Structure-property relationships in hybrid (C3H5N2)3[Sb2I9] and (C3H5N2)3[Bi2I9] isomorphs

Two hybrid crystals imidazolium iodoantimonate(III) and iodobismuthate(III), (C3H5N2)(3)[Sb2I9] (ImIA) and (C3H5N2)(3)[Bi2I9] (ImIB), have been synthesized and characterized in a wide temperature range (100-350 K) by means of X-ray diffraction, dielectric spectroscopy, proton magnetic resonance (H-1 NMR), FT-IR spectroscopy and optical observations. They undergo two temperature induced solid-solid structural phase transitions. The first one, quasi-continuous (with temperature hysteresis below 1 K), occurs at 324 K in ImIA and 327 K in ImIB, and the second one, clearly of the first order, at 273/278 (cooling/heating) and 291/295 K, in ImIA and ImIB, respectively. Ferroelastic properties are maintained in low-temperature phases. Both materials are isomorphic in the corresponding phases. High temperature phase I has a hexagonal P63/mmc symmetry, and phase II has orthorhombic Cmcm. The crystal architecture is composed of discrete, face-sharing bioctahedra [M2I9](3-) (M: Sb, Bi) and imidazolium cations which are highly disordered over phases I and II. The dynamics of the imidazolium cations has a prominent impact on the stability of the particular phases.