LaSr3 NiRuO4 H4 : A 4d Transition-Metal Oxide-Hydride Containing Metal Hydride Sheets

Is La3Ni2O6.5 a Bulk Superconducting Nickelate?

Superconducting states onsetting at moderately high temperatures have been observed in epitaxially stabilized RENiO2-based thin films. However, recently, it has also been reported that superconductivity at high temperatures is observed in bulk La3Ni2O7-δ at high pressure, opening further possibilities for study. Here we report the reduction profile of La3Ni2O7 in a stream of 5% H2/Ar gas and the isolation of the metastable intermediate phase La3Ni2O6.45, which is based on Ni2+. Although this reduced phase does not superconduct at ambient or high pressures, it offers insights into the Ni-327 system and encourages future study of nickelates as a function of oxygen content.

A new family of anti-perovskite oxyhydrides with tetrahedral GaO4 polyanions

New solid compounds A3-xGaO4H1-y (A = Sr, Ba; x ∼0.15, y ∼0.3), which are the first oxyhydrides containing gallium ions, have been synthesized by high-pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that the series adopts an anti-perovskite structure consisting of hydride-anion-centered HA6 octahedra with tetrahedral GaO4 polyanions, wherein the A- and H-sites show partial defect. Formation energy calculations from the raw materials support that stoichiometric Ba3GaO4H is thermodynamically stable with a wide band gap. Annealing the A = Ba powder under flowing Ar and O2 gas suggests topochemical H- desorption and O2-/H- exchange reactions, respectively.

Microstructural Activation of a Topochemical Reduction Reaction

The progress of the topochemical reduction reaction that converts LaSrNiRuO₆ into LaSrNiRuO₄ depends on the synthesis conditions used to prepare the oxidized phase. Samples of LaSrNiRuO₆ that have been quenched from high temperature can be readily and rapidly converted into LaSrNiRuO₄. In contrast, samples that have been slow-cooled cannot be completely reduced. This reactivity difference is attributed to the differing microstructures of the quenched and slow-cooled samples, with the former having much smaller average crystalline domain sizes and larger lattice strains than the latter. A mechanism to explain this effect is presented, in which the greater "plasticity" of small crystalline domains helps lower the activation energy of the reduction reaction. In addition, we propose that the enhanced lattice strain in quenched samples also acts to destabilize the host phase, further enhancing reactivity. These observations suggest that the microstructure of a material can be used to "activate" topochemical reactions in the solid state, expanding the scope of phases that can be prepared by this type of reaction.

Hole and Electron Doping of Topochemically Reduced Ni(I)/Ru(II) Insulating Ferromagnetic Oxides

La_xSr_2-xNiRuO₆, La_xSr_4-xNiRuO₈, and La_xSr_3-xNiRuO₇ are, respectively, the n = ∞, 1, and 2 members of the (La_x/2Sr_1-(x/2))_nSr(Ni_0.5Ru_0.5)_nO_3n+1 compositional series. Reaction with CaH₂, in the case of the La_xSr_2-xNiRuO₆ perovskite phases, or Zr oxygen getters in the case of the La_xSr_4-xNiRuO₈ and La_xSr_3-xNiRuO₇ Ruddlesden-Popper phases, yields the corresponding topochemically reduced (La_x/2Sr_1-(x/2))_nSr(Ni_0.5Ru_0.5)_nO_3n-1 compounds (La_xSr_2-xNiRuO₄, La_xSr_4-xNiRuO₆, and La_xSr_3-xNiRuO₅), which contain Ni and Ru cations in square-planar coordination sites. The x = 1 members of each series (LaSrNiRuO₄, LaSr₃NiRuO₆, and LaSr₂NiRuO₅) exhibit insulating ferromagnetic behavior at low temperature, attributable to exchange couplings between the Ni¹⁺ and Ru²⁺ centers they contain. Increasing the La³⁺ concentration (x > 1) leads to a reduction of some of the Ru²⁺ centers to Ru¹⁺ centers and a suppression of the ferromagnetic state (lower T_c, reduced saturated ferromagnet moment). In contrast, increasing the Sr²⁺ concentration (x < 1) oxidizes some of the Ru²⁺ centers to Ru³⁺ centers and enhances the ferromagnetic coupling (increased T_c, increased saturated ferromagnet moment) for the n = ∞ and n = 2 samples but appears to have no influence on the magnetic ordering temperature of the n = 1 samples. The magnetic couplings and influence of doping are discussed on the basis of superexchange and direct exchange couplings between the square-planar Ni and Ru centers.

