LaOBr:Tb and LaOCl:Tb are efficient cathodo-and x-ray phosphors.They have the type of structure as PBFC1 and consist of alternative layers of La-O-La-X-La. Their tetragonal unit cell is shown in Fig.1

[1]

. Each La

3+

ion is 9-coordinated by 4 oxide ions on one side and 5 halide

ions on the other side. Four halide ions locate at equal distance to La

3+

and the fifth one locates further. The doped Tb

3+

ions replace the La

3+

ions in a point site of C4v symmetry. Holsa and coworkers have studied the splitting of the ground

7

F

j

stats of Tb

3+

in LaOCl matrix

[2]

. The authors have investigated the physico-chemical properties and luminescence of LaOBr

1-x

Cl

x

:Tb system in detail

[3]

. The lattice constants a and c change linearly with the composition

x

. This shows that the system forms complete solid solution.