Luminescence properties of undoped langasite crystals

The optical and luminescence properties of La$_{3}$Ga$_{5}$SiO$_{14}$ lanthanum–gallium silicate crystals grown in atmospheres of argon and argon with the addition of oxygen are investigated. The results of calculations of the structure of energy bands are presented, obtained using the CASTEP module in the framework of the generalized gradient approximation and the local density approximation. The width of the optical band gap of the crystal is determined to be $E_{g}^{\operatorname{опт}}$ = 5.1 eV. Upon interband excitation, La$_{3}$Ga$_{5}$SiO$_{14}$ crystals grown in argon atmosphere show a luminescence band with a maximum at 430 nm, whereas for a crystal grown in argon with addition of oxygen, two luminescence bands with maxima at 470 and 530 nm dominate in the luminescence spectrum. The nature of the luminescence centers responsible for these bands is discussed with the help of the data for electronic structure calculations. The effect of temperature on the luminescent properties of La$_{3}$Ga$_{5}$SiO$_{14}$ is demonstrated. The presence of traps in La$_{3}$Ga$_{5}$SiO$_{14}$ is shown using thermally stimulated luminescence, and their activation energy is determined.