Raman study of (NH4)2CuCl4 · 2H2O—II: Order-disorder transition

Thc results of a Raman scattering study of the order-disorder phase transition in (NH₄)₂CuCL₄ · 2H₂O are presented. From the variation of intensity of some selected modes in the temperature range 300-100°K, we have established the temperature dependence of the long range order parameter m, defined as the difference between the fractional number of NH₄ ions in the two possible orientations. Since the ammonium network in (NH₄)₂CuCL₄ · 2H₂O is “quasi one-dimensional”, in contrast to the three-dimensional network in NH₄Cl, we discuss the consequence of this feature on the evolution of m.