The Hydride Anion in an Extended Transition Metal Oxide Array: LaSrCoO ₃ H _0.7

Abstract

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...

Ambient Pressure Synthesis and H^− Conductivity of LaSrLiH₂O₂

“…However, the preparation of oxyhydrides is mainly required special synthesis conditions such as a solid state reaction at high-pressure [6][7][8] or a strong reducing reaction using metal hydrides as the reductant. [9][10][11][12][13] Although the synthesis of the LSLHO system by a conventional solid-state reaction at ambient pressure has been reported, it is only suitable for a standard composition of La 2 LiHO 3 , which contains the lowest concentration of H ¹ in this system. 14 ¹ and (LaSrO 2 ) + layers along the c-axis (Fig.…”

Topotactic synthesis of strontium cobalt oxyhydride thin film with perovskite structure

“…In transition metal oxyhydrides, the metal-H − bond is known to be much shorter than the metal-oxygen bond, whereas the latter lengthens after hydride substitution. [4][5][6][7]12 For example, in the case of LaSrCoO 3 H 0.7 having Co-H The obtained SrCoO x H y film was found to be insulating with a resistivity ρ of >10…”

“…Figure 3(c) shows the Co L 2,3 -edge XAS spectra of the precursor SrCoO 2. 5 and SrCoO x H y films with Sr 2 CoO 3 Cl as a high-spin (HS) Co 3+ reference and CoO as a Co 2+ reference. 20 Each spectrum has peaks around 780 eV and 795 eV, which correspond to the Co L 3 and Co L 2 edges, respectively.…”

“…[5][6][7][8][9] These cobalt oxyhydrides with low Co valence states were obtained as bulk phases from the corresponding oxides, LaSrCoO 4 and Sr 3 Co 2 O 5.8 , respectively, by topotactic reaction using CaH 2 , which served not only as a strong reducing agent but also a source of hydride ions. In general, topotactic reactions for thin film samples have two advantages: considerably higher reactivity than bulk samples owing to the larger surface area/volume ratio, and stabilization of the perovskite-based framework arising from the epitaxial effect.…”

Correction

“…The authors form their unusual product by removing an oxide ion from the starting material, a process that is coupled to a redox reaction in which Fe 4+ ions are converted into Fe 2+ ions. The overall process uses a recently discovered reagent (calcium hydride) for the kinetically controlled removal of oxygen from oxides 6 , and it occurs at the remarkably low temperature of 280 °C. These conditions provide insufficient thermal energy to rearrange the structure of the starting material completely -only the relatively mobile oxide ions can change position (Fig.…”