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A central challenge for hydrogen generation via electrolytic water splitting is the identification of efficient oxygen evolution reaction (OER) catalysts; a key aspect of the challenge hinges on an ability to relate atomic-scale structure to observed activities. Amorphous iron-based oxide(hydroxides) prepared by photochemical metal organic decomposition (PMOD) are proven OER catalysts, but their atomistic structures have been elusive. Here, a combination of powder diffraction and pair distribution function (PDF) analyses enables the formulation of a set of structural characteristics that capture the salient features of amorphous iron oxide(hydroxide) (a-FeOx), a model compound for this class of materials. a-FeOx contains only octahedrally coordinated iron atoms, which form clusters of both edge- and corner-sharing octahedra. A degree of “eutaxy” with predominantly ABC-type anion stacking persists at length scales beyond the dimensions of cluster domains–consistent with thermally induced crystallization in...

Rational preparation of materials by design is a major goal of inorganic, solid-state, and materials chemists alike. Oftentimes, the use of nonmetallurgical reactions (e.g., chalcogenide fluxes, hydrothermal syntheses, and in this case solid-state metathesis) alters the thermodynamic driving force of the reaction and allows new, refractory, or otherwise energetically unfavorable materials to form under softer conditions. Taking this a step further, alteration of a metathesis reaction pathway can result in either the formation of the equilibrium marcasite polymorph (by stringent exclusion of air) or the kinetically controlled formation of the high-pressure pyrite polymorph of CuSe2 (by exposure to air). From analysis of the reaction coordinate with in situ synchrotron X-ray diffraction and pair distribution function analysis as well as differential scanning calorimetry, it is clear that the air-exposed reaction proceeds via slight, endothermic rearrangements of crystalline intermediates to form pyrite, whi...

Inorganic materials with organic constituents-hybrid materials-have shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C7H7)2SnI6, and tropylium lead iodide, C7H7PbI3, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic-inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications.

Die Tetraboride von Chrom und Mangan zeigen ein ungewohnliches Boratomgerust, das hypothetischem tetragonalen Diamant ahnelt. Es wird angenommen, dass sie sehr hart sind. Jetzt wurden Einkristalle von MnB4 erhalten. Die Verbindung kristallisiert im monoklinen Kristallsystem (P21/c) mit einer Struktur, die vier kristallographisch unabhangige Boratom-Positionen aufweist, was in Ubereinstimmung mit den Ergebnissen der 11B-MAS-NMR-Spektroskopie steht. Ein unerwartet kurzer Abstand zwischen Mn-Atomen deutet auf eine Mn-Mn-Doppelbindung hin und wird durch Peierls-Verzerrung verursacht. Die Struktur wurde mit Gruppe-Untergruppe-Beziehungen bestimmt. DFT-Rechnungen lassen auf MnI und Paramagnetismus schliesen, letzterer wurde durch magnetische Messungen bestatigt. Die Zustandsdichten zeigen eine Pseudo-Bandlucke am Fermi-Niveau; fur MnB4 wurde halbleitendes Verhalten beobachtet.

Tetraborides of chromium and manganese exhibit an unusual boron-atom framework that resembles the hypothetical tetragonal diamond. They are believed to be very hard. Single crystals of MnB4 have now been grown. The compound crystallizes in the monoclinic crystal system (space group P21 /c) with a structure that has four crystallographically independent boron-atom positions, as confirmed by (11) B MAS-NMR spectroscopy. An unexpected short distance between the Mn atoms suggests a double Mn-Mn bond and is caused by Peierls distortion. The structure was solved using group-subgroup-relationships. DFT calculations indicate Mn(I) centers and paramagnetism, as confirmed by magnetic measurements. The density of states shows a pseudo-band gap at the Fermi energy and semiconducting behavior was observed for MnB4 .

