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, B4 = -2.3153 +/- 0.015 MHz and B6 = -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 & Aring;.