The role of the perovskite lattice oxygen in the oxygen evolution reaction (OER) is systematically studied in the PrBaCo2O5+δ family. The reduced number of physical/chemical variables combined with in‐depth characterizations such as neutron dif‐fraction, O K‐edge X‐ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), magnetization and scanning transmission electron microscopy (STEM) studies, helps investigating the complex correlation between OER activity and a single perovskite property, such as the oxygen content. Larger amount of oxygen vacancies appears to facilitate the OER, possibly contributing to the mechanism involving the oxidation of lattice oxygen, i.e., the lattice oxygen evolution reaction (LOER). Furthermore, not only the number of vacancies but also their local arrangement in the perovskite lattice influences the OER activity, with a clear drop for the more stable, ordered stoichiometry.

The origin of the metal-to-insulator transition (MIT) in RNiO 3 perovskites with R = trivalent 4 f ion has challenged the condensed matter research community for almost three decades. A drawback for progress in this direction has been the lack of studies combining physical properties and accurate structural data covering the full nickelate phase diagram. Here we focus on a small region close to the itinerant limit (R = Pr, 1.5 K < T < 300 K), where we investigate the gap opening and the simultaneous emergence of charge order in PrNiO 3 . We combine electrical resistivity, magnetization, and heat capacity measurements with high-resolution neutron and synchrotron x-ray powder diffraction data that, in contrast to previous studies, we analyze in terms of symmetryadapted distortion modes. Such analysis allow us to identify the contribution of the different modes to the global distortion in a broad temperature range. Moreover, it shows that the structural changes at the MIT, traditionally described in terms of the evolution of the interatomic distances and angles, appear as abrupt increases of all nonzero mode amplitudes at T MIT = T N ∼ 130 K accompanied by the appearance of new modes below this temperature. A further interesting observation is the existence of a nearly perfect linear correlation between the amplitude of the breathing mode associated to the charge order and the staggered magnetization below the MIT. Our data also uncover a previously unnoticed anomaly at T * ∼ 60 K (∼0.4 × T MIT ), clearly visible in the electrical resistivity, lattice parameters and some mode amplitudes. Since phase coexistence is only observed in a small temperature region around T MIT (∼ ± 10 K), these observations suggest the existence of a hidden symmetry in the insulating phase. We discuss some possible origins, among them the theoretically predicted existence of polar distortions induced by the noncentrosymmetric magnetic order [Perez-Mato et al.,

Fe 4 Nb 2 O 9 was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at T N ≈ 90 K for an antiferromagnetic transition and T str ≈ 77 K of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below T N and uncovered the origin of the second dielectric anomaly as a structural phase transition across T str . In the antiferromagnetically ordered state below T N , both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below T str , the symmetry of crystal structure is lowered from trigonal P-3c1 to monoclinic C2/c, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of ∼1.7 • . The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from 2.1(1)μ B at 95 K to 3.83(4)μ B at 10 K assuming an equal moment configuration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between T N and T str are proved to be spin-current and d-p hybridization mechanisms.

Electric properties Electric properties D 8000Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series. -The series La 10-x (GeO 4 ) 6 O 3-1.5x (0 ≤ x ≤ 0.67) is synthesized from La2O3 and GeO2 by the ceramic method (1523 K, 6 h). Neutron and powder synchrotron X-ray diffraction reveal that the phases are hexagonal (space group P63/m) for 0.40 ≤ x ≤0.48 and triclinic (space group P1) for 0.25 ≤ x ≤ 0.34. Impedance measurements indicate that the samples are bulk oxide ion conductors. The conductivities are almost independent of the oxygen partial pressure under oxidizing conditions, which suggests pure oxide-ion conduction with negligible electronic contribution. -(LEON-REINA, L.; MARTIN-SEDENO, M. C.; LOSILLA, E. R.; CABEZA, A.; MARTINEZ-LARA, M.; BRUQUE, S.; MARQUES, F. M. B.; SHEPTYAKOV, D. V.; ARANDA*, M. A. G.; Chem.

One of the most important scientific problems faced by our society is how to convert and store clean energy. In order to achieve a significant progress in this field we need to understand the fundamental dynamical processes that govern the transfer of energy on an atomic scale. For many energy devices such as solid-state batteries and solid-oxide fuel cells, this means understanding and controlling the complex mechanisms of ion diffusion in solid matter. Because of the unusual evolution of correlated electronic properties (frustrated magnetism and superconductivity), the layered Co-oxide family NaxCoO2 (0<x<1), object of this work, has been extensively studied during the last decade. More recently it has also attracted the attention of applied sciences, mainly because of its structural similarity with LixCoO2, one of the most common Li-ion battery electrodes. In view of the larger abundance of Na in the earth crust with respect to Li, Na-ion batteries enjoy an increased attention. Hence we decided to investigate the Na-ion diffusion in this material, whose possible use as cathode for solid-state rechargeable batteries has recently been proposed [1]. The present study reports the observation of a crossover from quasi-1D to 2D Na-ion diffusion in Na0.7CoO2. High resolution neutron powder diffraction data indicate the existence of two structural transitions at T1=290K and T2=400K [2]. We present here evidence indicating that both transitions are closely related to changes in the Na-ion mobility. Analysis of the anomalies in the Na-Na distances, the Debye-Waller factors and the scattering density in the paths connecting neighbouring Na sites strongly suggest that Na-ion diffusion starts at T1, although for T1<T<T2 it occurs preferentially along quasi-1D paths. A fully isotropic diffusion is only observed for T>T2, coinciding with the equalization of all first-neighbor Na-Na distances in the structure [2]. These findings provide new insight on the subtle mechanisms controlling the Na-ion diffusion in the NaxCoO2 family and could be used for the design of related energy materials with improved functional properties. Fig. 1. Fourier difference maps of the z = 0.25 Na planes at T = 50, 320 and 450 K showing the evolution of the residual scattering density in the paths connecting the Na1 and Na2 sites (from ref.[2]).

The magnetic and structural properties of polycrystalline Co 4−x Ni x Nb 2 O 9 (x = 1, 2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x = 1, the compound crystallizes in the trigonal P3c1 space group. Below T N = 31 K it develops a weakly noncollinear antiferromagnetic structure with magnetic moments in the ab plane. The compound with x = 2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T C = 47 K. For this compound a weakly noncollinear ferrimagnetic structure with two possible configurations in the ab plane was derived from neutron diffraction study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provides further information on the magnetic structure of the compounds.

The substitution of hydrogen for oxygen atoms in metal oxides provides opportunities for influencing the solid-state properties. Such hydride oxides (or oxyhydrides) are potential functional materials and scarce. Here, we present the synthesis and characterization of holmium hydride oxide with the stoichiometric composition HoHO. It was prepared by the reaction of Ho 2 O 3 with either HoH 3 or CaH 2 as a powder of light-yellow color in sunlight and pink color in artificial light (Alexandrite effect), which is commonly observed for ionic Ho(III) compounds. HoHO crystallizes with an ordered fluorite superstructure (F4̅ 3m, a = 5.27550(13) Å, half-Heusler LiAlSi type), as evidenced by powder X-ray and neutron powder diffraction on both hydride and deuteride and supported by quantum-mechanical calculations. HoHO is the first representative with considerable ionic bonding for this structure type. The thermal stability and inertness toward air are remarkably high for a hydride because it reacts only above 540 K to form Ho 2 O 3 . At 294(1) K and 25(3)% relative humidity, HoHO is stable for at least 3 months. HoHO is paramagnetic with μ eff (Ho 3+ ) = 10.41(2) μ B without any sign of magnetic ordering down to 2 K.

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