Diffusional Dynamics of Hydride Ions in the Layered Oxyhydride SrVO2H

Perovskite-type oxyhydrides are hydride-ion-conducting materials of promise for several types of technological applications; however, the conductivity is often too low for practical use and, on a fundamental level, the mechanism of hydride-ion diffusion remains unclear. Here, we, with the use of neutron scattering techniques, investigate the diffusional dynamics of hydride ions in the layered perovskite-type oxyhydride SrVO₂H. By monitoring the intensity of the elastically scattered neutrons upon heating the sample from 100 to 430 K, we establish an onset temperature for diffusional hydride-ion dynamics at about 250 K. Above this temperature, the hydride ions are shown to exhibit two-dimensional diffusion restricted to the hydride-ion sublattice of SrVO₂H and that occurs as a series of jumps of a hydride ion to a neighboring hydride-ion vacancy, with an enhanced rate for backward jumps due to correlation effects. Analysis of the temperature dependence of the neutron scattering data shows that the localized jumps of hydride ions are featured by a mean residence time of the order of 10 ps with an activation energy of 0.1 eV. The long-range diffusion of hydride ions occurs on the timescale of 1 ns and with an activation energy of 0.2 eV. The hydride-ion diffusion coefficient is found to be of the order of 1 × 10^-6 cm² s^-1 in the temperature range of 300-430 K, which is similar to other oxyhydrides but higher than for proton-conducting perovskite analogues. Tuning of the hydride-ion vacancy concentration in SrVO₂H thus represents a promising gateway to improve the ionic conductivity of this already highly hydride-ion-conducting material.

Evidence of Paracrystalline Cation Order in the Ruddlesden-Popper Phase LaSr3NiRuO8 through Neutron Total Scattering Techniques

Cation ordering in perovskite-derived phases can lead to a wealth of tunable physical properties. Ordering is typically driven by a large difference between the cation size and charge, but many Ruddlesden-Popper phases A_n+1B_nO_3n+1 appear to lack such B-site ordering, even when these differences are present. One such example is the "double" Ruddlesden-Popper n = 1 composition LaSr₃NiRuO₈. In this material, a lack of B-site ordering is observed through traditional crystallographic techniques, but antiferromagnetic ordering in the magnetism data suggests that B-site cation ordering is indeed present. Neutron total scattering, particularly analysis of the neutron pair distribution function, reveals that the structure is locally B-site-ordered below 6 Å but becomes slightly disordered in the midrange structure around 12 Å. This provides evidence for paracrystalline order in this material: cation ordering within a single perovskite sheet that lacks perfect registry within the three-dimensional stack of sheets. This work highlights the importance of employing a structural technique that can probe both the local and midrange order in addition to the crystallographic structure and provides a structural origin to the observed magnetic properties of LaSr₃NiRuO₈. Further, it is proposed that paracrystalline order is likely to be common among these layered-type oxides.

Fluorination and reduction of CaCrO3 by topochemical methods

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure.

Trigonal-Planar Low-Spin Co2+ in a Layered Mixed-Polyhedral Network from Topotactic Reduction

Topotactic reduction of the perovskite oxide TbBaCo₂O_5.5 with CaH₂ leads to a new crystalline phase TbBaCo₂O_4.5, adopting a 2 × 2 × 1 superstructure compared to TbBaCo₂O_5.5. The structure consists of a corner-shared network of square pyramidal CoO₅ and trigonal planar CoO₃ units. Magnetic susceptibility and variable temperature neutron diffraction data reveal that TbBaCo₂O_4.5 adopts a G-type antiferromagnetically ordered structure (T_N ∼ 322 K). The ordered moments are consistent with the presence of low-spin Co²⁺ (S = ¹/₂) in trigonal-planar coordination and high-spin Co²⁺ centers in square pyramidal coordination. TbBaCo₂O_4.5 shows lower conductivity than TbBaCo₂O_5.5, which is consistent with the p-type conduction behavior. The unique anion vacancy arrangements in TbBaCo₂O_4.5 further complement the role of A-cations in controlling the oxygen vacancy distribution in LnBaCo₂O_5+δ series and demonstrate more opportunity to tune the structural and physical properties based on cationic and anionic lattice coupling.

Rhodium-containing oxide-hydrides: covalently stabilized mixed-anion solids

The first rhodium-containing oxide-hydride phases, LaSrCo0.5Rh0.5O3H and La0.5Sr1.5Mn0.5Rh0.5O3H, have been prepared via topochemical anion exchange. This clearly demonstrates the ability of rhodium, a late 4d transition metal, to kinetically stabilize oxide-hydride lattices, reinforcing the paradigm of covalent stabilization of transition-metal oxide-hydride phases.