EPR studies of phase transitions in cadmium calcium acetate hexahydrate as a function of different paramagnetic impurity-ion concentrations

The phase transition in cadmium calcium acetate hexahydrate (CCDAH) has been studied in detail with electron paramagnetic resonance (EPR) as a function of two different paramagnetic ion concentrations, namely, Cu²⁺ and Mn²⁺ ions. The change in transition temperature (122--143 K) with Cu²⁺ ion concentrations is explained in terms of mean-field theory and a soft vibrational mode of the -Ca-Cd_1-xCuₓ-Ca- chain along the c axis of the crystal. While the same theory can also explain our observed transition temperature (118--128 K) as a function of the Mn²⁺ ion concentration in this crystal, it does not explain why the limiting value of the transition temperature (i.e., 145 K) of CaCd_1-xCuₓ(CH₃COO)₄·6H₂O as x tends to zero, is strikingly different from the limiting value of (~128.4K) of CaCd_1-xMnₓ(CH₃COO)₄·6H₂O as x tends to zero. The same theory also successfully explains the absence of any phase transition in isomorphous CaCu(CH₃COO)₄·6H₂O. The value of -dTc/dx is significantly higher with Mn²⁺ than with Cu²⁺ in CCDAH.