Order‐disorder phase transition in diethylenetriammonium chlorocadmate single crystals determined by Raman spectroscopy

The Raman spectra of diethylenetriammonium chlorocadmate [(denH 3 )CdCl 5 ] single crystals were studied in the Raman shift range from 250 to 500 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 bands were sensitive to temperature; they correspond to the torsion motions of NH 3 + at 353 cm −1 and to the skeleton deformations of the (denH 3 ) 3+ cations at 336 cm −1 to which NH 3 + belongs. The analysis of the NH 3 + torsion mode based on an order‐disorder model allowed the determination of the activation energy U (300 K) = 0.91 kcal mol −1 The assumption of cluster fluctuations also allowed the critical exponents to be obtained for the transition δ = 0.71 and β = 0.35, the orientation correlation time τ c = 0.5 × 10 −12 s at 300 K and the correlation length ζ 0 ≈ 13 b , b being the lattice parameter. It is concluded that the skeleton deformation mode is a localized excitation coupled to jumps of the torsion mode of NH 3 + .