The energy levels of Er 3+ ions have been determined in lithium yttrium borate (Li 6 Y(BO 3 ) 3 )single crystals in a wide spectral range between 6000 -40000 cm -1 together with an analysis of the ground state, using optical and EPR spectroscopy. The crystal field splittings of the 4 I 15/2 ground state and those of nearly all excited states up to the 4 D 7/2 manifold have been obtained at low temperature from luminescence (T = 5 K) and absorption (T = 9 K) measurements, respectively. The numbers of experimentally observed Stark sublevels agree well with those expected theoretically for Er 3+ ions occupying a single low symmetry (C 1 ) site. A full set of g-factor and 167 Er hyperfine tensor parameters are presented for the ground state characterized by EPR; the measured Orbach-type spin-relaxation rates viz. the involved activation energies correspond to the optically derived lowest excited sublevels.

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