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...

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ABSTRACT:The coefficient of thermal expansion of ZrMgMo 3 O 12 has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (α = (1.6 ± 0.2) × 10 −7 K −1 from 25 to 450°C by X-ray diffraction (XRD)). ZrMgMo 3 O 12 belongs to the family of AMgM 3 O 12 materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo 3 O 12 by nuclear magnetic resonance (NMR) crystallography, using 91 Zr, 25 Mg, 95 Mo, and 17 O magic angle spinning (MAS) and 17 O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo 3 O 12 shows significant ionic conductivity, a property that is also related to its structure.

The compounds Na 2 Ti 2 Pn 2 O (Pn ) As, Sb) crystallize in the anti-K 2 NiF 4 structure type in the space group, I4/mmm, with Z ) 2 and the lattice parameters a ) 4.0810(9) Å and c ) 15.311(3) Å for the As analogue at 310 K and a ) 4.160(2) Å and c ) 16.558(7) Å for the Sb analogue at 150 K. The structure consists of edge-shared ∞ 2 [Ti 4/2 Pn 2 O 4/4 ] 2-layers separated by double layers of Na + . These compounds exhibit an anomalous transition in the temperature-dependent magnetic susceptibility at T c onset ) 330 K for the As analogue and T c onset ) 120 K for the Sb analogue. Temperature-dependent powder neutron diffraction has been performed to investigate the magnetic spin ordering and structure symmetry breakdown of the compounds; however, no scattering due to magnetic spin ordering or symmetry change has been detected. The temperature-dependent electrical resistivity of these compounds also exhibits an anomaly reminiscent of CDW (charge-density-wave)/SDW (spin-density-wave) materials. The As analogue shows an insulator-to-insulator transition around 135 K whereas the Sb analogue shows a metal-to-metal transition around 120 K that corresponds well to the onset of the anomaly in the magnetic susceptibility. The similarity and difference in the physical properties between the As and Sb analogues as well as related compounds will be discussed.

The modulation of atomic positions in CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) was studied using synchrotron radiation powder diffraction below 250 and 220 K, respectively. The copper-rich member CaCu(x)Mn(7-x)O12 (x = 0.23) does not show any modulation of the atomic positions at temperatures as low as 10 K. Using low-temperature neutron powder diffraction the modulation of the magnetic moments of Mn ions in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) has been investigated. Long-range modulated magnetic ordering in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) is observed below 90.4, 89.2 and 78.1 K. (0,0,q(p)) and (0,0,q(m)) are the propagation vectors describing the modulations of the atomic positions and the magnetic moments. For CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) the magnetic modulation and atomic modulation lengths are related by a factor of 2 satisfying the relation (1-q(p)) = 2(1-q(m)).