EPR of a Gd 3+ ‐Doped NH 4 Pr(SO 4 ) 2 · 4 H 2 O Single Crystal. Study of Phase Transition and Superposition Model Calculation of Zero‐Field Splitting Parameters

X‐band EPR spectra of a single crystal of Gd ++ ‐doped NH 4 Pr (SO 4 ) 2 · 4 H 2 O (APST) are recorded in the temperature range 4.2 to 410 K. The angular variation of the EPR line positions indicates the presence of two magnetically‐inequivalent Gd 3+ ions. The orientations of their principal axes are determined. The Gd 3+ spin‐Hamiltonian parameters are evaluated using a rigorous least‐squares‐fitting procedure. The superposition model is applied to calculate the values of the zero‐field splitting parameters b   2 0 , b   4 0of Gd 3+ , and those of the intrinsic parameters b 2 and b 4 . The temperature variation of the EPR spectra reveals the occurrences of two first‐order phase transitions in APST crystal at 266 and 155K.