D

4

→

7

F

6

7F5 transitions

and the weaker emission of

5

D

3

→

7

F

3

7

F

2

and

5

D

4

→

7

F

3

7

F

4

transitions under excitation with 254nm radiation. There are four types of excited state to produce

5

D

4

→

7

F

i

emissions: (a) the excited state of O

2

-

(2p)→W

6+

(5d) charge-transfer in tungstate groups (the excitation energy can be transferred from the host lattice to the

5

D

4

energy level of Tb

3+

ion)

(b) that of O

2

-

(2p)→Tb

3+

(4f) charge-transfer

(c) that of 4f

8

→4f

7

5d transition of Tb

3+

ion and (d) 4f→4f transition of Tb

3+

ion. In the case of

5

D

3

→

7

F

i

emission

the excited state only has three types of (b)

(c) and (d). The excitation energy levels of (a) can not be transfered to energy levels of

5

D

3

and above.