Low temperature anomalous field effect in Sr_xBa_(1-x)Nb_20_6 uniaxial relaxor ferroelectric seen via acoustic emission

摘要

Sr_(0.75)Ba_(0.25)Nb_2O_6 [100]-oriented uniaxial tungsten bronze relaxor crystals have been studied by means of dedicated acoustic emission during their thermal cycling in 150-300 K temperature range under dc electric field (E). A 1~s) order transition in a modulated incommensurate tetragonal phase has been successfully detected at T_m~I = 198 K on heating and T_m~I = 184 K on cooling, respectively. As field E enhances, a thermal hysteresis gradually narrows and vanishes in the critical point at E_(th) = 0.31kV/cm, above which a phase transition becomes to 2nd order. The T_m~I(E) dependence looks as a V-shape dip, not similar that previously has been looked as a smeared minimum between both the two polar and nonpolar tetragonal phases near T_m = 220 ÷ 230 K in the same crystals (Dul'kin et al, J Appl. Phys. 110, 044106 (2011)). Due to such a V-shape dip is characteristic for Pb-based multiaxial perovskite relaxor, a rhombohedral phase is waited to be induced by a field E in the critical point temperature range. The emergence of this rhombohedral phase as a crucial evidence of an orthorhombic phase presumably existing within the modulated incommensurate tetragonal phase in tungsten bronze Sr_xBa_(1-x)Nb_20_6 relaxor is discussed.