PULSED EPR SPECTROSCOPY OF GD3-XRXF2+X (R=LA, Y) MIXED-CRYSTALS( CENTERS IN CA1)

A study of Gd3+ centers in Ca1-xRxF2+x (R = La, Y) crystals using puls ed EPR spectroscopy is presented. The echo-induced EPR (ESE-EPR) spect rum shows, besides the signal of slightly perturbed cubic Gd3+ centers , a broad signal at g approximate to 2 due to Gd3+ centers at low symm etry sites. To describe the effects of R3+ ions on the EPR Gd3+, a mod el, including cubic and linear R3+-2F(i)(-) centers, is developed. Its predictions are compared with the experimental results. The compositi on dependence of the EPR signal due to slightly perturbed cubic Gd3+ c enters 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 fo r 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 innoc ent'' paramagnetic probe for structural studies in these mixed crystal s.