Li3SO3N has been prepared by the mechanochemical reaction between lithium sulfamate (LiSO3NH2) and lithium amide or lithium hydride. This is the first example of an anhydrous compound containing the rare [SO3N]3− polyanion. Li3SO3N adopts the low-temperature P m n 2 1 β structure of isoelectronic Li3PO4. Structural refinement from neutron powder diffraction data supports disordering of nitrogen over two of the three anion sites, although a fully disordered model cannot be conclusively refuted. While the compound is a poor ionic conductor, it is electrochemically stable to at least 5 V vs. Li+/Li0. This simple mechanochemical pathway should be suitable for preparing a host of other alkali-rich nitrogen-containing polyanionic compounds.

Solid-state diffusion is often the primary limitation in the synthesis of crystalline inorganic materials and prevents the potential discovery and isolation of new materials that may not be the most stable with respect to the reaction conditions. Synthetic approaches that circumvent diffusion in solid-state reactions are rare and often allow the formation of metastable products. To this end, we present an in situ study of the solid-state metathesis reactions MCl2 + Na2S2 → MS2 + 2 NaCl (M = Fe, Co, Ni) using synchrotron powder X-ray diffraction and differential scanning calorimetry. Depending on the preparation method of the reaction, either combining the reactants in an air-free environment or grinding homogeneously in air before annealing, the barrier to product formation, and therefore reaction pathway, can be altered. In the air-free reactions, the product formation appears to be diffusion limited, with a number of intermediate phases observed before formation of the MS2 product. However, grinding the reactants in air allows NaCl to form directly without annealing and displaces the corresponding metal and sulfide ions into an amorphous matrix, as confirmed by pair distribution function analysis. Heating this mixture yields direct nucleation of the MS2 phase and avoids all crystalline binary intermediates. Grinding in air also dissipates a large amount of lattice energy via the formation of NaCl, and the crystallization of the metal sulfide is a much less exothermic process. This approach has the potential to allow formation of a range of binary, ternary, or higher-ordered compounds to be synthesized in the bulk, while avoiding the formation of many binary intermediates that may otherwise form in a diffusion-limited reaction.

Substituted metal oxides containing ionic species have been attracting a great deal of attention because of their potential ability to reduce the usage of precious metals in heterogeneous catalysts. We investigate Pd-substituted CeO2 for C–H bond activation reactions including the partial oxidation and dry reforming of CH4. This catalyst has been previously studied for CO oxidation, NOx reduction, and the water-gas shift reaction. Pd-substituted CeO2, Ce1–xPdxO2−δ, was prepared as a powder with high surface area and a hollow sphere morphology using ultrasonic spray pyrolysis. The catalysts were extensively characterized using synchrotron X-ray diffraction and other techniques, confirming phase pure samples up to 10 mol % Pd substitution. Ce0.95Pd0.05O2−δ was found to be active for partial oxidation of CH4 around 500 °C and higher. Our studies, including postcatalytic synchrotron diffraction, suggest that the single-phase Ce1–xPdxO2−δ material is not the active species and that catalysis occurs instead ove...

Complex oxides--containing at least two different cations on crystallographically distinct sites--have recently been shown to display redox cycling of platinum group metals (PGMs), such as Pd; for example, Pd-substituted complex oxides can reversibly extrude metallic Pd under reducing conditions and then reincorporate Pd(2+) ions into the lattice under oxidizing conditions. The title compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) (0 ≤ x ≤ 0.07) crystallizing in the noncentrosymmetric YMnO(3) structure, were prepared using a sol-gel process at 800 °C, and the structures were refined from high-resolution synchrotron X-ray powder diffraction data. Their redox cycling behavior was monitored using synchrotron X-ray diffraction and EXAFS studies. In contrast to the previously studied complex oxide host compounds, YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) is only modestly tolerant to cycling: repeated redox cycling leads to the formation of PdO, which, on the time-scale of the oxidation cycles, does not reincorporate in the complex oxide lattice. Both oxidized and reduced samples were tested for the oxidation of CO to CO(2) under CO-lean conditions. YMn(0.5)Fe(0.5-x)Pd(x)O(3-δ) performs essentially as well as previously studied YFe(1-x)Pd(x)O(3-δ). The CO oxidation light-off characteristics of the hexagonal hosts are very similar to finely dispersed PdO. Despite evidence that Pd is almost fully dispersed as divalent ions in the host lattice, which is presumably accompanied by the concurrent creation of oxygen vacancies (2 Pd(2+):1 V(O(2-))), the as-prepared hexagonal materials do not display any significant improvement in catalytic activity as a function of Pd substitution level. This suggests that the corner-connected trigonal bipyramids that characterize this structural family do not enable the transport of oxygen through the bulk of the lattice. The study casts light on factors in the solid-state chemistry of precious metal-substituted complex oxides that influence the efficacy of redox cycling of the precious metal, and catalytic performance.

