PHENOMENOLOGICAL THEORY FOR PREDICTING THE TEMPERATURE DEPENDENCE OF ELASTIC, DIELECTRIC AND PIEZOELECTRIC PROPERTIES IN SIMPLE PROPER FERROELECTRIC CRYSTALS.

The Landau:Ginsburg:Devonshire phenomenological theory for proper ferroelectrics has been extended to include sixth order electrostrictive coupling in the prototype (paraelectric) structure. These higher order terms lead to contributions to the stiffened elastic compliances which are quadratic in polarization P, so that in the ferroelectric phase the strong temperature dependence of P//s the spontaneous polarization is reflected into the elastic response. The analysis has been applied to the tungsten bronze ferroelectric Ba//0//. //3//9Sr//0//. //6//1Nb//2O//6 and sixth order electrostriction constants measured in the prototype phase used to derive elastic behavior in the ferroelectric form.