Elastic constants of congruent and nearly stoichiometric lithium niobate crystals by Brillouin light scattering up to 800 °C

The temperature dependence of the elastic constants C11E, C12E, C14E, and C44E as well as the piezoelectrically stiffened constants C13D and C33D of lithium niobate (LN) was determined using polarized Brillouin light scattering. The congruent (48.38 mol. % of Li2O) and nearly stoichiometric (49.5 mol. %) compositions of LN were studied from room temperature up to 800 and 900 °C, respectively. Brillouin measurements were taken on X-, Y-, Z-, and (YXl)/45° LN cuts using temperature-resolved refractive index data and corrected for density variations. At room temperature, the measured elastic constants of congruent LN are generally in agreement with the values previously reported in the literature. Variations of C11E, C12E, C13D, and C33D with temperature are observed to be approximately linear up to 800 °C. In contrast, shear elastic constants C14E and C44E exhibit a strong nonlinear behavior above 400 °C. This coincides with an inversion in the thermal expansion of the c axis lattice parameter. The thermal activation of a shear optical phonon mode, which is estimated to be at 14 THz, may explain these structural and mechanical changes while preserving the trigonal structure. This change is decorrelated from the second-order phase transition driven by spontaneous polarization near the Curie temperature.