Electron paramagnetic resonance and photoluminescence study on local structure of Gd ions in Gd-doped CaF crystals.

Employing electron paramagnetic resonance (EPR) and excitation and photoluminescence (PL) spectra, changes of the local structure of Gd ions were investigated for the CaF crystals containing 0.00015, 0.17, 1.22, 5.75 at% Gd ions, respectively. The obtained spin Hamiltonian parameters of the cubic configuration Gd monomer are = 1.9862, = -2.3153 ± 0.015 MHz and = -0.0005 ± 0.001 MHz. The peak-to-peak width of the EPR lines of the cubic configuration Gd is significantly broadened and partially quenched with the increase of the Gd concentration, which indicates that the exchange and dipole interactions between Gd-Gd are enhanced. From theoretical calculations, combined with experimentally measured angular dependence, we found that for the 1.22 at% Gd:CaF crystals, Gd remains essentially cubic, and the majority of the Gd ions are distributed as a dimer conformation along the [110] direction and at a distance of about 7.7 Å.