The influence of co-doping on the luminescence and thermoluminescence properties of Cu-containing fluoride borate crystals

Abstract

The results of this study provide data on the luminescence and thermoluminescence properties of fluoride borate Ba₁₂(BO₃)₆[BO₃][LiF₄] crystals co-doped with copper, rare earth elements, and strontium. The crystals were grown from high temperature solutions. This study revealed a considerable difference in decay time: about 60 ns for Cu⁺ photoluminescence at 412 nm in Ba₁₂(BO₃)₆[BO₃][LiF₄]:Cu crystals after excitation at 263 nm, and about 1.8 ms for Eu³⁺ photoluminescence at 612 nm in Ba₁₂(BO₃)₆[BO₃][LiF₄]:Cu,Eu crystals after excitation at 300, 325, and 395 nm. The absence of short-wavelength luminescence of Cu⁺ centers in photoluminescence spectra under 325 nm excitation in Ba₁₂(BO₃)₆[BO₃][LiF₄]:Cu,Eu and Ba₁₂(BO₃)₆[BO₃][LiF₄]:Cu,Eu,Tb,Ce crystals is associated with the nonradiative energy transfer from the excited electronic state of Cu⁺ ions to the energy levels of the rare earth elements. The thermoluminescence curves of these crystals lack a peak associated with capture centers. Co-doping with copper and strontium (Ba₁₂(BO₃)₆[BO₃][LiF₄]:Cu,Sr, P4₂/mbc, a = 13.5174 (3) Å, c = 14.9399 (3) Å) has a noticeable effect on both photo- and thermoluminescence properties. This fosters the formation of deeper capture centers and promotes an increase in the temperature interval between thermoluminescence peaks, which is essential for the stable storage of dosimetric information. First-principles density functional theory investigation indicates that the presence of monovalent copper ions narrows the bandgap width compared to that of undoped Ba₁₂(BO₃)₆[BO₃][LiF₄] crystals due to the transitions from the Cu-3d to Ba-5d levels. The presence of bivalent copper in the structure results in unfilled defect levels inside the bandgap. This contributes to the absorption in the visible range due to the electronic transitions from the O-2p to Cu-3d levels.