| Popis: |
In the series of molecular composites of perovskite-type layer structures NH3—(CH2)n—NH3 MCl4, the derivative with n = 4 and M = Cd appears as a particular case, compared to other homologous compounds (M = Mn). The distance between two successive perovskite layers, which is dependent on the molecular chain length, appears very short in the crystalline initial form. The polymorphism has been studied by thermal and crystallographic investigations on both powder and single-crystal materials. Five crystalline forms V, IV, III, II and I have been evidenced. The irreversibility or reversibility of the different phase transitions has been established. Structure determinations of the “limit” phases V (initial room-temperature form), III (high-temperature form) and I (new room-temperature form) allow us to correlate the different phase transitions of this complex to the conformational evolutions of the organic part [NH3—(CH2)4—NH3]2+. The results show that a temperature increase on form V leads to an irreversible transformation from gauche to all-trans conformation in the organic chain which keeps the definitive conformation in further phase transitions. Orientational disorder of the chain appears in the high-temperature phase and its evolution with decreasing temperature has been analysed from entropy change measurements. |
