Thermal vibration in CsSCN below the structural phase transition at 470 K

Abstract

Caesium thiocyanate undergoes a phase transition at Tc =470 K from an orthorhombic (Pnma) form to a cubic modification. Single-crystal neutron diffraction data have been obtained for CsSCN within the range 300 to 441 K and, together with reported data at 453 K, are analysed to study thermal motion in the crystal preceding the onset of the phase transition. For the SCN- ions, the data are consistent with large rigid-body translations and librations, with only small contributions from internal modes. From 440 K to To, the SCN- ions exhibit enhanced amplitudes of translational and librational motion; the Cs ÷ ions exhibit a concomitant anomalous increase in vibrational amplitudes. This behaviour is highly anharmonic and is indicative of the onset of the phase