Electronic states of lanthanide in the ternary thiogallate CaGa2S4

The electronic states of lanthanide (Ln) doped CaGa2S4 are investigated by the molecular orbital calculations for a spherical cluster of LnCa8Ga12S24 using the FORTRAN program DVSCAT on the basis of the Discrete Variational method with Xα potentials (DV-Xα). In view of the SCF convergence, the Ln-doped lattice should contract to 85–90% of the mother crystal around the Ln atom for the lightweight lanthanides from Ce to Sm. On the other hand, the lattice contraction is very small for the heavyweight lanthanides, especially for Er, Tm and Yb in contrast to the generally known lanthanide contraction for Ln3+ ions. This is probably attributed to the effective charges of Ln atoms calculated here to be less than +1 for all lanthanides contrary to the chemically accepted value of +3. The energy level scheme of 4f and 5d related molecular orbitals is proposed for each Ln substituting Ca in CaGa2S4, showing that the optical processes relating to the 5d→4f transition must be complicated especially for the lightweight Ln-doped CaGa2S4.