Low temperature anomalous field effect in Sr _xBa _1-xNb ₂O ₆ uniaxial relaxor ferroelectric seen via acoustic emission

Sr _0.75Ba _0.25Nb 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 ^st 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 0.31 kV/cm, above which a phase transition becomes to 2 ^nd 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, 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-xNb ₂O ₆ relaxor is discussed.