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Single crystals of Cs 3 Bi 2 I 9 have been grown. Polarized-optical, dielectric, and calorimetric studies have been performed, along with measurements of birefringence, and the coefficients of elasticity c 44 , and linear expansion a over a wide temperature range. It is found that an intrinsic (pseudo-intrinsic) second-order phase transition to the ferroelastic phase takes place at T 0 =220.0±0.6 K accompanied by a change of the crystal symmetry from 6/ mmc to 2/ m . The transition parameter depends linearly on the shear strain x 5 and varies with temperature according to a power law with exponent β =0.5.
Mel’nikova et al. (1997) studied this question.