VIBRATIONAL-SPECTRA AND STRUCTURAL ORDER IN THE LITHIUM RARE-EARTH SULFIDES

Raman spectra have been measured on a series of sulfide compounds LiLn S2, where Ln is one of the rare earths between Pr and Yb. Compounds of the smaller rare earths have the ordered alpha-NaFeO2 structure and g ive two-band Raman spectra in agreement with group theoretical calcula tions. Compounds of the larger rare earths have a disordered NaCl stru cture which, contrary to the NaCl selection rules, give an intense bro ad Raman band spanning the wavenumber region between the bands of the ordered structure. The broad band is interpreted as a density of state s function with contributions of phonons from throughout the Brillouin zone. The massive breakdown of the k = 0 selection rule occurs becaus e of the charge mismatch between monovalent and trivalent ions distrib uted over the crystalographically equivalent sites of the NaCl structu re. (C) 1994 Academic Press, Inc.