Optical Transparency of Rare-Earth Iron Garnets

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Abstract
Optical absorption and electrical resistivity have been measured for single-crystal flux grown rare-earth iron garnets doped with Si4+, Sn4+, Ge4+, and Ca2+. Strong optical absorption near 9000 cm−1 is observed due to relaxation mechanisms involving transfer of an electron from Fe2+ ions in the case of Si, Ge, Sn doping, or a hole from Fe4+ ions in the case of Ca doping to Fe3+ ions in the vicinity. This is accompanied by n-type electrical conductivity for crystals containing Fe2+ introduced by Si, Sn, Ge, and by p-type conductivity for those containing Fe4+ produced by Ca doping. We find that pure rare-earth iron garnets contain small amounts (0.004 atom/formula) of Fe4+ producing undesirable optical absorption, and this can be eliminated by small additions of tetravalent Si, Sn, or Ge.