Thermal Expansion of Lithium Tantalate and Lithium Niobate Single Crystals

Abstract

The principal thermal expansion coefficients of single crystals of LiTaO₃, LiNbO₃ and α quartz (for reference purposes) have been determined by an x‐ray diffraction technique in the temperature range 25°–500°C. All three materials crystallize in the trigonal system, the first two in class 3m and the last one in class 32, and are characterized by only two principal expansion coefficients, parallel and normal to the c or Z axis. The Bragg x‐ray angle data were fitted to suitable polynomials in the temperature deviations from an arbitrary reference of 25°C, and the expansion coefficients up to second order calculated from the coefficients of these polynomials. The Z‐axis expansion of LiTaO₃ showed a definite maximum at about 250°C. Similar behavior was indicated in the Z‐axis expansion of LiNbO₃, but at a much higher temperature (at the high extreme of the measurement range). The expansion of all three crystals normal to the Z axis was quite regular.