The optical and luminescence properties of La_3Ga_5SiO_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 $$E_{g}^{{{\text{opt}}}}$$ = 5.1 eV. Upon interband excitation, La_3Ga_5SiO_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_3Ga_5SiO_14 is demonstrated. The presence of traps in La_3Ga_5SiO_14 is shown using thermally stimulated luminescence, and their activation energy is determined.

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Optical properties of lanthanum-gallium silicate (La3Ga5SiO14), lanthanum-gallium tantalate (La3Ga5.5Ta0.5O14), calcium-tantalum-gallium silicate (Ca3TaGa3Si2O14) crystals in the near ultraviolet (UV) and visible wavelength ranges (200-800 nm) have been studied. For the first time the angles of deviation of the moments of the dipole transition spectral dependences theta(λ) were used to characterize the absorption processes in these crystals. These dependences make it possible to reveal absorption bands that are weakly noticeable on the transmission coefficients spectral dependences, and to estimate the degree of deviation of the dispersion dependence theta(λ) from the value theta=45o corresponding to an ideal ordered crystal. Analysis of theta(λ) dependencies showed that in the studied wavelength range, the lowest deviation from theta =45o is observed in Ca3TaGa3Si2O14, and the highest - in La3Ga5SiO14. The coefficients of the specific rotation angle of the polarization plane rho were obtained by the spectrophotometric interference method in polarized light. The measured values are best approximated by the Vyshin equation. For the first time, dependences of the modified Drude formula 1/rho=f(λ2) were calculated for these crystals. These dependences should be linear in case of an ideal crystal. The largest deviation from the linear dependence is observed for La3Ga5SiO14 crystals, and the lowest for Ca3TaGa3Si2O14. The discrete values of the refractive coefficients No and Ne were measured by the prism method. La3Ga5.5Ta0.5O14 crystals have the highest refractive coefficients, Ca3TaGa3Si2O14 crystals have the lowest. The obtained values are approximated by the Sellmeier equation.