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The synthesis of the first 4d transition metal oxidehydride,LaSr 3 NiRuO 4 H 4 ,isprepared via topochemical anion exchange.Neutron diffraction data showthat the hydride ions occupyt he equatorial anion sites in the host lattice and as aresult the Ru and Ni cations are located in aplane containing only hydride ligands,au nique structural feature with obvious parallels to the CuO 2 sheets present in the superconducting cuprates.DFT calculations confirm the presence of S = 1 = 2 Ni + and S = 0, Ru 2+ centers,b ut neutron diffraction and mSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3d z 2 and H1si nthe super-exchange pathways.
Complex transition-metal oxides continue to be the subject of extensive study because they exhibit aw ide variety of interesting physical and chemical properties.T hese range from magnetoresistance,high-temperature superconductivity, and collective magnetism, to ferroelectricity,ionic conductivity,a nd unusual catalytic and photocatalytic behavior. [4] Theresulting metallic state exhibits significant twodimensional character because the hydride ions limit dispersion of the p-symmetry bands,which span the Fermi level, along the z-axis.Despite their many attractive features,o nly ah andful of transition-metal oxide-hydride phases have been prepared to date.The few reported systems can be separated into different groups on the basis of the mechanism by which the metastable oxide-hydride phase is stabilized with respect to decomposition to the elemental transition metal and water (AMO x H y ! Themost obvious difference between oxide and hydride ions is their charge,a saresult of which hydride-for-oxide substitution necessarily involves reduction, allowing access to unusually low transition-metal oxidation states.T he conversion of insulating A II TiO 3 oxides into metallic A II TiO 3Àx H y oxide-hydrides is aclassic example of the use of hydride-foroxide substitution to modify materials properties.