Owing to gold's oxophobicity, its oxide chemistry is rather limited, and elevated oxygen pressures are usually required to prepare ternary and quaternary oxide compounds with gold ions. The Au{sup 3+} oxide, La{sub 4}LiAuO{sub 8}, is remarkable both because it can be prepared at ambient pressure in air, and because of its unusual stability toward thermal decomposition and reduction. The structure of La{sub 4}LiAuO{sub 8} was established by Pietzuch et al. using single crystal X-ray diffraction [1]. The compound adopts an ordered modification of the Nd{sub 2}CuO{sub 4} structure, containing two-dimensional sheets in which AuO{sub 4} square planes are separated from one another by LiO{sub 4} square planes. In light of the meager X-ray scattering factors of Li and O, relative to La and Au, we report here a neutron powder diffraction study of La{sub 4}LiAuO{sub 8}, definitively confirming the structure. To our knowledge, this is the first reported neutron diffraction study of any stoichiometric oxide compound of gold. X-N maps, which make use of nuclear positions obtained from Rietveld refinement of time-of-flight neutron diffraction data and electron densities obtained from synchrotron X-ray powder diffraction data, point to the highly covalent nature of the Au-O bonding in La{sub 4}LiAuO{sub 8}. Thismore » is in good agreement with charge densities and Bader charges obtained from full density functional relaxation of the structure.« less

La4LiAuO8 is a stable Au3+ oxide that was recently examined as a possible model compound for the role of Au3+ in heterogeneous catalysis. Due to the paucity of thermodynamic data, the energetics of La4LiAuO8 and its likely decomposition product, LiLaO2, were investigated. The ΔHf−ox, of La4LiAuO8 and LaLiO2 are both exothermic at −187.7 ± 5.8 and −41.4 ± 9.6 kJ/mol, respectively. From the thermodynamic data, the decomposition temperature of La4LiAuO8 was calculated as either 979 ± 95 or 1331 ± 43 °C for the formation of LiLaO2 or Li2O, respectively. Thus, LiLaO2 is the expected decomposition product.

The precipitation of crystals with stoichiometric and ordered arrangements of distinct metal cations often requires carefully designed molecular precursors and/or sufficient activation energy in addition to the necessary mass transport. Here, we study the formation of ordered double perovskite hydroxides, MnSn(OH)(6) and CoSn(OH)(6), of the generic chemical formula, BB'(OH)(6) (no A site), using kinetic control of aqueous hydrolysis from simple metal salt solutions. We find that the precipitation yields ordered compounds only when the B ion is Mn(II) or Co(II), and not when it is any other divalent transition metal ion, or Zn(II). The key step in forming the compounds is the prevention of rapid and uncontrolled hydrolysis of Sn(IV), and this is achieved by a fluoride counteranion. The two compounds, MnSn(OH)(6) and CoSn(OH)(6), are studied by high-resolution synchrotron X-ray diffraction and from the temperature dependence of magnetic behavior. From maximum entropy image restoration of the electron density and from Rietveld analysis, the degree of octahedral distortion and tilting and the small extent of anti-site disorder are determined. From the nonoverlapping electron density, we infer strongly ionic character of bonding. As the first magnetic study of such materials, we report simple paramagnetic behavior with no long-range magnetic order down to 2 K for the Mn(II) compound, while the cobalt compound presents uncompensated antiferromagnetic interactions, attributed to the single-ion anisotropy of octahedral Co(II).

