Exploring Reversible Redox Behavior in the 6H-BaFeO3-δ (0 < δ < 0.4) System: Impact of Fe3+/Fe4+ Ratio on CO Oxidation

This work is devoted to evaluating the relationship between the oxygen content and catalytic activity in the CO oxidation process of the 6H-type BaFeO3-δ system. Strong evidence is provided about the improvement of catalytic performance with increasing Fe average oxidation state, thus suggesting the involvement of lattice oxygen in the catalytic process. The compositional and structural changes taking place in both the anionic and cationic sublattices of the catalysts during redox cycles have been determined by temperature-resolved neutron diffraction. The obtained results evidence a structural transition from hexagonal (P63/mmc) to orthorhombic (Cmcm) symmetry. This transition is linked to octahedra distortion when the Fe3+ concentration exceeds 40% (δ values higher than 0.2). The topotactical character of the redox process is maintained in the δ range 0 < δ < 0.4. This suggests that the cationic framework is only subjected to slight structural modifications during the oxygen exchange process occurring during the catalytic cycle.

Synergy of diffraction and spectroscopic techniques to unveil the crystal structure of antimonic acid

The elusive crystal structure of the so-called 'antimonic acid' has been investigated by means of robust and state-of-the-art techniques. The synergic results of solid-state magic-angle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochlore-type structure of stoichiometric formula (H₃O)_1.20(7)H_0.77(9)Sb₂O₆. Some protons belong to heavily delocalized H₃O⁺ subunits, while some H⁺ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O-H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bond-valence energy landscape analysis. X-ray absorption spectroscopy results corroborate the structural data around Sb⁵⁺ ions at short-range order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Identifying the presence of magnetite in an ensemble of iron-oxide nanoparticles: a comparative neutron diffraction study between bulk and nanoscale

Scientific interest in iron-oxides and in particular magnetite has been renewed due to the broad scope of their fascinating properties, which are finding applications in electronics and biomedicine. Specifically, iron oxide nanoparticles (IONPs) are gathering attraction in biomedicine. Their cores are usually constituted by a mixture of maghemite and magnetite phases. In view of this, to fine-tune the properties of an ensemble of IONPs towards their applications, it is essential to enhance mass fabrication processes towards the production of monodisperse IONPs with controlled size, shape, and stoichiometry. We exploit the vacancy sensitivity of the Verwey transition to detect the presence of magnetite. Here we provide direct evidence for the Verwey transition in an ensemble of IONPs through neutron diffraction. This transition is observed as a variation in the Fe magnetic moment at octahedral sites and, in turn, gives rise to a change of the net magnetic moment. Finally, we show this variation as the microscopic ingredient driving the characteristic kink that hallmarks the Verwey transition in thermal variation of magnetization.

Structural, electrical characterization and oxygen-diffusion paths in LaSrGa1−xMgxO4−δ (x = 0.0–0.2) layered perovskites: an impedance spectroscopy and neutron diffraction study

We present the diffusion pathways in perovskite-like structures. The modules of Gfourir and BondStr of Fullprof are feasible to obtain different paths in first approximation.

Topotactic Oxidation of Perovskites to Novel SrMo1-xMxO4-δ (M = Fe and Cr) Deficient Scheelite-Type Oxides

New polycrystalline SrMo_1−xM_xO_4−δ (M = Fe and Cr) scheelite oxides have been prepared by topotactical oxidation, by annealing in air at 500 °C, from precursor perovskites with the stoichiometry SrMo_1−xM_xO_3−δ (M = Fe and Cr). An excellent reversibility between the oxidized Sr(Mo,M)O_4−δ scheelite and the reduced Sr(Mo,M)O_3−δ perovskite phase accounts for the excellent behavior of the latter as anode material in solid-oxide fuel cells. A characterization by X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) has been carried out to determine the crystal structure features. The scheelite oxides are tetragonal, space group I4₁/a (No. 88). The Rietveld-refinement from NPD data at room temperature shows evidence of oxygen vacancies in the structure, due to the introduction of Fe³⁺/Cr⁴⁺ cations in the tetrahedrally-coordinated B sublattice, where Mo is hexavalent. A thermal analysis of the reduced perovskite (SrMo_1−xM_xO_3−δ) in oxidizing conditions confirms the oxygen stoichiometry obtained by NPD data; the stability range of the doped oxides, below 400–450 °C, is lower than that for the parent SrMoO₃ oxide. The presence of a Mo⁴⁺/Mo⁵⁺ mixed valence in the reduced SrMo_1−xM_xO_3−δ perovskite oxides confers greater instability against oxidation compared with the parent oxide. Finally, an XPS study confirms the surface oxidation states of Mo, Fe, and Cr in the oxidized samples SrMo_0.9Fe_0.1O_4-δ and SrMo_0.8Cr_0.2O_4-δ.

