EPR CHARACTERIZATION OF Gd3+ IMPURITY IONS IN Li6Y(BO3)3 SINGLE CRYSTALS

Borates are excellent nonlinear optical materials with high optical quality and wide UV transparency range. Double-borates, like lithium yttrium orthoborate (Li6Y(BO3)3, LYB) or the isostructural lithium gadolinium orthoborate (Li6Gd(BO3)3, LGB) are good candidates for both laser host and scintillation applications. Rare-earth ions (e.g. Ce, Nd, Eu, Er, Tm, Yb, etc.) can be easily incorporated into LYB and LGB single crystals having a monoclinic crystal structure belonging to the P21/c space group. Though a large number of papers have already been published on absorption and luminescence measurements concerning the energy levels of rare-earth ions, much less is known about their exact lattice position and ground state. Electron paramagnetic resonance (EPR) spectroscopic results showed the presence of both isolated and paired ytterbium ions substituted for Y3+ in LYB heavily doped with Yb3+.1,2 For LYB single crystals doped with a much lower amount of Er3+ ions, both the optical and EPR spectra indicate that the Er3+ dopant substitutes at an essentially unperturbed Y3+ site.3 The aim of the present work is to determine the spin-Hamiltonian parameters of Gd3+ ions in nominally pure and Fe-doped LYB crystals. Gd3+ has a 8S7/2 ground state with high spin (S=7/2). On the one hand, zero field splitting complicates the spin-Hamiltonian analysis of the EPR spectra. The orbitally non-degenerate S ground state allows spectra to be measured at room temperature. Experiments were carried out at 34 GHz, where the microwave energy is considerably larger than the zero field splitting. They reveal one dominant site for the Gd3+ impurity with low symmetry like for Y3+. The information obtained from the spin-Hamiltonian analysis of Gd3+ is expected to be more directly related with the geometry of the impurity site than for Er3+ or Yb3+ ions, with their orbitally degenerate ground state and effective spin ½. Via a comparison of the EPR characteristics for these three lanthanide impurities in LYB, an attempt is made to further interpret the electronic ground states of Er3+ and Yb3+ in this insulating host. 1. V. Jubera, M. Chavoutier, A. Artemenko, P. Veber, M. Velazquez and A. Garcia, ChemPhysChem 12 (2011) 1288. 2. S. Arceiz Casas, G. Corradi, L. Kovacs, E. Tichy-Racs and S. Greulich-Weber, DPG-Fruhjahrstagung (DPG Spring Meeting) of the Condensed Matter Section (SKM) Dresden, 19-24. 03. 2017 3. L. Kovacs, S. Arceiz Casas, G. Corradi, E. Tichy-Racs, L. Kocsor, K. Lengyel, W. Ryba-Romanowski, A. Strzep, A. Scholle, and S. Greulich-Weber, Opt. Mater. 72 (2017) 270.