Attribution of the absorption bands of ruthenium-doped yttrium gallium garnet crystals to Ru3+, Ru4+, and Ru5+ 4d-ions by MCD

Ruthenium-doped yttrium gallium garnet single crystals were grown from a PbO/PbF2/B2O3 flux. Most samples are blue, occasionally green, orange or lemon yellow, depending upon the growth temperature and the amount of a divalent or tetravalent counterion. A spectroscopic study was carried out using several complementary techniques (optical absorption, MCD, and EPR). Low temperature MCD enabled discrimination among those absorption bands correlated to paramagnetic ions (Ru3+ or Ru5+) or diamagnetic Ru4+ at the octahedral site. Ru3+ dominates in the lemon yellow sample with a nominal Ge/Ru ratio of ≈7, whereas Ru5+ is responsible for the orange colour in a crystal with Ca/Ru ≈7. The strongest MCD features above 2.2 eV could be rationalised in terms of oxygen-to-Run+ (n = 3-5) charge transfer transitions in octahedral complexes, whereas the 2 eV band of blue crystals is tentatively assigned to an intervalence transition implying Ru4+. It is suggested that YIG films co-doped with ruthenium and a large amount of calcium or germanium might present interesting Faraday effect properties.