EPR investigation of Gd³⁺ and Eu²⁺ in the α‐ and β‐phases of lead phosphate

The X‐band EPR spectra of Gd³⁺ and Eu²⁺ diluted in Pb₃(PO₄)₂ crystals are studied. Lead phosphate exhibits a ferroelastic phase transition at 180°C and the EPR spectra obtained in each phase differ from each other. The spectra are very complex because the zero field splitting has the same order of magnitude as the Zeeman term. The spin Hamiltonian parameters and the energy levels are computed. “Forbidden” or “missing” transitions and line intensities can be explained. Very likely Gd³⁺ substitute a Pb_I ion. For Gd³⁺ in β‐phase g ≈ 2 and b 20 = 990 × × 10⁻⁴ cm⁻¹ (200°C); for Gd³⁺ in α‐phase at 25°C g ≈ 2, b 20 = 930 × 10⁻⁴ cm⁻¹ and b 22 = −77 × × 10⁻⁴ cm⁻¹.