Optical spectra and thermal Schottky levels in the high temperature γ-phase of the lanthanide sesquisulfides Original Research Article

The detailed crystal-field splitting of the energy levels of the trivalent lanthanide ions (Ln3+) in the high temperature γ-phase of the lanthanide sesquisulfides (Ln2S3) are compared with calculations that employ lattice-sum and three-parameter theory models. The results are also compared with the thermal Schottky levels deduced from heat capacity measurements on samples of Ln2S3 having similar composition and physical properties. Good agreement is reached between the calculated levels and the experimental levels that come from the two independent sources of data. It appears that the Ln3+ site symmetry (S4) in the Th3P4 defect structure of the high-temperature phase of Ln2S3 can be represented by a slightly higher symmetry (D2d) that reflects the symmetry of the Ln3+ polyhedra in a lattice that has random cation vacancies throughout the unit cell.