On structural phase transitions in piperidinium halogenoantimonates(III) and bismuthates(III): X-ray, calorimetric, dilatometric, dielectric and Raman studies

Three piperidinium analogues: (C5H10NH2)(2)BiCl5, (C5H10NH2)(2)BiBr5 and (C 5H10NH2)(2)SbBr5 have been studied by means of differential scanning calori metry, thermal expansion, dielectric and Raman scattering techniques. Each piperidinium salt undergoes one high-temperature structural phase transitio n, which has been classified as an "order-disorder" type. All transitions a re connected with onset of reorientational motion of the organic cations. T he X-ray studies on (C5H10NH2)(2)BiCl5 show that it crystallises in orthorh ombic Pna2(1) space group. The structure has been refined to R = 0.0336. It consists of one-dimensional (BiCl52-)(n) polyanionic chains and two non-eq uivalent piperidinium cations, one ordered and one disordered. The disorder is realised by splitting all atoms between two positions with half occupan cy. The Raman studies of the polycrystalline (C5H10NH2)(2)BiCl5 performed i n the frequency region between 3500 and 50 cm(-1) show that the vibrational state of the piperidinium cation changes around the phase transition point . (C) 2000 Elsevier Science Ltd. All rights reserved.