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 degrees 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 C-44, (C-11-C-12)/2, and C-11 + 2C(12), (C-11-C-12)/2 was the smallest and showed the largest softening at high temperatures above the dielectric maximum temperature (T-m). The softening of (C-11-C-12)/2 resulted in the substantial increase of the elastic anisotropy parameter defined as 2C(44)/(C-11-C-12). 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 similar to 500 degrees 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deviate from the cubic phase because the premise of three independent elastic constants for the ideal cubic phase was no longer valid below T-m. Published under license by AIP Publishing.