Spectroscopy at high pressures. Part 42.—The Raman spectrum of mercury(II) thiocyanate

Abstract

A first-order phase transition has been found in Hg(SCN)₂. It shows exceptional hysteresis, appearing at ca. 35 kbar with increasing pressure but stable down to ca. 3 kbar under decompression. The spectra are consistent with phase II having the polymeric Cd(SCN)₂ structure. The transition would thus require a quadrupling of the primitive cell of Hg(SCN)₂: a plausible route and mechanism are suggested, the phase change being triggered by a Z-point instability in the parent structure. A full assignment is given for phase I and is shown to require the space group C³_2h rather than either of two alternatives suggested by the X-ray work.