STRUCTURAL AND VIBRATIONAL STUDY OF THE PHASE-TRANSITIONS IN CRYSTALLINE BIS(N-DECYLAMMMONIUM)TETRACHLOROCUPRATE

Five crystalline phases have been characterized in the perovskite laye r compound (C10H21NH3)2CuCl4, using differential scanning calorimetry, X-ray diffraction and infrared spectroscopy. The ordered phase V cont ains three types of hydrocarbon chains; two of them have an almost ext ended configuration with a gauche bond in the proximity of the NH3 gro up, the other one is the planar all-trans form. The proportion of the different forms varies as a function of temperature from 100 K to 306 K within phase V. The crystallographic symmetry of phases V and IV is triclinic while phases II and I belong to the monoclinic and orthorhom bic systems respectively. When raising temperature, two discontinuous transitions, V-III and III-II, affect the interlayer distances and are related to the presence of conformational defects of the type gt3g- a nd gtg-, respectively, as well as to a relative disorder in the chain orientations. When decoupling the III-V transition by cooling, the abr upt change in the lattice parameters occurs between phases III and IV. The high temperature II-I transition is athermic and implies only a c ontinuous structural modification from the monoclinic to the orthorhom bic symmetry.