The negative thermal expansion coefficient in lithium niobate single crystals

Thermal expansion coefficients of lithium niobate single crystals having congruent composition ([Li]/{[Li]+[Nb]} = 48.56%) have been measured in the temperature range 50 to 1000°C. The magnitude of these anisotropic coefficients varies with respect to temperature, in quite different manner. While the magnitude of expansion coefficient along x or y axes slowly increases with temperature monotonically, it's value along the z axis decreases with increase in temperature. At about 650°C, the expansion coefficient becomes zero and thereafter attains negative values (the material shrinks) with further increase in temperature. We discuss the peculiar behavior of expansion coefficient with respect to temperature along z axis, in relation with the crystal structure of lithium niobate single crystals.