Thermal expansion and transitions of single crystal lithium niobates from -60 to 250°C

The thermal expansion of single crystals of lithium niobate was investigated over the temperature range from -60 to 250°C along both the a- and c-crystallographic axes. Samples of as-grown crystals having the congruent composition and samples after the in-diffusion of Li2O to give the stoichiometric composition were measured. The data were successfully fitted to a third order polynomial equation.An anomalous decrease in the thermal expansion coefficient along the c-axis was observed near 120°C for the congruent crystal. Differential thermal analysis, electrical capacitance and acoustic velocity measurements all show weak but repeatable effects in this same temperature range for congruent material. The nature of the transition which gives rise to these changes has not yet been explained but is speculated to arise from some defect order/disorder process.