Electric properties of KTiOPO₄ and NaTiOPO₄ from temperature-dependent X-ray diffraction

Single crystals of KTiOPO₄ (KTP) and NaTiOPO₄ (NaTP) show pronounced pyroelectric behaviour. In order to determine the origin of this property on an atomic scale, X-ray diffraction measurements have been carried out at several temperatures between 100 and 600 K. Modelling of the electron density and the evolution of the structure as a function of temperature has enabled the determination of values for the spontaneous polarization of the compounds and the pyroelectric coefficient of KTP, principally due to the alkaline-ion displacements with a value of 2.0 nC cm^-2 K^-1. Structure modifications, compared with NaTiOPO₄, and the calculation of the electrostatic potential explain the anisotropic behaviour of ionic conductivity of KTP single crystals.