Abstract: The phase transition sequences of two members of the tetramethylammonium tetrachlorometallate(III) family of hybrid organic-inorganic salts have been determined and structurally characterized as a function of temperature for the first time. Unusually, a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase is observed. Two additional intermediate phases are observed for Fe 3+. First-principles calculations and the presence of short Cl?Cl contacts for Ga 3+ suggest the [GaCl 4]-anion to be conformationally hindered due to stronger lone-pair-?-hole interactions. The conformationally more flexible Fe 3+ structures show sublattice melting with the onset of rotational disorder in the [NMe 4] + cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl 4]-sublattice.

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