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

Enhanced Magnetic Interaction by Face-Shared Hydride Anions in 6H-BaCrO2H

Studies on magnetic oxyhydrides have been almost limited to perovskite-based lattices with corner-sharing octahedra with a M-H-M (M: transition metal) angle of θ ∼ 180°. Using a high-pressure method, we prepared BaCrO₂H with a 6H-type hexagonal perovskite structure with corner- and face-sharing octahedra, offering a unique opportunity to investigate magnetic interactions based on a θ ∼ 90° case. Neutron diffraction for BaCrO₂H revealed an antiferromagnetic (AFM) order at T_N ∼ 375 K, which is higher than ∼240 K in BaCrO_3-xF_x. The relatively high T_N of BaCrO₂H can be explained by the preferred occupancy of H^- at the face-sharing site that provides AFM superexchange in addition to AFM direct exchange interactions. First-principles calculations on BaCrO₂H in comparison with BaCrO₂F and BaMnO₃ further reveal that the direct Cr-Cr interaction is significantly enhanced by shortening the Cr-Cr distance due to the covalent nature of H^-. This study provides a useful strategy for the extensive control of magnetic interactions by exploiting the difference in the covalency of multiple anions.

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.

LaSrCo0.5Rh0.5O3.25 and LaSrNi0.5Rh0.5O3.25:Topochemically Reduced, Mixed Valence Rh(I)/Rh(III) Oxides

Topochemical reduction of the n = 1 Ruddlesden-Popper phases LaSrCo_0.5Rh_0.5O₄ and LaSrNi_0.5Rh_0.5O₄ with Zr yields LaSrCo_0.5Rh_0.5O_3.25 and LaSrNi_0.5Rh_0.5O_3.25, respectively. Magnetization and XPS data reveal that while the rhodium centers in LaSrCo_0.5Rh_0.5O_3.25 and LaSrNi_0.5Rh_0.5O_3.25 have an average oxidation state of Rh²⁺, these are actually mixed valence Rh(I,III) compounds, with the disproportionation of Rh²⁺ driven by the favorability of locating d⁸ Rh¹⁺ and d⁶ Rh³⁺ cations within square-planar and square-based pyramidal coordination sites, respectively.