Structure Analysis of the Phase Transition in KSCN

Crystal structures of KSCN are investigated with an X-ray diffraction method at several temperatures in the orthorhombic and the tetragonal phases. The nature of the order-disorder transition for SCN - ions is confirmed above and below the transition temperature. Thermal mortion of K + ions shows a high anharmonicity in the a – b plane of the tetragonal unit cell. The spontaneous displacement of K + in the orthorhombic phase correlates to the order parameter of SCN - . It is suggested that the phase transition is explained by the orientation-phonon coupling system.