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

TL;DR

This work reports the first structure solution of the hydrated alkali ruthenate Rb2Ru2O7·H2O by 3D electron diffraction, revealing a monoclinic $C2/c$ framework with two Ru sites arranged in edge-sharing Ru–Ru motifs that form infinite zig-zag chains. Magnetic data show near-diamagnetic behavior with only a small Ru-centered moment, consistent with a local singlet arising from Ru–Ru overlap and high Ru oxidation states (Ru$^{+5}$ to Ru$^{+6}$) confirmed by bond-valence-sum analysis. Structural validation by PXRD and electron microscopy demonstrates coherence between the 3D ED model and bulk material, and establishes 3D ED as a powerful tool for solving hydrated, hydroxide-containing transition-metal oxides in polycrystalline form. The study highlights a structure–magnetism relationship governed by direct Ru–Ru interactions in edge-sharing networks and expands the known chemistry of alkali ruthenates.

Abstract

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.

Figures (8)

• Figure 1: Polyhedral representation of (a) 3D ED structure solution projected onto $(100)$ for Rb2Ru2O7 . H2O in the monoclinic C2/c space group ($a=7.841(3)$ Å, $b=12.500(3)$ Å, $c=8.392(2)$, Z = 4, $\beta=93.57(4)^\circ$), (b) Zig-zag chain of alternating RuO6/RuO5 projected onto $(010)$. (c) Edge-sharing unit that comprises the nominal molecule labeled by crystallographically unique atoms.
• Figure 2: PXRD pattern of bulk Rb2Ru2O7 . H2O with a Rietveld refinement using the structure solved by 3D ED (ICSD deposition: CSD 2514706) without internal standard. Rietveld refinement: $\chi^2 = 1.8$, $R_\mathrm{wp} = 4.5$, $R_\mathrm{exp} = 3.4$.
• Figure 3: (a) Temperature dependence of the molar susceptibility, $\chi(T)$, for bulk polycrystalline Rb2Ru2O7 . H2O, zero field cooled with applied field of 1 mT. (b) Isothermal magnetization, $M(\mu_{0}H)$, at selected temperatures. All $\chi$ and $M$ data are normalized to the crystalline Rb2Ru2O7 . H2O mass fraction obtained by Rietveld refinement with an Al2O3 internal standard ($f_\mathrm{cryst}=0.25$) and values are reported per Ru in the crystalline phase.
• Figure 4: Images of the three crystallites from which the data are collected. The 1 $\mu$m scale bar is shown bottom-left of each image, alongside the tilt angle at which the image is taken, while the green circle in the center represents the approximate location of the selected area aperture used in each experiment.
• Figure 5: PXRD pattern of bulk Rb2Ru2O7 . H2O with internal standard $\mathbf{Al_2O_3}$ Internal standard (ICSD 130950).