Thermally stimulated recombination processes and luminescence in Li6(Y,Gd,Eu)(BO3)3 crystals

Abstract

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li6(Y,Gd,Eu)(BO3)3 have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li6Gd(BO3)3, Li6Eu(BO3)3, Li6Y0.5Gd0.5(BO3)3: Eu, and Li6Gd(BO3)3: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the 6PJ → 8S7/2 transitions in the Gd3+ ion and the group of lines at 580–700 nm due to the 5D0 → 7FJ transitions (J = 0–4) in the Eu3+ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.