Synthesis, EPR and optical spectroscopy of the Cr-doped tetraborate glasses

► Glasses with Li2B4O7:Cr and KLiB4O7:Cr compositions of high optical quality were obtained. ► EPR, optical spectra, and luminescence kinetics were studied and compared with similar borate glasses. ► Spectroscopy shows Cr3+ centers in low-field and high-field octahedral sites of glass structure. ► Parameters of the Cr3+ paramagnetic and luminescence centers were obtained and analyzed. ► The Li2B4O7:Cr and KLiB4O7:Cr glasses can be promising for tunable lasers of the NIR region. Electron paramagnetic resonance (EPR), optical absorption and photoluminescence spectra as well as luminescence kinetics of the Li2B4O7:Cr and KLiB4O7:Cr tetraborate glasses were investigated at T=300K. The Li2B4O7:Cr and KLiB4O7:Cr glasses containing 0.4 and 1.6mol.% Cr2O3 of high optical quality were obtained from polycrystalline compounds by fast cooling of the melts. The X-band EPR spectroscopy shows that the Cr impurity is incorporated in the tetraborate glass network as isolated Cr3+ centers and Cr3+–Cr3+ pairs coupled by magnetic dipolar and exchange interactions. The EPR spectral parameters (geff and ΔBpp) of both Cr3+ and Cr3+–Cr3+ centers in the Li2B4O7:Cr and KLiB4O7:Cr glasses were measured and analyzed. All transitions in optical absorption, luminescence excitation and emission spectra of these glasses are identified. Broad complex bands that peak near 615, 405, and 350nm in optical absorption and luminescence excitation spectra correspond to the 4A2g(F)→4T2g(F), 4A2g(F)→4T1g(F), and 4A2g(F)→4T1g(P) spin-allowed transitions of the Cr3+ centers in distorted octahedral sites of the tetraborate glass network. The octahedral (cubic) crystal field strength (10Dq) and Racach parameters (B and C) for Cr3+ centers in Li2B4O7:Cr and KLiB4O7:Cr glasses are estimated. Narrow and broad emission bands in red – NIR regions are assigned to the 2Eg(F)→4A2g(F) (R1 line) and 4T2g(F)→4A2g(F) (electron-vibration) transitions, which correspond to the Cr3+ centers in high-field and low-field sites, respectively. All observed emission bands are characterized by non-exponential decay. Measured average lifetimes and local structure of the Cr3+ centers in high-field and low-field sites of the Li2B4O7:Cr and KLiB4O7:Cr glass network have been discussed.