Boosting Oxygen Evolution Reaction by Creating Both Metal Ion and Lattice-Oxygen Active Sites in a Complex Oxide

Developing efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance to many chemical and energy transformation technologies. The diversity and flexibility of metal oxides offer numerous degrees of freedom for enhancing catalytic activity by tailoring their physicochemical properties, but the active site of current metal oxides for OER is still limited to either metal ions or lattice oxygen. Here, a new complex oxide with unique hexagonal structure consisting of one honeycomb-like network, Ba₄ Sr₄ (Co_0.8 Fe_0.2 )₄ O₁₅ (hex-BSCF), is reported, demonstrating ultrahigh OER activity because both the tetrahedral Co ions and the octahedral oxygen ions on the surface are active, as confirmed by combined X-ray absorption spectroscopy analysis and theoretical calculations. The bulk hex-BSCF material synthesized by the facile and scalable sol-gel method achieves 10 mA cm^-2 at a low overpotential of only 340 mV (and small Tafel slope of 47 mV dec^-1 ) in 0.1 m KOH, surpassing most metal oxides ever reported for OER, while maintaining excellent durability. This study opens up a new avenue to dramatically enhancing catalytic activity of metal oxides for other applications through rational design of structures with multiple active sites.

Water insertion and combined interstitial-vacancy oxygen conduction in the layered perovskites La1.2Sr0.8−xBaxInO4+δ

H₂O molecules are split within the structure with protons bonded to the axial oxygens of the InO₆ octahedra, and with OH units occupying the interstitial space.

Atomically Resolved Short-Range Order at the Nanoscale in the Ca-Mn-O System

The elucidation of the reaction mechanisms involving redox processes in functional transition-metal oxides, which usually start in areas of very few nanometers in size, is yet a challenge to be satisfactorily achieved. Atomically resolved HAADF and EELS have provided both chemical and structural information at the nanoscale, which reveal the preservation of short-range cationic order in areas of 2-3 nm length as the driving force behind the reversibility of the Ca₂Mn₃O₈-Ca₂Mn₃O₅ redox process. Oxygen evolution is accommodated by cationic diffusion along the Ca and Mn layers of the cation-deficient Ca₂Mn₃O₈ delafossite related structure, whereas Mn remains octahedrally coordinated.

Ionic conductivity enhancement in Ti-doped Sr11Mo4O23 defective double perovskites

A substantially higher ionic motion can be achieved by partially replacing Mo(vi) by Ti(iv) cations in the novel Sr₁₁Mo_4−xTi_xO_23−δ (with x = 0.0, 0.5 and 1.0) electrolyte oxides, successfully enhancing the oxygen vacancy level.

Giant Seebeck effect in Ge-doped SnSe

Thermoelectric materials may contribute in the near future as new alternative sources of sustainable energy. Unprecedented thermoelectric properties in p-type SnSe single crystals have been recently reported, accompanied by extremely low thermal conductivity in polycrystalline samples. In order to enhance thermoelectric efficiency through proper tuning of this material we report a full structural characterization and evaluation of the thermoelectric properties of novel Ge-doped SnSe prepared by a straightforward arc-melting method, which yields nanostructured polycrystalline samples. Ge does not dope the system in the sense of donating carriers, yet the electrical properties show a semiconductor behavior with resistivity values higher than that of the parent compound, as a consequence of nanostructuration, whereas the Seebeck coefficient is higher and thermal conductivity lower, favorable to a better ZT figure of merit.

A new LaCo0.71(1)V0.29(1)O2.97(3) perovskite containing vanadium in octahedral sites: synthesis and structural and magnetic characterization

In the course of an investigation to prepare the hypothetic new double perovskite La(3)Co(2)VO(9) with Co(2+) and V(5+) in octahedral sites, we obtained the new simple perovskite LaCo(0.71(1))V(0.29(1))O(2.97(3)) as the main phase. The pure compound was then synthesized by the citrate decomposition method. The crystal structure was studied by X-ray (PXRD) and powder neutron diffraction (PND). Physical properties were characterized by X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and thermogravimetric analysis (TGA). Rietveld refinements were performed in the orthorhombic space group Pnma (#62). Refined cell parameters were a = 5.4762(2) Å, b = 7.7609(2) Å and c = 5.5122(1) Å. Magnetization measurements showed that this perovskite is an antiferromagnet with a Neel temperature of 15 K. At high T the magnetization follows the Curie-Weiss law corrected by temperature independent paramagnetism (TIP) showing an effective magnetic moment of 3.03μ(B) well described by the contribution of Co(2+) (HS), Co(3+) (IS), V(3+) and V(4+) ions. The crystallographic formula was refined by PND and oxidation state distribution was determined by the combination of PND, XAS, TGA and magnetic measurements.

