Gd³⁺ EPR study of phase transitions in a NH₄Eu(SO₄)₂.4H₂O single crystal: a superposition-model calculation of zero-field splitting parameters

Detailed X-band EPR studies of the Gd³⁺ ion have been carried out in a europium ammonium sulphate single crystal between liquid-helium and room temperature. The behaviours of the overall splitting of EPR lines and the EPR linewidth indicate the occurrences of two first-order phase transitions at 273+or-0.5 and 166+or-0.5 K, respectively. The spin-Hamiltonian parameters for Gd³⁺ at room temperature have been evaluated. The intrinsic parameter b₂ for Gd³⁺ in NH₄Eu(SO₄)₂.4H₂O crystal has been calculated by the use of the superposition model of Newman (1975), and found to be -5.802 GHz, with the exponent t₂=9.0.