We present the synthesis and structural characterisation of a transition metal oxide hydride, LaSrCoO 3 H 0.7 , which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a twodimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering up to at least 350 K. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal-containing electronic and magnetic materials.The covalent interaction between the O 2− anion and the d-orbitals of the transition metal cation is at the heart of the remarkable electronic properties of the transition metal oxides(1, 2): even in mixed-anion oxyhalides(3), it is the metal-oxide interactions which dominate the physical properties. Developing synthetic routes to materials in which other anions partially replace oxide could open up the possibility of preparing entirely novel families of electronically active transition metal compounds. The hydride anion, H − , with a 1s 2 electronic configuration, is known to engage in strong covalent bonding with transition metal centres in discrete molecular species(4) and would be an excellent candidate for the transmission of exchange interactions or electron delocalization between transition metal cations in an oxide hydride, if the formidable synthetic difficulties barring the path to such a phase could be overcome. The problem is that H − , unlike O 2− or halide anions such as F − and Cl − , is a powerful reducing agent and would be expected to transform the transition metal component of a typical hightemperature ternary transition metal oxide synthesis into the metal, defeating most possible synthetic strategies. Here we demonstrate a lowtemperature topotactic route to insert H − anions directly into an extended transition metal oxide array, and show that H − transmits exchange interactions between the transition metal cations at least as effectively as O 2− , opening up a new mechanism for designing co-operative effects in solids.We have recently shown that NaH is an effective low-temperature reducing agent for ternary transition metal oxides. At temperatures below 190• C NaH affords the Ni(I)(5) and Co(I)(6) oxidation states, but at higher temperatures completely reduces the metal because of the presence of hydrogen gas in thermal equilibrium with the hydride salt. In order to study the solidstate reactivity of H − with ternary transition metal oxides at higher temperatures, we used the more thermally stable CaH 2 (with a decomposition temperature of 885• C compared with 210• C for NaH). CaH 2 was reacted with the Co(III) oxide LaSrCoO 4 , which adopts the layered K 2 NiF 4 structure with square planar CoO 2 sheets alternating with (La/Sr) O rock-salt layers and octahedral coordination around Co(III). Reaction for two periods of 4 days at 450• C in a sealed Pyrex tube with intermediate grinding afforded a mixture of CaO and an orthorhombic phase 1 (7). The orthorhombic phas...

structure structure (solids and liquids) D 2000 -006The Hydride Anion in an Extended Transition Metal Oxide Array: LaSrCoO 3 H 0.7 .-The title compound is prepared by reaction of CaH 2 and LaSrCoO 4 (sealed Pyrex tube, 450 • C, 8 d). LaSrCoO 3 H 0.7 crystallizes in the orthorhombic space group Immm, as revealed by powder synchrotron XRD. The structure contains oxide chains bridged by hydride anions to form a two-dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering at temperatures up to at least 350 K. The preparation method may prove to be a general route to transition metal oxide hydrides, opening up previously uncharted areas in electronic and magnetic materials synthesis. -(HAYWARD, M. A.; CUSSEN, E. J.; CLARIDGE, J. B.; BIERINGER, M.; ROSSEINSKY, M. J.; KIELY, C.

We present the results of muon-spin-relaxation experiments for two materials which show geometric frustration. ZnCr 2 O 4 has a spinel structure with S = 3 2 spins on a lattice of corner-sharing tetrahedra. Our experiments show that a local magnetic field which is quasi-static on the muon timescale develops below T c = 12.5 K, a transition which has been associated with a three-dimensional analogue of the spin-Peierls transition. In contrast, Gd 3 Ga 5 O 12 has a garnet structure with S = 7 2 spins arranged on interpenetrating triangular sublattices. In this material the muon data exhibit a temperature-dependent spin-relaxation rate indicative of slow spin fluctuations. We discuss these differing behaviours and relate them to the underlying physics in the two materials.

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