Structure and properties of [(CH₃)₄P]₃[Sb₂Cl₉] and [(CH₃)₄P]₃[Bi₂Cl₉]

Two new phosphonium chloroantimonate(III) and chlorobismuthate(III) crystals ([(CH₃)₄P]₃[Sb₂Cl₉] (PCA) and [(CH₃)₄P]₃[Bi₂Cl₉] (PCB)) have been synthesized and their structure determined by means of single-crystal x-ray diffraction. The structure of the title compounds contains discrete [M₂Cl₉]³⁻ anions in general positions and disordered [(CH₃)₄P]⁺ cations. The two compounds are isomorphous in the room temperature phase and crystallize in the polar trigonal space group, P31c. The phase situation in PCA and PCB is established on the basis of differential scanning calorimetry and dilatometric studies. Two phase transitions are found: at 534/534 K () and at 135/138 K () for PCA and at 550/550 K () and at 151/155 K () for PCB (on cooling/heating). The values of the transition entropies (ΔS_tr) obtained suggest that the transitions are of the order–disorder type. The dielectric dispersion measurements have revealed a relaxation process in the PCB crystal. The pyroelectric measurements of both compounds have confirmed the polar nature of the low temperature phase (III). The ferroic (ferroelastic) properties were found over phase III for both crystals.