Pulsed EPR spectroscopy of Gd3+ centers in Ca1−xRxF2+x (R=La, Y) mixed crystals

A study of Gd3+ centers in Ca1−xRxF2+x (R=La, Y) crystals using pulsed EPR spectroscopy is presented. The echo-induced EPR (ESE-EPR) spectrum shows, besides the signal of slightly perturbed cubic Gd3+ centers, a broad signal at g≈2 due to Gd3+ centers at low symmetry sites. To describe the effects of R3+ ions on the EPR Gd3+, a model, including cubic and linear R3+−2Fi− centers, is developed. Its predictions are compared with the experimental results. The composition dependence of the EPR signal due to slightly perturbed cubic Gd3+ centers in mixed Ca1−xRxF2+x crystals is explained taking into account the different clustering tendency in La and Y crystals. Moreover, the formation of mixed clusters involving R3+ and Gd3+ ions is proposed for both series of samples. A greater clustering trend is found in the Y crystals than in the La ones. Gd3+ ions are found to be a “non innocent” paramagnetic probe for structural studies in these mixed crystals.