Structure and phase transitions in tetramethylammonium tetrabromoindate(III) and tetraethylammonium tetrabromoindate(III) crystals

Abstract

Two alkylammonium bromoindate(III) crystals: [(CH3)4N]InBr4 and [(C2H5)4N]InBr4 have been studied by differential scanning calorimetry, thermal expansion, dielectric, IR and X-ray diffraction methods. In the tetramethylammonium analogue three thermal anomalies have been detected at 219, 255 and 332K indicating complex sequence of phase transitions. The tetraethylammonium analogues have revealed one structural phase transition at 225K. All phase transitions of first-order type are characterised by a large transition entropy indicating a considerable disordering of the high temperature phases of both compounds. The dielectric anomalies accompanying the phase transitions are typical for crystals exhibiting high temperature “rotator phase”. The infrared changes observed at the phase transition point in [(C2H5)4N]InBr4 indicate that the mechanism of this transition is governed by a disordering process of the cationic sublattice.