Low Temperature Elastic Constants and Piezoelectric Coefficients of LiNbO3 and LiTaO3: Resonant Ultrasound Spectroscopy Measurement and Lattice Dynamics Analysis

The complete sets of elastic constants C-ij and piezoelectric coefficients e(ij) for LiNbO3 and LiTaO3 single crystals have been determined by resonant ultrasound spectroscopy (RUS) from ambient temperature to 6 K. Both C-ij (T) and e(ij) (T) of the two crystals monotonically increased as the temperature decreased. The Einstein temperature estimated from Varshni's equation revealed that e(15) and e(22) of LiNbO3 have remarkably low values compared with the acoustic Debye temperature. In addition, the lattice anharmonicity of these piezoelectric coefficients was also extraordinarily low. An analysis based on the group theory and lattice dynamics revealed that both LiNbO3 and LiTaO3 crystals have three types of internal displacement modes: A(1), A(2), and E, and only the E mode affects e(15) and e(22). Therefore, it is reasonable to suppose that the E mode internal displacement is responsible for the unusual behaviors of the tow piezoelectric coefficients. (C) 2012 The Japan Society of Applied Physics