Transition-metal oxyhydrides are of considerable current interest due to the unique features of the hydride anion, most notably the absence of valence p orbitals. This feature distinguishes hydrides from all other anions, and gives rise to unprecedented properties in this new class of materials. Here we show via a high-pressure study of anion-ordered strontium vanadium oxyhydride SrVO2H that H− is extraordinarily compressible, and that pressure drives a transition from a Mott insulator to a metal at ~ 50 GPa. Density functional theory suggests that the band gap in the insulating state is reduced by pressure as a result of increased dispersion in the ab-plane due to enhanced Vdπ-Opπ-Vdπ overlap. Remarkably, dispersion along c is limited by the orthogonal Vdπ-H1s-Vdπ arrangement despite the greater c-axis compressibility, suggesting that the hydride anions act as π-blockers. The wider family of oxyhydrides may therefore give access to dimensionally reduced structures with novel electronic properties.

Topochemical reduction of the ordered double perovskite LaSrNiRuO6 with CaH2 yields LaSrNiRuO4, an extended oxide phase containing infinite sheets of apex-linked, square-planar Ni(1+)O4 and Ru(2+)O4 units ordered in a checkerboard arrangement. At room temperature the localized Ni(1+) (d(9), S = (1)/2) and Ru(2+) (d(6), S = 1) centers behave paramagnetically. However, on cooling below 250 K the system undergoes a cooperative phase transition in which the nickel spins align ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. Features of the low-temperature crystal structure suggest a symmetry lowering Jahn-Teller distortion could be responsible for the observed diamagnetism of the ruthenium centers.

Topochemical reduction of the double-perovskite oxide Sr 2 FeIrO 6 under dilute hydrogen leads to the formation of Sr 2 FeIrO 4 . This phase consists of ordered infinite sheets of apex-linked Fe 2+ O 4 and Ir 2+ O 4 squares, stacked with Sr 2+ cations, and is the first report of Ir 2+ in an extended oxide phase. Plane-wave DFT calculations indicate high-spin Fe 2+ (d 6 , S = 2) and low-spin Ir 2+ (d 7 , S = ½) configurations for the metals and confirm that both ions have a doubly occupied orbital, a configuration that is emerging as a consistent feature of all layered oxide phases of this type. The stability and double occupation of in the Ir 2+ ions invites a somewhat unexpected analogy to the extensively studied Ir 4+ ion as both ions share a common near-degenerate ( / / ) 5 valence configuration. On cooling below 115 K Sr 2 FeIrO 4 enters a magnetically ordered state in which the Ir and Fe sub-lattices adopt type II antiferromagnetically coupled networks which interpenetrate each other leading to frustration in the nearest-neighbor Fe-O-Ir couplings, half of which are ferromagnetic and half anti-ferromagnetic. The spin frustration drives a symmetry-lowering structural distortion in which the four equivalent Ir-O and Fe-O distances of the tetragonal I4/mmm lattice split into two mutually trans pairs in a lattice with monoclinic I112/m symmetry. This strong magneto-lattice coupling arises from the novel local electronic configurations of the Fe 2+ and Ir 2+ cations and their cation-ordered arrangement in a distorted perovskite lattice.

The synthesis of the first 4d transition metal oxide-hydride, LaSr NiRuO H , is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO sheets present in the superconducting cuprates. DFT calculations confirm the presence of S=1/2  Ni and S=0, Ru centers, but neutron diffraction and μSR 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 3dz2 and H 1s in the super-exchange pathways.

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. [1] Ty pically the properties of complex oxides are tuned via cation substitutions,b ut modifications to the anion lattice, either by the introduction of anion vacancies or by substituting non-oxide heteroanions,c an also be used to modify the chemical and physical behavior of oxides.F or example,anion doping allows metal oxidation states to be adjusted, the onsite electronic configuration of transition metal centers to be modified (through ligand-field interactions) and inter-cation couplings to be tuned (though cation-anion-cation linkages), providing access to novel electronic states.Thecontrasting features of oxide (O 2À )and hydride (H À ) anions offer many attractive opportunities to modify the electronic properties of host oxide phases by anion substitution. 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. [2] Thelower electronegativity of hydride compared to oxide also implies ah igher degree of covalency and orbital mixing in M À H bonds compared to M À Oanalogues,and as aresult the band structures of oxide-hydrides will be qualitatively different from their parent oxides.F urthermore,m agnetic coupling strengths can be strongly enhanced in oxide-hydride phases, resulting in the high magnetic ordering temperatures observed for LaSrCoO 3 H 0.7 ,S rVO 2 H, and SrCrO 2 H. [3] A final, more subtle difference between oxide and hydride ions is the absence of p-symmetry valence orbitals on the H À anion. Theo rbital connectivity of ap hase can therefore be altered dramatically,e specially if the oxide and hydride anions adopt an ordered arrangement....