Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ-irradiation

Recharging processes of chromium ions were investigated for Mg2SiO4:Mg, Cr single crystals using annealing in 02 and in air and gamma-irradiation, as compare to YAG:Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg2SiO4 :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O2- vacancy existing in the crystal, but also to the external field such as gamma-irradiation. The additional absorption after gamma-irradiation shows the decrease in intensity of the absorption of Cr3+ and Cr4+ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr3+ and Cr4+. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr4+ giving rise to Cr4+ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after gamma-irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of gamma-iffadiation suggests that gamma-irradiation ionizes only Cr ions.