Beyond layer stacking: molecular Ru₂O₉ dimer correlations in pressure-synthesized Ba₃NbRu₂O₉

Ruthenium-based oxides featuring face-sharing octahedra provide a powerful chemical platform for tuning correlated electronic states through direct metal-metal interactions and valence control. Here we report the high-pressure, high-temperature synthesis and comprehensive characterization of Ba₃NbRu₂O₉, a previously unreported 6H-type hexagonal perovskite. Single-crystal and powder X-ray diffraction measurements confirm a fully ordered structure composed of face-sharing Ru₂O₉ dimers separated by corner-sharing NbO₆ octahedra, crystallizing in the P6₃/mmc space group. Charge balance analysis reveals an unusually low mixed Ru³⁺/Ru⁴⁺ oxidation state (nominal Ru^3.5+), representing a reduced Ru valence realized within the Ba₃MRu₂O₉ family. Magnetic susceptibility measurements show Curie-Weiss behavior at high temperatures with an effective magnetic moment of 2.39μ_B per formula unit and a negative Curie-Weiss temperature, indicating predominant antiferromagnetic interactions. Below ∼15 K, a bifurcation between zero-field-cooled and field-cooled susceptibilities emerges, while the absence of a frequency-dependent shift in AC susceptibility and the lack of a λ-type anomaly in specific heat down to 1.8 K indicate a frozen spin state with short-range correlations rather than long-range magnetic order or a canonical spin-glass transition. Electrical transport measurements reveal a broad resistivity maximum near 35 K followed by a low-temperature upturn and pronounced positive magnetoresistance. Remarkably, the temperature dependence of resistance closely resembles that of the nine-layer BaRuO₃ polytype, despite the structural similarity of Ba₃NbRu₂O₉ to the four-layer phase. These results demonstrate that the electronic and magnetic properties of Ba₃NbRu₂O₉ are governed primarily by the molecular electronic state of the Ru₂O₉ dimers rather than by crystallographic stacking alone, underscoring the central role of dimer-based correlations in low-valence ruthenates.