Research highlights: {yields} New triple-layer Dion-Jacobson layered perovskite solid solutions synthesized. {yields} New series of Ta-doped layered perovskite solid acids, HCa{sub 2}Nb{sub 3-x}Ta{sub x}O{sub 10}. {yields} New series of Sr-doped layered perovskite solid acids, HCa{sub 2-x}Sr{sub x}Nb{sub 3}O{sub 10}. {yields} Layered perovskites with highest Ta content are weaker solid acids than HCa{sub 2}Nb{sub 3}O{sub 10}. -- Abstract: Dion-Jacobson type layered perovskites such as A'Ca{sub 2}Nb{sub 3}O{sub 10} (A' = K, Rb, H) have continued to be of great interest due to their compositional variability, rich interlayer chemistry, and wide range of physical properties. In this study, we investigated the range and effects of substitutional doping of Ta{sup 5+} for Nb{sup 5+} and of Sr{sup 2+} for Ca{sup 2+} in A'Ca{sub 2}Nb{sub 3}O{sub 10}. We have prepared and characterized three new solid solutions: KCa{sub 2}Nb{sub 3-x}Ta{sub x}O{sub 10}, RbCa{sub 2}Nb{sub 3-x}Ta{sub x}O{sub 10}, and RbCa{sub 2-x}Sr{sub x}Nb{sub 3}O{sub 10}. These materials all readily undergo proton exchange to form two new series of hydrated solid acid phases, which in most cases can be dehydrated to form stable HCa{sub 2}Nb{sub 3-x}Ta{sub x}O{sub 10} and HCa{sub 2-x}Sr{sub x}Nb{sub 3}O{sub 10} compounds. Intercalation studies with n-hexylamine and pyridine were carried out to gauge the relativemore » Bronsted acidities across the HCa{sub 2}Nb{sub 3-x}Ta{sub x}O{sub 10} series, and we determined that materials with the highest tantalum contents are weaker acids than the parent compound HCa{sub 2}Nb{sub 3}O{sub 10}. Preliminary intercalation studies with pyridine for the HCa{sub 2-x}Sr{sub x}Nb{sub 3}O{sub 10}.yH{sub 2}O solid acids, however, showed no significant difference in acidity with varying strontium content.« less

La(4)LiAuO(8) and La(2)BaPdO(5), two previously known oxides, are presented as model compounds for examining the role of isolated and immobilized Au(3+) and Pd(2+) ions in heterogeneous catalysis. Structural characterization, stability, surface composition, and electronic structure of these compounds are presented. These are examined in studies ranging from synchrotron X-ray scattering, including pair distribution function (PDF) and maximum entropy method (MEM) analysis, to density functional calculations of the electronic structures. The exceptional stability displayed by these compounds as verified by thermogravimetric analysis can be attributed to the presence of covalent Au-O and Pd-O interactions revealed in MEM studies, which suggests a criterion for stabilizing these highly oxophobic transition metals in oxide environments. Catalytic testing of the two compounds as heterogeneous catalysts in the oxidation of CO to CO(2) are presented. La(2)BaPdO(5) appears to be an effective catalyst for CO oxidation, despite the low surface area of the oxide being used. This is the first time that a fully ordered (rather than doped) Pd(2+) oxide had been used to catalyze CO oxidation. La(4)LiAuO(8) on the other hand, is much less effective at catalyzing CO oxidation. Differences in the reactivities of the two compounds are discussed with respect to differences in their density functional electronic structures.

