On the ordering in [(C2H5)(4)N]2CuCl4 crystal: An X-ray study and theoretical considerations

The crystal structure of [(C2H5)(4)N]2CUCl4 has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P4(2)/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at T-c = 258K is of improper ferroelastic type and it is associated with the ordering of the CuCl42- and a partial ordering of the [(C2H5)(4)N](+) ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.