Investigation of the ferroelectric-ferroelastic phase transition in KH ₂ PO ₄ and RbH ₂ PO ₄ by means of γ-ray diffractometry

Without application of an external electric field, the temperature dependence of the shear angle u xy , which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma -radiation in KH 2 PO 4 and RbH 2 PO 4 . Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a first-order one. The modified Slater's theory provides for RbDP a good fit to the experimental data u xy (T) in the whole temperature range investigated. The fact that beta =0.5 near T c can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.