Vibrational Spectra and Structural Order in the Lithium-Rare Earth Sulfides

Abstract

Raman spectra have been measured on a series of sulfide compounds LiLnS2, where Ln is one of the rare earths between Pr and Yb. Compounds of the smaller rare earths have the ordered αNaFeO2 structure and give two-band Raman spectra in agreement with group theoretical calculations. Compounds of the larger rare earths have a disordered NaCl structure which, contrary to the NaCl selection rules, give an intense broad Raman band spanning the wavenumber region between the bands of the ordered structure. The broad band is interpreted as a density of states function with contributions of phonons from throughout the Brillouin zone. The massive breakdown of the k = 0 selection rule occurs because of the charge mismatch between monovalent and trivalent ions distributed over the crystallographically equivalent sites of the NaCl structure.