Aqueous Synthesis of Strontium Ruthenate(VI) Oxyhydroxides and Their Crystal Structure Solution from Microcrystals

Abstract

Investigation of the earliest stages of hydrothermal reactions between KRuO4 and either Sr(NO3)2 or SrO2 allows the isolation of phase-pure SrRuO3(OH)2, a hitherto uncharacterized Ru(VI) oxyhydroxide, previously mentioned in the literature as SrRuO4·H2O. Further exploration of synthesis conditions with excess of strontium salt yields a second Ru(VI) phase, Sr3Ru2O8(OH)2 at 200 °C. Both structures were solved by 3D-electron diffraction and refined using powder neutron diffraction (SrRuO3(OH)2: monoclinic P21/n, a = 9.9903(3) Å, b = 7.7023(2) Å, c = 17.3677(6) Å, β = 89.353(2)°; Sr3Ru2O8(OH)2: tetragonal P4̅, a = 13.2206(5) Å c = 5.4852(2) Å). The two Ru(VI) materials each contain isolated trigonal bipyramidal ruthenium centers with axially positioned hydroxides with longer Ru–OH bond distances than the equatorial Ru–O distances. Infrared spectroscopy corroborates the presence of hydroxide, with O–H stretching bands that are red-shifted upon deuteration. X-ray absorption near-edge spectroscopy at the Ru K-edge confirms the +6 oxidation state of Ru, and magnetization measurements show two unpaired electrons per Ru associated with the 4d2 electronic configuration in both materials. The crystal structures, particularly the local environment of Ru, are compared with other reported Ru(VI) compounds, and derived bond valence sum parameters give consistent results for ruthenate(VI) oxides and oxyhydroxides.