Thomas C. Brunold and Hans U. Güdel
Ruthenium(VI) Doped into Single Crystals with the BaSO₄ and b-K₂SO₄ Structures:
Optical Absorption Spectra of RuO₄^2-
Inorg. Chem. 36, 2084-2091 (1997)                 

Abstract: By establishing suitable high-temperature preparation conditions we were able to grow the first RuO₄^2--doped crystals of K₂CrO₄, K₂SeO₄, Cs₂MoO₄, BaSO₄, BaCrO₄, and BaSeO₄. Their polarized absorption spectra at low temperatures are reported and discussed. These are very different from the ruthenate(VI) spectra in the literature, and the lack of resemblance between calculated and measured spectra of ruthenate(VI)-assumed to be RuO₄^2- in the past-is easily understood. The ³A₂ ¹A₁ spin-flip transition of RuO₄^2- peaks at 7100 cm^–1. Stronger bands due to ³A₂ ³T₂ and ³T₁ are observed in the vis around 13 500 and 16 500 cm^–1, respectively, giving rise to the green-blue color of most samples. In the BaSO₄ host the ¹A₁ absorption line is split into three differently polarized components exhibiting a strong temperature dependence. This can be analyzed in terms of a Boltzmann population with three levels at 0, 13, and 18 cm^–1, corresponding to the three spinor components of the ³A₂ ground state. In order to better understand the major changes which occur on going from the 3d² to the 4d² electron configuration the spectroscopic data of RuO₄^2- are compared with those of CrO₄^4-, MnO₄^3-, and FeO₄^2-.