Structure and phase transitions in [(CH3)(4)P](3)[Sb2Br9] and [(CH3)(4)P](3)[Bi2Br9]

摘要

Two new phosphonium bromoantimonate(III) and bromobismuthate(III) [(CH3)(4)P](3)[Sb2Br9] (PBA) and [(CH3)(4)P](3)[Bi2Br9] (PBB) crystals have been synthesized and their structure determined by single-crystal X-ray diffraction. Both compounds are isomorphous in the room temperature phase and crystallize in the trigonal space group (polar), P31c. The structure consists of discrete [M2X9](3-) anions and [(CH3)(4)P](+) cations. A sequence of structural phase transitions in PBA and PBB is established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible phase transitions are found: (I <----> II) at 540/540 K and (II <----> III) at 193/195 K for PBA and (I <----> II) at 550/550 K and (II <----> III) at 205/207.5 K for PBB (on cooling/heating). The pyroelectric measurements of both compounds have confirmed the polar nature of phases (11) and (111). Proton spin-lattice relaxation time of polycrystalline PBA have been studied in temperatures 77-400 K. A dynamic inequivalence of two tetramethylphosphonium cations has been detected. A ferroelastic domain structure was found over the lowest temperature phase (III) of both crystals studied. The possible mechanisms of the phase transitions are discussed on the basis of the presented results. (C) 2003 Elsevier Inc. All rights reserved.