Paramagnetic resonance of 155Gd3+ in a ThO2 single crystal: study of the hyperfine structure (allowed and forbidden lines)

The hyperfine structure of ¹⁵⁵Gd³⁺ ions in Th⁴⁺ substitutional cubic sites of a single crystal of ThO₂ (fluorite structure) has been studied by means of electron-paramagnetic-resonance techniques. The observation of "forbidden" transitions traditionally labeled by Δ m=1 or 2 is reported. Theoretical calculations using first-order perturbation theory for the hyperfine structure and nuclear Zeeman terms after a numerical diagonalization of the remaining (electronic Zeeman plus crystal-field terms) spin Hamiltonian are in good agreement with experimental results.