THERMO-OPTIC STUDIES OF STRUCTURAL PHASE TRANSITIONS IN CRYSTALS OF THE KTiOPO₄ FAMILY. M. N. Womersley, P. A. Thomas, Department of Physics, University of Warwick, Coventry, CV4 7AL, England

A uniquely-modified Kösters prism interferometer is used for simultaneous and separate measurement of the change in refractive index, dn, and the thermal expansion coefficient with temperature, with a resolution of 4x10^-7, on 1mm³ biaxial crystals, is described. The application of this technique to new nonlinear optical materials of the KTiOPO₄ (KTP) family is made with the aim of elucidating and modelling the onset and increase of the alkali-metal mobility in the temperature range from 300deg.C to 900deg.C and studying the high-temperature ferroelectric to paraelectric phase transition.

The evolution of the refractive index as a function of temperature, n(T), is plotted for a number of materials, with the high-temperature high-symmetry component above the phase transition deducted from the low-temperature data and is fitted to a model describing both the extrinsic (structure-dependent) and intrinsic (polarizability-dependent) contributions to the thermo-optic effect. The results are reviewed within the context of structure-property relationships and the high-temperature phase transitions in the KTP family.