Phase Coexistence in Cs3Bi2I9 Ferroelastics: Optical, Dilatation and Ultrasonic Velocity Studies

The results for dilatometric, ultrasonic velocity and the domain struct-ure studies of Cs3Bi2I9 crystals in the course of ferroelastic phase trans-ition are presented. It is shown that the phase transition in Cs3Bi2I9 is of the first order. Basing on the thermal expansion results and the data obtained for the elastic stiffness of Cs3Bi2I9 crystals, the latent heat Q=79J/kg at Tc is calculated. The region between 183K and 221K (at cooling) is identified as a heterostructure region, where ferroelastic and paraelastic phases coexist.