Experimental visualization of the diffusion pathway of sodium ions in the Na3[Ti2P2O10F] anode for sodium-ion battery

Sodium-ion batteries have attracted considerable interest as an alternative to lithium-ion batteries for electric storage applications because of the low cost and natural abundance of sodium resources. The materials with an open framework are highly desired for Na-ion insertion/extraction. Here we report on the first visualization of the sodium-ion diffusion path in Na₃[Ti₂P₂O₁₀F] through high-temperature neutron powder diffraction experiments. The evolution of the Na-ion displacements of Na₃[Ti₂P₂O₁₀F] was investigated with high-temperature neutron diffraction (HTND) from room temperature to 600°C; difference Fourier maps were utilized to estimate the Na nuclear-density distribution. Temperature-driven Na displacements indicates that sodium-ion diffusion paths are established within the ab plane. As an anode for sodium-ion batteries, Na₃[Ti₂P₂O₁₀F] exhibits a reversible capacity of ~100 mAh g⁻¹ with lower intercalation voltage. It also shows good cycling stability and rate capability, making it promising applications in sodium-ion batteries.

k=0 magnetic structure and absence of ferroelectricity in SmFeO3

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675  K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

Visualization by neutron diffraction of 2D oxygen diffusion in the Sr(0.7)Ho(0.3)CoO(3-δ) cathode for solid-oxide fuel cells

Sr0.7Ho0.3CoO3-δ oxide has been recently described as an excellent cathode material (1274 mW cm(-2) at 850 °C with pure H2 as fuel1) for solid oxide fuel cells (SOFCs) with LSGM as electrolyte. In this work, we describe a detailed study of its crystal structure conducted to find out the correlation between the excellent performance as a cathode and the structural features. The tetragonal crystal structure (e.g., I4/mmm) basically contains layers of octahedrally coordinated Co2O6 units alternated with layers of Co1O4 tetrahedra sharing corners. An "in situ" neutron power diffraction (NPD) experiment, between 25 and 800 °C, reveals the presence of a high oxygen deficiency affecting O4 oxygen atoms, with large displacement factors that suggest a large lability and mobility. Difference Fourier maps allow the visualization at high temperatures of the 2D diffusion pathways within the tetrahedral layers, where O3 and O4 oxygens participate. The measured thermal expansion coefficient is 16.61 × 10(-6) K(-1) between 300 and 850 °C, exhibiting an excellent chemical compatibility with the electrolyte.

Ferromagnetic Cu-O-Cu coupling in CaCu3Sn4O12 probed by neutron diffraction

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T(C) = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 μ(B)/Cu atom.

Dimerization and charge order in hollandite K₂V₈O₁₆

The metal-insulator transition occurring in hollandite K₂V₈O₁₆ has been studied by means of neutron and x-ray diffraction as well as by thermodynamic and electron-spin resonance measurements. The complete analysis of the crystal structure in the distorted phase allows us to identify dimerization as the main distortion element in insulating K₂V₈O₁₆. At low-temperature, half of the V chains are dimerized perfectly explaining the suppression of magnetic susceptibility due to the formation of spin singlets. The dimerization is accompanied by the segregation of charges into chains.

Magnetic structure of LaCrO3 perovskite under high pressure from in situ neutron diffraction

The temperature-pressure phase diagram for both the crystal and magnetic structures of LaCrO(3) perovskite has been mapped out by in situ neutron-diffraction experiments under pressure. The system offers the opportunity to study the evolution of magnetic order, spin direction, and magnetic moment on crossing the orthorhombic-rhombohedral phase boundary. Moreover, a microscopic model of the superexchange interaction has been developed on the basis of the crystal structure obtained in this work to account for the behavior of T(N) under high pressure.

CaCrO3: an anomalous antiferromagnetic metallic oxide

Combining infrared reflectivity, transport, susceptibility, and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. Local spin density approximation calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizable pd hybridization remarkably strong next-nearest-neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magnetoelastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.

Zener polaron ordering in half-doped manganites

We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.

On the Location of Li(+) Cations in the Fast Li-Cation Conductor La(0.5)Li(0.5)TiO(3) Perovskite

Lithium cations sit in the center of the oxygen "windows" defined by the vertex-sharing TiO(6) octahedra of the title compound (see picture), as shown from neutron diffraction data. The octahedra are tilted to optimize the bond distances between the atoms. The unusual coordination and the partially unoccupied sublattice (occupancy factors 1/6 and (1/2) for Li and La, respectively) account for the high mobility of the Li(+) cations.

Neutron powder diffraction study of α-Ti(HPO4)2.H2O and α-Hf(HPO4)2.H2O; H-atom positions. Erratum

The H-atom coordinates were omitted from Tables 2 and 3 on p. 897 [Salvadó, Pertierra, García-Granda, García, Rodríguez & Fernández-Díaz, Acta Cryst. (1996), B52, 896–898]. The values are now listed.

Neutron powder diffraction study of α-Ti(HPO4)2.H2O and α-Hf(HPO4)2.H2O; H-atom positions

The complete crystal structures, including H-atom positions, of α-Ti(HPO4)2.H2O (α-TiP) and α-Hf(HPO4)2.H2O (αHfP) were determined by Rietveld refinement and Fourier synthesis, using constant-wavelength neutron diffraction data. This work is one of few recent examples of the determination of H atoms using neutron powder diffraction with an intense source. The orientation of water molecules in the cavities of α-TiP is similar to that previously found for α-ZrP. For α-HfP, although the hydrogen-bonding scheme is identical, the orientation of water molecules in similar cavities was found to be different.