Optical spectra and thermal Schottky levels in the high temperature gamma-phase of the lanthanide sesquisulfides

The detailed crystal-field splitting of the energy levels of the trivalent lanthanide ions (Ln(3+)) in the high temperature gamma-phase of the lanthan ide sesquisulfides (Ln(2)S(3)) are compared with calculations that employ l attice-sum and three-parameter theory models, The results are also compared with the thermal Schottky levels deduced from heat capacity measurements o n samples of Ln(2)S(3) having similar composition and physical properties. Good agreement is reached between the calculated levels and the experimenta l levels that come from the two independent sources of data. it appears tha t the Ln(3+) site symmetry (S-4) in the Th3P4 defect structure of the high- temperature phase of Ln(2)S(3) can be represented by a slightly higher symm etry (D-2d) that reflects the symmetry of the Ln(3+) polyhedra in a lattice that has random cation vacancies throughout the unit cell. (C) 2000 Elsevi er Science Ltd. All rights reserved.