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The four dopants (Nd, Gd, Dy, and Er) substitutionally occupy the Li + sites in lithium tetraborate (Li 2 B 4 O 7 : RE) glasses as determined by analysis of the extended X-ray absorption fine structure. The dopants are coordinated by 6-8 oxygen at a distance of 2.3 to 2.5 Å, depending on the rare earth. The inverse relationship between the RE-O coordination distance and rare earth (RE) atomic number is consistent with the expected lanthanide atomic radial contraction with increased atomic number. Through analysis of the X-ray absorption near edge structure, the rare earth dopants adopt the RE 3+ valence state. There are indications of strong rare earth 5d hybridization with the trigonal and tetrahedral formations of BO 3 and BO 4 based on the determination of the rare earth substitutional Li + site occupancy from the X-ray absorption near edge structure data. The local oxygen disorder around the RE 3+ luminescence centers evident in the structural determination of the various glasses, and the hybridization of the RE 3+ dopants with the host may contribute to the asymmetry evident in the luminescence emission spectral lines. The luminescence emission spectra are indeed characteristic of the expected f-to-f transitions; however, there is an observed asymmetry in some emission lines.

The electron paramagnetic resonance spectra of the glasses with Li 2 B 4 O 7 and KLiB 4 O 7 compositions doped with Mn and Cu were investigated. On the basis of obtained EPR spectra analysis it was shown that the Mn and Cu impurities are incorporated into the glass network as Mn2+ EPR spectra in the glasses with Li2B4O7 and KLiB4O7 compositions are characterised by the following parameters, measured at T = 300 K: isotropic g-factor (giso = 2.00 ± 0.05), isotropic hyperfine constant (Aiso = (8.65 ± 0.05) mT) and peak-to-peak linewidth of hyperfine components ∆Bpp = (3.50 ± 0.05) mT. The Cu 2+ EPR spectra in the glasses with Li2B4O7 and KLiB4O7 compositions, registered at T = 300 K are characterised by the same g-values (g = 2.34 ± 0.05, g ⊥ = 2.06 ± 0.05) and peak-to-peak line width of hyperfine components (∆B pp = (5.11 ± 0.05) mT, ∆B ⊥ pp = (1.80 ± 0.05) mT), whereas anisotropic hyperfine constants show some differences (A = (14.28 ± 0.05) mT, A ⊥ = (2.34 ± 0.05) mT for glass with Li 2 B 4 O 7 composition and A = (14.21 ± 0.05) mT, A ⊥ = (2.55 ± 0.05) mT for glass with KLiB 4 O 7 composition). The possible local structure of the Mn 2+ and Cu 2+ centres in the lithium and potassium-lithium tetraborate glass network has been considered.