Investigation of the ferroelastic phase transition in the SrMgF$_4$ pyroelectric crystal

A SrMgF$_4$ compound has been synthesized and a high optical-quality crystal has been grown. Optical-polarization observations, X-ray diffraction analysis, and the measurement of the birefringence $\Delta n_i(t)$ in the SrMgFF$_4$ crystal have been carried out in the temperature range of 90–1200 K. A second-order improper ferroelastic phase transition accompanied by birefringence anomalies and the symmetry change $P$112$_1$ ($Z$ = 12) $\leftrightarrow Cmc2_1$ ($Z$ = 4) has been discovered at $T_0$ = 478 $\pm$ 1 K. The crystal remains pyroelectric in both phases. Considerable contributions of the fluctuations of the order parameter have been observed in the temperature ranges of $(T_0-T)<$ 15 K and $(T-T_0)<$ 60 K.