Phase transition (PT) was found in Cs3Bi2I9 single crystal at T0=223 K. Optical birefringence, dielectric and elastic constants and differential scanning calorimetry have been studied. It was found that birefringence Δnc, and twinning picture appeared on (001) — and (100) — plates below T0, as well as softening of elastic stiffness c44 above T0. It allows to classify the PT as proper ferroelastic one with symmetry change 6/mmm ↔2/m.
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Mel’nikova et al. (1996) studied this question.
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