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Anomalous behaviors of elastic constants of lead-free piezoelectric (Na1/2Bi1/2)TiO3-5%BaTiO3 single crystals studied by Brillouin spectroscopy

The acoustic properties of unpoled (Na1/2Bi1/2)TiO3-5%BaTiO3 (NBT-5%BT) lead-free single crystals were investigated over a wide temperature range from 740 °C to room temperature. Temperature dependences of sound velocities, acoustic attenuation coefficients, and independent elastic constants were determined by using Brillouin spectroscopy. Among the three symmetrized eigen elastic constants C44, (C11 − C12)/2, and C11 + 2C12, (C11 − C12)/2 was the smallest and showed the largest softening at high temperatures above the dielectric maximum temperature (Tm). The softening of (C11 − C12)/2 resulted in the substantial increase of the elastic anisotropy parameter defined as 2C44/(C11 − C12). This suggested that the local symmetry and the associated structural instability of local polar distortions were tetragonal. The onset temperature of quasistatic dynamics of polar nanoregions was ∼500 °C at which all four attenuation coefficients began to increase noticeably. The broad and diffused acoustic anomalies indicated that the low-temperature phase is a typical relaxor state without any long-range order. However, the noncubic local distortions at low temperatures seem to drive the average symmetry of NBT-5%BT to deviate from the cubic phase because the premise of three independent elastic constants for the ideal cubic phase was no longer valid below Tm.