Caesium hydrogensulphide CsS(H,D): Hydrogen bonds and disorder of the hydrogensulphide ions

Abstract

Abstract IR and Raman spectra (4000–50 cm −1 ) of caesium hydrogensulphide and also deuterated specimens were recorded in the temperature range 6–520 K. The spectra confirm the presence of SH − …︁SH − hydrogen bonds in CsSH in contrast to other MSH compounds (M = alkali metal). The temperature evolutions of the wavenumbers and half‐widths of the SH stretching and librational modes clearly display the partly second‐order phase transitions. These are driven by thermally activated disordering of the SH − ions from the antiferroelectrically ordered low‐temperature polymorph (LTM) via a one‐dimensionally disordered middle‐temperature polymorph (MTM) to the three‐dimensionally disordered room‐temperature form (RTM) of the CsCl type. The transition temperatures between these polymorphs differ depending on the degree of deuteration, viz. 105 and 115 K and 190 and 210 K for CsSH and CsSD, respectively, owing to the greater strengths of SD − …︁SD − bonds compared with SH − …︁SH − hydrogen bonds. In the RTM, the fraction of SH − ions which form hydrogen bonds decreases with increase in temperature. The strength (ν SH = 2480–2550 cm −1 , r H…︁S = 275–300 pm) and geometry (H…︁SH angle ≈90°) of SH − …︁SH − hydrogen bonds is also discussed.