Electron paramagnetic resonance and photoluminescence study on local structure of Gd³⁺ ions in Gd-doped CaF₂ crystals

Employing electron paramagnetic resonance (EPR) and excitation and photoluminescence (PL) spectra, changes of the local structure of Gd³⁺ ions were investigated for the CaF₂ crystals containing 0.00015, 0.17, 1.22, 5.75 at% Gd ions, respectively. The obtained spin Hamiltonian parameters of the cubic configuration Gd³⁺ monomer are g = 1.9862, B ₄ = -2.3153 ± 0.015 MHz and B ₆ = -0.0005 ± 0.001 MHz. The peak-to-peak width of the EPR lines of the cubic configuration Gd³⁺ is significantly broadened and partially quenched with the increase of the Gd³⁺ concentration, which indicates that the exchange and dipole interactions between Gd³⁺-Gd³⁺ are enhanced. From theoretical calculations, combined with experimentally measured angular dependence, we found that for the 1.22 at% Gd:CaF₂ crystals, Gd³⁺ remains essentially cubic, and the majority of the Gd³⁺ ions are distributed as a dimer conformation along the [110] direction and at a distance of about 7.7 Å.