Screening Order–Disorder Phase Transition in 1-D Perovskite-like Crystals of [Azetidinium]CdBr3

In search for new polar compounds, we have synthesized the organic–inorganic hybrid [C3H8N]CdBr3 (AZECdBr3). This is a bromide analog of [C3H8N]CdCl3, which has been studied by us earlier. The chloride compound exhibited ferroelectric properties, so it seemed reasonable to check the properties of AZECdBr3. The AZECdBr3 crystals reveal three phase transitions (PTs): the first one (I → II) of the second order at 437.0/436.7 K (heating/cooling), the next (also of the second order) at 231.0 K (II → III), and the last one (classified as a first-order transition) at 197.0/194.0 K (III → IV). On the basis of the DSC data, the thermodynamic parameters have been estimated. The single-crystal X-ray measurements indicate orthorhombic symmetry in all phases investigated, i.e., space groups: Cmcm (phase II 250.0(1) K) and Pbnm for structures at 215.0(1) K (phase III) and 100.0(1) K (phase IV). The anionic substructure is composed of distorted [CdBr6]4– octahedra forming infinite one-dimensional [{CdBr3}−]n chains arranged in the c-direction. The azetidinium cations, C3H8N+, are located between these inorganic chains. The dielectric permittivity, ε′, measured at frequencies from the range between 500 Hz and 2 MHz obtained for single crystals along the [100], [010], and [001] directions evidenced a strong anisotropy. The ferroelastic nature of phases II, III, and IV was confirmed by the polarized light microscopy photographs. On the basis of the obtained results, the mechanisms of the solid–solid PTs have been proposed.