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 meta l hydrogensulfides MSH (M = Na, K, and Rb) as well as of deuterated sp ecimens are recorded in the temperature range 10-540 K. The temperatur e evolutions of frequencies and half-widths of the SH-stretching and l ibrational modes clearly display the partly second-order phase transit ions of the polymorphic title compounds. These phase transitions are d riven by thermally activated disordering of the SH- ions from the anti ferroelectrically ordered low-temperature polymorphs (LTM) via the two -dimensionally disordered middle-temperature polymorphs (MTM) to three -dimensionally disordered high-temperature polymorphs (HTM). Because o f the lack of full translational symmetry in the case of the disordere d 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 dis ordered SH- ions are observed additionally to the bands due to the ord ered ions. With the exception of NaSH HTM, the SH- ions are not involv ed in hydrogen bonds. This is revealed by both the SH-stretching frequ encies, which are larger than that of free SH- ions, and their negativ e temperature shifts. (C) 1994 Academic Press, Inc.