Phase transition sequences in tetramethylammonium tetrachlorometallates by X-ray diffraction and spectroscopic measurements

The phase transition sequences of two members of the tetramethylammonium tetrachlorometallate(III) family of hybrid organic-inorganic salts have been determined and structurally-characterised as a function of temperature for the first time. Unusually, we observe a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase. Two additional intermediate phases are observed for M = Fe3+. First-principles calculations and the presence of short Cl…Cl contacts for M = Ga3+ suggest the [GaCl4]− anion to be conformationally hindered due to stronger lone-pair-σ-hole interactions. The conformationally more flexible M = Fe3+ structures show sublattice melting with the onset of rotational disorder in the [NMe4]+ cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl4]− sublattice