What is it about?
Due to the good thermal-mechanical and luminescence properties, Yb:YAG ceramics are suitable as thin-disk lasers; however, their efficiency is limited by the presence of Yb2+ ions, which entail parasitic energy transfer from Yb3+ to Yb2+. This article focuses on the Yb2+ formation in Yb:YAG ceramics prepared by solid-state reaction sintering. The samples were prepared by air annealing, the oxidation of the material leads to recharging Yb2+ ion to its trivalent state. The activation energy was determined by Jander to be Ea(D) = 2.7 ± 0.2 eV, which is in good agreement with the activation energy for oxygen diffusion in the YAG lattice. It was concluded that the recharging of Yb2+ ion to its trivalent state in YAG ceramics is limited by the oxygen self-diffusion through the grain volume, and the oxygen vacancy alone and/or together with the presence of antisites can be proposed as Yb2+ charge compensation mechanism in the YAG ceramics, unlike the YbAG single crystals, where tetravalent impurities are responsible for charge compensation.
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Why is it important?
The garnet ceramics of Yb:YAG were synthesized by vacuum reaction sintering at high temperature, the time-dependence of Yb2+ concentration was investigated by air annealing in a special furnace. After vacuum sintering, the absorption spectra of the Yb:YAG ceramics contained four absorption bands which disappeared under air annealing and correspond to the absorption of the Yb2+ ions and the F+ colour centres, moreover, the absorption bands of Yb3+ slightly increased in intensity. A longer air annealing led to a decrease in the Yb2+ amount, whereas a higher temperature sped up the oxidation velocity. Only 0.25 at% of Yb ions existed in the divalent state before oxidation. The Yb2+ time-dependence was described by the Jander model, allowing to conclude that the Yb2+ to Yb3+ oxidation process is limited by the diffusion of oxidative agents. The EG of Yb2+ oxidation was Ea(D) = 2.7 ± 0.2 eV, which corresponds to the activation energy EG for volume oxygen diffusion in the YAG lattice. We concluded that the Yb2+ oxidation in the Yb:YAG ceramics is limited by oxygen diffusion, meaning that oxygen vacancies and/or oxygen vacancies with antisites compensate for the Yb2+ charge, in contrast to the YbAG single crystal where tetravalent impurities are responsible for it.
Perspectives
The formation of Yb2+ ions is an important part of the energy loss process due to the energy transfer in the Yb3+-Yb2+ pairs, therefore, understanding the nature of Yb2+ charge compensation mechanism would allow decreasing the occurrence of this parasitic process in laser materials. To our knowledge, few works related to the Yb2+-Yb3+ ions valence transformation process have been published [14,15], and a detailed mechanism of Yb2+ ions charge compensation has not yet been proposed due to the lack of supporting information.
Mykhailo Chaika
Institute of Low Temperature and Structure Research
Read the Original
This page is a summary of: Dynamics of Yb2+ to Yb3+ ion valence transformations in Yb:YAG ceramics used for high-power lasers, Optical Materials, March 2020, Elsevier,
DOI: 10.1016/j.optmat.2020.109774.
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