Synthesis and spectroscopy of tetraborate glasses doped with copper

Lithium and potassium–lithium tetraborate glasses doped with Cu (Li2B4O7:Cu and KLiB4O7:Cu) of high optical quality were obtained from polycrystalline compounds by fast cooling of the corresponding melt. Cu impurity was added to the Li2B4O7 and KLiB4O7 compounds in the form of a CuO oxide in the amounts of 0.4 and 1.6 mol.%. On the basis of EPR and optical spectroscopy (absorption, emission and luminescence excitation and kinetics) data analysis it was shown that the Cu impurity was incorporated into the tetraborate glass network as Cu2+ (3d9) and Cu+ (3d10) ions. The EPR spectra of Cu2+ centers were almost identical in glasses with the Li2B4O7:Cu and KLiB4O7:Cu compositions and were characteristic for glassy compounds. The Cu2+ EPR spectra parameters (g-values, hyperfine constants and peak-to-peak linewidths) in the Li2B4O7:Cu and KLiB4O7:Cu glasses were obtained at T = 300 K. The characteristic broad absorption band peaked near 750 nm was assigned to the 2B1g → 2B2g transition of the Cu2+ centers. An intense absorption in the UV region (λ < 350 nm) was related to the Cu2+ → O2− charge-transfer band. Broad emission bands with the maxima near 420 and 465 nm were observed in the luminescence spectra of the Li2B4O7:Cu and KLiB4O7:Cu glasses. The emission bands, peaked near 420 and 465 nm, were characterized by single exponential decay with lifetimes τ = 23 and 27 μs, respectively for the Li2B4O7:Cu and KLiB4O7:Cu glasses (Cu content—1.6 mol.%) at T = 300 K. Both emission bands belonged to the Cu+ centers with different local environments or their distortion. A possible local structure for two types of Cu+ centers in a tetraborate glass network is discussed.