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

Two alkylammonium bromoindate(III) crystals: [(CH3)(4)N]InBr4 and [(C2H5)(4 )N]InBr4 have been studied by differential scanning calorimetry, thermal ex pansion, dielectric, IR and X-ray diffraction methods, fn the tetramethylam monium analogue three thermal anomalies have been detected at 219, 255 and 332 K indicating complex sequence of phase transitions. The tetraethylammon ium analogues have revealed one structural phase transition at 225 K. All p hase transitions of first-order type are characterised by a large transitio n entropy indicating a considerable disordering of the high temperature pha ses of both compounds. The dielectric anomalies accompanying the phase tran sitions are typical for crystals exhibiting high temperature "rotator phase ". The infrared changes observed at the phase transition point in [(C2H5)(4 )N]InBr4 indicate that the mechanism of this transition is governed by a di sordering process of the cationic sublattice. (C) 2000 Elsevier Science B.V . All rights reserved.