The impact of yttrium aluminum garnet stoichiometry deviation on the conversion efficiency of tetravalent chromium ions

In this work, the conversion efficiency of Cr4+ ions and the optical properties of ceramics based on chromium-doped yttrium aluminum garnet were investigated. Increasing the conversion efficiency of Cr3+ to Cr4+ opens up broad prospects for using YAG ceramics as saturable absorbers for passive Q-switching of Nd- and Yb-solid-state lasers. The aim of this work was to study the effect of magnesium oxide concentration on the conversion efficiency of Cr3+ to Cr4+ under conditions of Al3+ (4.8 mol.%) and Y3+ (2.9 mol.%) cations excess in the garnet structure as well as the stoichiometric ratio Y3+/Al3+ = 3/5. The possibility of changing the concentration of Cr4+ ions in both the octahedral and tetrahedral positions due to the formation of magnesium substitution defects when the composition deviates from the yttrium-aluminum garnet stoichiometry has been studied. Chromium-doped transparent optical ceramics based on yttrium-aluminum garnet with different ratios of Y3+/Al3+ cations was obtained by the two-stage coprecipitation method. Magnesium oxide was used as a sintering additive and charge compensator for converting chromium ions from the trivalent to the tetravalent state in concentrations of 0 to 0.2 wt.%. It was found that aluminum excess in the yttriumaluminum garnet matrix leads to a decrease in the Cr3+ to Cr4+ conversion efficiency. A deviation of stoichiometry towards an excess of yttrium leads to a decrease of air annealing temperature for oxidizing chromium ions to the tetravalent state and their transition to the tetrahedral position. However, the samples optical transmittance with an excess of yttrium was lower than in the cases of an excess of aluminum and stoichiometry. It was found that with an increase in the vacuum sintering temperature from 1780 to 1820 °C in the samples with an excess of aluminum, an increase in the concentration of Cr4+ ions in the tetrahedral position occurs, as evidenced by higher absorption intensity at a wavelength of 1030 nm. For the cases of stoichiometry and an excess of yttrium in garnet, this effect is less pronounced. Samples of optical ceramics of yttrium-aluminum garnet with a light transmission of more than 75 % in the visible and near-IR ranges with pronounced absorption bands of chromium have been obtained. The results of the work can be applied to the creation of passive switches for solid-state lasers.