Acta Cryst. (2008). B64, 558-566 [ doi:10.1107/S0108768108027353 ]
Abstract: [(CH3)3PH]3[Sb2Cl9] experiences four phase transitions which were found by means of calorimetry, thermogravimetry and X-ray diffraction. The crystal structure was solved in the space group P63/mmc at 382 K (phase I), Pnam at 295 K (phase II) and Pna21 at 175 K (phase V). We observed an unusual increase in symmetry from the monoclinic to the orthorhombic form at the IV
V transition. The parent hexagonal high-temperature phase I consists of highly disordered [(CH3)3PH]+ cations and [Sb2Cl9]3- anions with an octahedral environment of SbIII. The transition from phases I to II is associated with the ordering of [(CH3)3PH]+ cations. Moreover, the successive transformations from phases I to V are related to the change in the arrangement of Cl atoms in [Sb2Cl9]3- anions from the discrete `face-sharing bioctahedra' (phase I) to two corner-sharing square pyramids. A mechanism for the phase transitions is proposed. It is observed that weak C-H
Cl interactions are responsible for the structure arrangement in low-temperature phases.
Keywords: phase transitions; symmetry; calorimetry; thermogravimetry; X-ray diffraction.
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| Structure factor file (CIF format) (34.3 kbytes) |
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