Abstract
A SrMgF4 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 Δni(t) in the SrMgF4 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 P1121 (Z = 12) ↔ Cmc21 (Z = 4) has been discovered at T0 = 478 ± 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 (T0 − T) < 15 K and (T − T0) < 60 K.
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