ULTRASONIC MEASUREMENTS OF THE ELASTIC-CONSTANTS AND THEIR TEMPERATURE-DEPENDENCE FOR NEARLY PERFECT SINGLE-CRYSTALS OF POTASSIUM TETRACHLOROZINCATE (K2ZNCL4)

Ultrasonic and thermal expansivity measurements have been used to calc ulate the elastic properties of nearly perfect single crystals of pota ssium tetrachlorozincate. Determination of the nine components of the elastic tensor indicates that this material has lower elastic constant s than comparable tetrahedral compounds containing sulfate anions [von Hausshul (1965). Acta Cryst. 18, 841-845]. The largest elastic consta nt is along the crystallographic a axis, along which are aligned the p seudoequivalent tetrachlorozincate anions associated with the structur al modulation. Conversely, the b axis exhibits the smallest of the thr ee principal elastic constants and the highest thermal-expansion coeff icient. Measurements of the temperature dependence of the elastic cons tants as the incommensurate phase transition is approached show simila r behaviour for most of the components with the exception of the off-d iagonal component C-13, where a significantly different temperature se nsitivity is observed. Comparison of the calculated elastic constant v alues with those for other A(2)BX(4) systems indicates that the introd uction of the ZnCl4 grouping has a significant effect in lowering the C-44 constant. A reduction in these constants is a reflection of the e ffects that this grouping has on the overall lattice energy, leading e ventually to the formation of incommensurate phase behaviour. The unus ual temperature dependence of the C-13 component is indicative of soft -mode generation, which may play a role in the onset of the ferroelect ric-to-incommensurate phase transformation.