Pressure-induced configuration distortion and phase transition inK2Hg(CN)4investigated by Raman scattering

The effect of hydrostatic pressure on the Raman scattering due to the internal and lattice modes of vibration in crystalline ${\mathrm{K}}_2$Hg${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ has been investigated. The ferroelastic phase transition ∼1.5 kbar and room-temperature exhibits a pseudo-second-order behavior. Two first-order structural phase transitions have been observed at ∼5 and ∼12 kbar in the present work which are presumably driven, respectively, by a soft optical phonon at 44 ${\mathrm{cm}}^{-1\mathrm{}}$ (value at 1 bar) and two at 38 and 44 ${\mathrm{cm}}^{-1\mathrm{}}$ (values at 8.6 kbar). The Hg${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4^{2-}$ tetrahedra in the crystal suffer a configuration distortion which is largely the deformation of the C-Hg-C angles at pressures below 12 kbar and the Hg-C-N angles above 12 kbar. The distortion originates from the nonisotropic lattice interactions arising presumably from the polyhedral tilting process induced by pressure which has been shown to be essentially consistent with the known structural properties and the Raman data of the crystal. In addition to the tilting process, the translational displacive process may also exist at pressures above 5 kbar.