Ferroelectric phase transition in Cs3Bi2I9

Single crystals of Cs₃Bi₂I₉ 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. Zh. éksp. Teor. Fiz. 39, 1850–1852 (October 1997)