Structure, phase transitions and molecular dynamics of [C(NH2)3]3[M2I9], M = Sb, Bi

Two novel guanidinium iodoantimonate(III) and iodobismuthate( III) crystals, [C(NH2)(3)](3)[Sb2I9] and [C(NH2)(3)](3)[Bi2I9], have been synthesized and their structures have been determined by means of single-crystal x-ray diffraction studies at three temperatures ( 293, 348 and 362 K). Both compounds appeared to be isomorphous in corresponding phases. The crystal structure of the title compounds is composed of discrete M2I93- ( M = Sb, Bi) anions and C(NH2)(3+) guanidinium cations. A non-equivalence of two guanidinium cations has been found. Both guanidinium analogs exhibit a rich sequence of phase transitions. In Gu(3)Sb(2)I(9), three solid-solid structural phase transformations of the first order type are detected at 119/121, 341/344 and 355/362 K ( on cooling/heating) by the DSC and dilatometric techniques. Gu(3)Bi(2)I(9) displays four first order phase transitions: 179/185, 202/215, 287/291 and 358/368 K. The low temperature phases appear to have ferroic (ferroelastic)properties. The prototypic paraelastic phase for both compounds belongs to hexagonal symmetry ( space group P6(3)/mmc). The dielectric response has been measured in a wide frequency region ( 100 Hz-1 MHz), but no dielectric dispersion has been detected. Possible mechanisms of the phase transitions in Gu(3)M(2)I(9) (M = Sb, Bi) are discussed on the basis of the presented results.