Low-temperature phase transition of a crystal in the lithium sodium niobate system

Abstract

We have studied a crystal of composition ${\mathrm{Li}}_{0.03}$${\mathrm{Na}}_{0.97}$${\mathrm{NbO}}_{3}$ that undergoes a first-order ferroelectric-ferroelectric phase transition at about 180 K on cooling and near 260 K on heating. Evidence of the phase transition comes from anomalies in the dielectric constant, dc electrical resistivity, pyroelectric coefficient, spontaneous polarization, specific heat, and thermal expansion. X-ray-diffraction data reveal that the room-temperature phase is orthorhombic with space group Pc${2}_{1}$b (${\mathit{C}}_{2\mathit{v}}^{5}$) and the low-temperature phase is rhombohedral with space group R3c (${\mathit{C}}_{3\mathit{v}}^{6}$). Relationships between their unit-cell parameter are obtained. The spontaneous polarization in these two phases is determined. The large thermal hysteresis of the transition is ascribed to the large structural change required at the transition.