Examining the contribution of Cu and Sr codoping on luminescence properties of borate crystals

Four crystals of Ba₁₂(BO₃)₆[BO₃][LiF₄] (LBBF) with different ratios of co-dopants, Cu and Sr, were grown from high-temperature solutions. The LBBF structure is built from a porous [Ba₁₂(BO₃)₆]⁶⁺ framework with channels along the optical axis filled with anionic groups. It was found that LBBF:Cu,Sr accommodates copper ions in the channels of the structure in both mono- and divalent states with the concentration of Cu ⁺ exceeding that of Cu²⁺, and Sr²⁺ replaces Ba²⁺ in the framework. At a constant concentration of copper and an increase in the concentration of strontium in the initial solution, the concentration of Cu⁺ in LBBF does not change monotonically but has a maximum. The broad photoluminescence band at about 410–440 nm is associated with the emission of Cu⁺ ions, while the bands at about 310–317 nm and 600 nm are related to Cu²⁺. All emissions demonstrate considerable temperature quenching in the range from 77 to 300 K. The LBBF:Cu crystals exhibit intense thermoluminescence (TL) in the range from 100 to 250 K after low-temperature X-ray irradiation, and peaks at about 315 and 380 K after room-temperature irradiation. With combined doping with strontium, the TL peaks shift to the region of higher temperatures, 365 and 410 K, without weakening the high-temperature component, which intensity linearly depends on radiation dose.