High-pressure synthesis of Ba2RhO4, a rhodate analog of the layered perovskite Sr-ruthenate

A layered perovskite-type oxide ${\mathrm{Ba}}_{2}{\mathrm{RhO}}_{4}$ was synthesized by a high-pressure technique with the support of convex-hull calculations. The crystal and electronic structure were studied by both experimental and computational tools. Structural refinements for powder x-ray diffraction data showed that ${\mathrm{Ba}}_{2}{\mathrm{RhO}}_{4}$ crystallizes in a ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$-type structure, isostructural to ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ and ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$. Magnetic, resistivity, and specific-heat measurements for polycrystalline samples of ${\mathrm{Ba}}_{2}{\mathrm{RhO}}_{4}$ indicate that the system can be characterized as a correlated metal. Despite the close similarity to its ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ counterpart in the electronic specific-heat coefficient and the Wilson ratio, ${\mathrm{Ba}}_{2}{\mathrm{RhO}}_{4}$ shows no signature of superconductivity down to 0.16 K. Whereas the Fermi surface topology has reminiscent pieces of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, an electronlike ${e}_{\mathrm{g}}\ensuremath{-}({d}_{{x}^{2}\ensuremath{-}{y}^{2}})$ band descends below the Fermi level, making this compound unique also as a metallic counterpart of the spin-orbit coupled Mott insulator ${\mathrm{Ba}}_{2}{\mathrm{IrO}}_{4}$.