From six- to five-coordinated Sb(III) in [(CH3)3PH]3[Sb2Cl9]: transition pathways from single-crystal X-ray diffraction

[(CH(3))(3)PH](3)[Sb(2)Cl(9)] 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 P6(3)/mmc at 382 K (phase I), Pnam at 295 K (phase II) and Pna2(1) 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 [(CH(3))(3)PH](+) cations and [Sb(2)Cl(9)](3-) anions with an octahedral environment of Sb(III). The transition from phases I to II is associated with the ordering of [(CH(3))(3)PH](+) cations. Moreover, the successive transformations from phases I to V are related to the change in the arrangement of Cl atoms in [Sb(2)Cl(9)](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.