On the specific features of anisotropy in piezoelectric properties of high-temperature ferroelectric ceramics

The best known representatives of ferroelectric ceramics (FC) with very different values of piezoelectric coefficients for different types of vibrations, i.e. with a high degree of anisotropy are the compositions based on lead titanate (PT) Recently, it has been found that other FC compounds and their solid solutions belonging to structural families other than PT: Ba and Pb niobates (tetragonal-tungsten-bronze type), Bi titanates and niobates (layered perovskite-like), Li niobate (pseudoilmenites) also possess a pronounced anisotropy in piezoelectric properties (PPA). All of these compounds have the same high value of Curie point temperature (T_c): with increasing T_c there is a tendency of PPA to increase, especially at T>T_c=400°C. It was shown that such a variation in PPA is due to the absence of low-temperature phase transitions in the FE region in corresponding compositions and, as a result, a decreased anisotropy in dielectric properties (DPA). Using the thermodynamic approach, the connection between PPA and DPA and the phase state of FC was considered