Hydrogen-bond formation in sodium and potassium hydrosulfides at high pressure

Sodium and potassium hydrosulfide were investigated at high pressure using a combination of infrared and Raman spectroscopy and energy dispersive x-ray powder diffraction. NaSH was found to undergo an ordering transition below 9 kbar to yield a cell equivalent to that of the low-temperature phase, but reduced in size. A hydrogen-bonded phase was formed above 83 kbar, the diffraction pattern of which could be fitted using a TlI-type structural model. The refined cell parameters at 130 kbar are a=7.362(60) Å, b=3.698(38) Å, c=5.615(50) Å, β=104.50(51)°, Z=4. The space group would be P$2_1$/a with bifurcated hydrogen-bonded chains running along b. KSH underwent two transitions on compression at 19 and 23 kbar, respectively. The first transition involved only a small volume change. The diffraction data for this phase could be indexed based on a TlI-type model structure, resulting in the following cell at 20 kbar: a=9.129(41) Å, b=4.181(18) Å, c=6.781(18) Å, β=106.47(27)°, Z=4. A large decrease in frequency, increase in bandwidth, and strongly negative dν/dp values for the infrared and Raman active S-H stretching vibrations indicated that strong hydrogen bonds were formed at the 23-kbar transition. A unit cell was obtained from the x-ray powder data with a=7.448(22) Å, b=7.328(26) Å, c=8.110(22) Å, β=97.46(36)°, Z=8 at 31 kbar. A structure model with a distorted CsCl structure, related to the hydrogen-bonded, low-temperature phase of CsSD containing four-membered, hydrogen-bonded ${\mathrm{S}}_4$ ${\mathrm{D}}_4^{4\mathrm{-}}$ units, is proposed. These and other recently reported results indicate that TlI-type structures are more common than previously believed and are, in fact, the favored intermediates between the NaCl- and CsCl-based structures for systems with nonspherical atoms or ions.