LATTICE VIBRATION-SPECTRA .78. POLYMORPHISM AND PHASE-TRANSITIONS OF THE ISOSTRUCTURAL MSH (M = NA, K, AND RB)

IR and Raman spectra (4000-50 cm-1) of the isostructural alkaline metal hydrogensulfides MSH (M = Na, K, and Rb) as well as of deuterated specimens are recorded in the temperature range 10-540 K. The temperature evolutions of frequencies and half-widths of the SH-stretching and librational modes clearly display the partly second-order phase transitions of the polymorphic title compounds. These phase transitions are driven by thermally activated disordering of the SH- ions from the antiferroelectrically ordered low-temperature polymorphs (LTM) via the two-dimensionally disordered middle-temperature polymorphs (MTM) to three-dimensionally disordered high-temperature polymorphs (HTM). Because of the lack of full translational symmetry in the case of the disordered compounds IR and Raman bands are observed, which are not allowed by group theory. In the case of the LTMs, IR and Raman bands owing to disordered SH- ions are observed additionally to the bands due to the ordered ions. With the exception of NaSH HTM, the SH- ions are not involved in hydrogen bonds. This is revealed by both the SH-stretching frequencies, which are larger than that of free SH- ions, and their negative temperature shifts. (C) 1994 Academic Press, Inc.