High-pressure Raman study of one-dimensional crystals of the very polar molecule hydrogen cyanide.

Abstract

At ambient temperature, hydrogen cyanide freezes at 0.2 GPa and undergoes a solid-solid transition at 0.8 GPa. Compression beyond 1.3 GPa induces polymerization. The crystal structures stable between 0.2 and 0.8 GPa and above 0.8 GPa were identified as the body-centered tetragonal and orthorhombic structures, respectively, that also occur at low temperatures and ambient pressure. Both structures consist of linear chains of molecules aligned parallel to the c axis of the unit cell. The Raman peak for the internal C-H stretching mode shifts to lower frequency with increasing pressure in both solid phases because compression strengthens the intrachain hydrogen bond. The frequency of the Raman-active libration increases rapidly with increasing pressure in the tetragonal phase and at an even greater rate in the orthorhombic phase. The LO-TO splitting of the librational mode, ${\mathrm{\ensuremath{\omega}}}_{\mathrm{LO}}$-${\mathrm{\ensuremath{\omega}}}_{\mathrm{TO}}$, decreases monotonically with increasing pressure, whereas the difference between the squares of these frequencies, ${\mathrm{\ensuremath{\omega}}}_{\mathrm{LO}}^{2}$-${\mathrm{\ensuremath{\omega}}}_{\mathrm{TO}}^{2}$, is independent of pressure.