EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn 2+ and Cu 2+ ions as probe

Results of temperature dependence of EPR spectra of Mn ²⁺ and Cu ²⁺ ions doped calcium cadmium acetate hexahydrate (CaCd(CH ₃COO) ₄·6H ₂O) have been reported. The investigation has been carried out in the temperature range between room temperature (∼ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH ₃ groups which become frozen at ∼ 128 K. The incorporation of Cu ²⁺ and Mn ²⁺ probes at Ca ²⁺ and Cd ²⁺ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca ²⁺ and Cd ²⁺ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.