Translational Twins Extraordinary Classification of Divalent Nitrates

Symmetry aspects of Pm3 → Pa3 and Pm3 → P213 phase transitions in Barium, Strontium and Lead nitrates are considered. It is noted that the Pm3 → Pa3 phase transition is proper for crystal class called nonferroics. The characteristic features of nonferroics are given. It has been shown that divalent nitrates have these features. The Pm3 → P213 phase transition is classified as a phase transition characteristic of second-order ferroics. Thus, it is concluded that divalent nitrates are second-order ferroics and nonferroics at the same time. It was reported that to the classification of higher-order ferroics, a spontaneous thermodynamic quantity described by a third-rank tensor should arise in the P213 phase. Such a tensor could be a tensor of piezoelectric coefficients, but in the P213 phase we were unable to detect the piezoelectric effect. It was suggested that in the P213 phase piezoelectric charges are localized on the walls of translational twins (domains), where they can be neutralized. Such domains arise as a result of the Pm3 → Pa3 phase transition. They were visualized by chemical etching. The temperature dependences of dielectric permeance in high-temperature region were also investigated. Small anomalies of the dielectric permeance were fond in the vicinity of the Pm3 → Pa3 phase transition for all tree compounds. It is noted that such anomalies are one of the signs of translational phase transitions.