Ground-State Splitting of Trivalent Gd and Cm in ZrSiO4, HfSiO4, and ThSiO4, Determined by ESR

The electron spin resonance spectrum of trivalent gadolinium impurity in the tetragonal host crystals of ZrSiO4, ThSiO4, and HfSiO4 has been observed at X band, and the spin-Hamiltonian parameters were obtained at various temperatures. The dominant second-order parameter B20 for Gd was observed to be negative in all three crystals. Hyperfine structure for the odd isotopes 155Gd and 157Gd was clearly resolved and measured. In ThSiO4 crystals doped with trivalent curium, no ESR signals were detected, supporting the assumption of a large negative B20 term for this ion in these hosts.