New data on the phase transition in KSCN

Abstract

Abstract The structural phase transition in single crystals of KSCN which can be classified as improper ferroelastic, has been investigated by accurate birefringence measurements, Brillouin scattering and far infrared spectroscopy. The optical study showed the transition to be of first order. The temperature dependence of the order parameter η cannot be described by the usual thermodynamic potential with terms up to sixth power. However, previously developed model theory in which the transition is connected with the ordering of thiocyanate ions, leads to an η(T) dependence agreeing well with our data in a wide range of temperatures. The transition induces an anomaly in the elastic constant o 33; the contribution of the transition to the acoustic mode frequency below T tr is positive and close to T tr it is proportional to η2. The IR spectroscopy brings new data on polar phonons in both phases. All modes show a pronounced increase of damping near T tr, giving evidence about fast reorientations of SCN molec...