Magnetic and Thermal Properties of Crystals Including Isolated Clusters. I. Heat Capacity and Infrared Spectrum of [Cr₃O(CH₃COO)₆(H₂O)₃]Cl·6H₂O Crystal between 1.5 and 280 K

Abstract

The heat capacity was measured between 1.5 and 280 K. At liquid helium temperatures a heat capacity anomaly was observed and a new phase transition was found at 211.4 K with a secondary C_p maximum at 215.5 K. The entropy and the enthalpy changes for the overall transition are ΔS_t=13.778 J K⁻¹ mol⁻¹ and ΔH_t=3322 J mol⁻¹, respectively.
The infrared spectra were measured at 120, 225 and 295 K. Many new absorption peaks appeared below the transition point. A mechanism for the phase transition is suggested in terms of ordering processes of crystalline water.
A new model is proposed for explaining the heat capacity anomaly at liquid helium temperature: The cluster ions with equilateral triangle symmetry at room temperature are slightly distorted through the phase transition processes resulting two kinds of cluster ions forming isosceles triangles with different exchange parameters. The closest agreement with the experimental data was obtained by assuming J₀=30 k and J₁=4.5 k for half moles of the cluster ions and J₀=30 k and J₁=−1.5 k for the remaining half moles.