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 (C5H10NH2)(2)SbBr5 have been studied by means of differential scanning calorimetry, thermal expansion, dielectric and Raman scattering techniques. Each piperidinium salt undergoes one high-temperature structural phase transition, which has been classified as an "order-disorder" type. All transitions are connected with onset of reorientational motion of the organic cations. The X-ray studies on (C5H10NH2)(2)BiCl5 show that it crystallises in orthorhombic 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-equivalent piperidinium cations, one ordered and one disordered. The disorder is realised by splitting all atoms between two positions with half occupancy. The Raman studies of the polycrystalline (C5H10NH2)(2)BiCl5 performed in 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.