Magnetism and 3D Electron Diffraction Solution of Hydrated Rubidium-Ruthenium Oxide Rb$_2$Ru$_2$O$_7$.H$_2$O

The crystal structure of Rb$_2$Ru$_2$O$_7$.H$_2$O was determined by three-dimensional electron diffraction from the individual crystallites of a solid-state powder product. Rb$_2$Ru$_2$O$_7$.H$_2$O crystallizes in space group \textit{C}2/\textit{c} ($a=7.841(3)$ Å, $b=12.500(3)$ Å, $c=8.392(2)$ Å, $β=93.57(4)^\circ$, Z=4). The structure contains infinite chains that run normal to the (101) plane and consist of alternating RuO$_6$ octahedra and square-pyramidal RuO$_5$ units connected via shared O-O edges. Magnetic properties were measured on the bulk powder, showing a diamagnetic baseline from 300 to 60 K with a small Curie tail below 55 K. The magnetic moment, calculated from the 1.8 K isotherm, saturates at $M=4.4\times10^{-3}\,μ_\mathrm{B}\,\mathrm{Ru}^{-1}$, much less than would be expected for $S=1$ ruthenium. Bond-valence-sum analysis indicates high-valent Ru, and the near-diamagnetic response is consistent with the edge-sharing Ru-Ru motif, where weak direct Ru-Ru overlap yields a local singlet.