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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Condensed Matter Physics

Background:

• Ruthenium-based oxides with face-sharing octahedra are key for tuning electronic states via metal-metal interactions.
• Controlling valence states in these materials is crucial for understanding their correlated electronic properties.

Purpose of the Study:

• To synthesize and characterize a novel hexagonal perovskite, Ba3NbRu2O9.
• To investigate the influence of structural ordering and valence states on the electronic and magnetic properties of this compound.
• To elucidate the role of Ru2O9 dimers in determining the material's behavior.

Main Methods:

• High-pressure, high-temperature synthesis.
• Single-crystal and powder X-ray diffraction for structural analysis.
• Charge balance analysis to determine oxidation states.
• Magnetic susceptibility (DC and AC) and specific heat measurements.
• Electrical transport and magnetoresistance measurements.

Main Results:

• Successfully synthesized and characterized Ba3NbRu2O9, a 6H-type hexagonal perovskite with an ordered structure (P63/mmc).
• Identified a low mixed Ru3+/Ru4+ valence state (nominal Ru3.5+), indicating reduced ruthenium.
• Observed antiferromagnetic interactions at high temperatures and a frozen spin state below 15 K, lacking long-range magnetic order.
• Electrical transport showed a resistivity maximum near 35 K, a low-temperature upturn, and significant positive magnetoresistance, resembling BaRuO3 polytypes.

Conclusions:

• The electronic and magnetic properties of Ba3NbRu2O9 are primarily governed by the molecular electronic state of the Ru2O9 dimers.
• Crystallographic stacking alone is not the dominant factor; dimer-based correlations are central to the behavior of low-valence ruthenates.
• This study provides insights into tuning correlated electronic states in ruthenium oxides through dimer engineering.