ELASTIC, THERMOELASTIC AND PIEZOELASTIC PROPERTIES OF CRYSTALS OF THEKSCN FAMILY (KN3, KHF2, KSCN, NH4HF2, NH4SCN)

Large single crystals of KN3, KHF2, NH4HF2, KSCN and NH4SCN, which are structurally related to those of the CsCl type, were grown from aqueo us solutions by controlled evaporation. Elastic, thermoelastic and pie zoelastic constants were determined from ultrasonic resonance frequenc ies of thick plane-parallel plates. In addition, the tensors of therma l expansion were measured. Elastic properties and thermal expansion re flect the close relationship to the CsCl-type structure. The anisotrop y is mainly caused by the preferential orientation of the long axes of the anionic dumb-bells within one distinct plane. KSCN exhibits weak elastic anomalies approaching the phase transition at 413 K. These eff ects, which are much smaller than those found in the high-temperature phases of the alkali cyanides, indicate, together with the almost miss ing hysteresis and the small transition enthalpy, a transition of main ly second order. The variation of the elastic behaviour of KHF2 and NH 4SCN passing through the transition at 471 K and 367 K, respectively, express first-order processes.