Non-trivial behavior of the low temperature maximum of dielectric constant and location of the end critical point in Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead free relaxor ferroelectrics crystals detected by acoustic emission

[001] lead free relaxor ferroelectrics crystals of Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ were studied by means of dielectric and acoustic emission methods in the temperature range of 25-240 °C and under a dc bias electric field up to 0.4 kV/cm. A temperature maximum of the dielectric constant was found near 170 °C, as well as the acoustic emission bursts pointed out to both the depolarization temperature near 120 °C and the temperature, corresponding to the maximum of dielectric constant, near 170 °C. While the depolarization temperature increased linearly, the temperature of the dielectric constant maximum was shown to exhibit a V-shape behavior under an electric field: it initially decreases, reaches a sharp minimum at some small threshold electric field of 0.15 kV/cm, and then starts to increase similar to the Curie temperature of the normal ferroelectrics, as the field enhances. Acoustic emission bursts, accompanying the depolarization temperature, weakened with the enhancing field, whereas the ones accompanying the temperature of the dielectric constant maximum exhibited two maxima: near 0.1 kV/cm and near 0.3 kV/cm. The meaning of these two acoustic emission maxima is discussed.