Optical Absorption of Tetrahedral Co3+ and Co2+ in Garnets

Optical spectra of single crystals of high-purity garnets YIG (Y3Fe5O12), YAG (Y3Al5O12), and YGG (Y3Ga5O12) containing cobalt ions have been investigated. With the pure materials the spectra for all three crystals are essentially the same, and the spectrum has been attributed to the rarely observed tetrahedral Co3+. Small amounts of tetrahedral Co2+ also in evidence in YAG and to a smaller extent in YGG can be eliminated by additions of Ca2+ with the cobalt. Addition of Si4+ with the cobalt produces a spectrum characteristic of tetrahedral Co2+ in YAG and YGG but not in YIG. In the latter Fe2+ is formed and the lines of tetrahedral Co2+ are missing. Evidence for lines due to octahedral and dodecahedral Co2+ and octahedral Co3+ is given together with the approximate crystal-field splitting parameter Dq for each. A qualitative interpretation of the fine structure in the spectra of the tetrahedral ions is discussed.