ORDER-DISORDER PHASE-TRANSITION IN DIETHYLENETRIAMMONIUM CHLOROCADMATE SINGLE-CRYSTALS DETERMINED BY RAMAN-SPECTROSCOPY

The Raman spectra of diethylenetriammonium chlorocadmate [(denH3)CdCl5 ] single crystals were studied in the Raman shift range from 250 to 50 0 cm-1 for temperatures between 10 and 428 K. A phase transition at T( c) = 168 K was observed and characterized. In particular, two Raman ba nds were sensitive to temperature; they correspond to the torsion moti ons of NH3+ at 353 cm-1 and to the skeleton deformations of the (denH- 3)3+ cations at 336 cm-1 to which NH3+ belongs. The analysis of the NH 3+ torsion mode based on an order-disorder model allowed the determina tion of the activation energy U(300 K) = 0.91 kcal mol-1. The assumpti on of cluster fluctuations also allowed the critical exponents to be o btained for the transition delta = 0.71 and beta = 0.35, the orientati on correlation time tau(c) = 0.5 x 10(-12) S at 300 K and the correlat ion length xi0 almost-equal-to 13b, b being the lattice parameter. It is concluded that the skeleton deformation mode is a localized excitat ion coupled to jumps of the torsion mode of NH3+.