ON the interconfigurational 4fn↔4fn−15d1 optical transitions of Ce3+ and Pr3+ in Ca9R3+ (PO4)7 [R3+=Al, Ga, Sc, Lu, Y, Gd, La]

A systematic study of the optical properties of Ce3+ and Pr3+ in Ca9R3+ (PO4)7 [R3+=Al, Sc, Lu, Y, Gd, La] is presented. The materials crystallize in the Whitlockite structure of β-Ca3(PO4)2. From the room temperature excitation spectra, the crystal-field splitting and centroid shift of the Ce3+ 5d1 configuration is estimated. The optical properties (crystal-field splitting, the centroid shift and Stokes shift of emission) of Ce3+ is independent of the R3+ cation in Ca9R3+ (PO4)7. It is concluded that in this family of materials and in the parent β-Ca3(PO4)2 lattice, the Ce3+ and Pr3+ preferentially occupy the eight coordinated 18(b) Ca(3) site of the R3c space group. Relative to other phosphates, the covalence of the Ce3+–O2− bonding is high in this family of orthophosphate materials.