We report x-ray total scattering and pair distribution function (PDF) studies of the structural relaxation around interstitial manganese (Mn$_i$) in ferromagnetic Mn$_{1+\delta}$Sb ($0.03 \le \delta \le 0.23$) alloys, guided by density functional theory (DFT). Refinements to the experimental PDF using a crystallographically constrained structural model indicate an expansion in the equatorial plane of the Mn$_i$Sb$_5$ trigonal bipyramidal site, which introduces significant positional disorder in addition to the nominally-random occupation of interstitial voids. Observation of a weak diffuse signal near the symmetry-forbidden (001) reflection position is indicative of correlated disorder from the clustering of Mn$_i$. Density functional relaxation of supercells approximating the $\delta = 0.08$, $0.15,$ and $0.23$ compositions provides improved models that accurately describe the short-range structural distortions captured in the PDFs. Such structural relaxation increases the DFT calculated moment on Mn$_i$, which aligns antiparallel to the primary Mn moments, but leads to insubstantial changes in the average Mn and Sb moments and moments of Mn and Sb proximal to interstitials, thus providing a more accurate description of the observed bulk magnetic properties., Comment: 10 pages, 9 figures. Supplemental material available upon request

Rational preparation of materials by design is a major goal of inorganic, solid-state, and materials chemists alike. Oftentimes, the use of nonmetallurgical reactions (e.g., chalcogenide fluxes, hydrothermal syntheses, and in this case solid-state metathesis) alters the thermodynamic driving force of the reaction and allows new, refractory, or otherwise energetically unfavorable materials to form under softer conditions. Taking this a step further, alteration of a metathesis reaction pathway can result in either the formation of the equilibrium marcasite polymorph (by stringent exclusion of air) or the kinetically controlled formation of the high-pressure pyrite polymorph of CuSe2 (by exposure to air). From analysis of the reaction coordinate with in situ synchrotron X-ray diffraction and pair distribution function analysis as well as differential scanning calorimetry, it is clear that the air-exposed reaction proceeds via slight, endothermic rearrangements of crystalline intermediates to form pyrite, which is attributed to partial solvation of the reaction from atmospheric humidity. In contrast, the air-free reaction proceeds via a significant exothermic process to form marcasite. Decoupling the formation of NaCl from the formation of CuSe2 enables kinetic control to be exercised over the resulting polymorph of these superconducting metal dichalcogenides.

Chromium tetraboride [orthorhombic, space group Pnnm (No. 58), a = 474.65(9) pm, b = 548.0(1) pm, c = 286.81(5) pm, and R value (all data) = 0.041], formerly described in space group Immm, was found not to be superhard, despite several theory-based prognoses. CrB(4) shows an almost temperature-independent paramagnetism, consistent with low-spin Cr(I) in a metallic compound. Conductivity measurements confirm the metallic character.

Li3SO3N is prepared by ball-milling LiSO3NH2 with a slight excess of LiNH2 or LiH for 4 h followed by annealing at 400—450 °C.

The platinum group metals (PGMs) are widely employed as catalysts, especially for the mitigation of automotive exhaust pollutants. The low natural abundance of PGMs and increasing demand from the expanding automotive sector necessitates strategies to improve the efficiency of PGM use. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime. An appealing strategy for reducing metal loading is the substitution of PGM ions into oxide hosts: the use of single atoms (ions) as catalytic active sites represents a highly atom-efficient alternative to the use of nanoparticles. This review addresses the crystal chemistry and reactivity of oxide compounds of precious metals that are, or could be relevant to developing an understanding of the role of precious metal ions in heterogeneous catalysis. We review the chemical conditions that facilitate stabilization of the notoriously oxophobic precious metals in oxide environments, and survey complex oxide hosts that have proven to be amenable to reversible redox cycling of PGMs.

The "lone" 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl(+), Pb(2+), and Bi(3+) with the [Xe]4f(14)5d(10)6s(2) electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au(-) ion with the [Xe]4f(14)5d(10)6s(2) electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au(-) anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au-Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic Cmcm structure featuring zigzag Au-Au chains.

The single crystal structure refinement of CrB4 is repeated and it is confirmed that the compound crystallizes in the orthorhombic space group Pnnm with Z = 2.

Substituted N-alkyldinaphthocarbazoles were synthesized using a key double Diels-Alder reaction. The angular nature of the dinaphthocarbazole system allows for increased stability of the conjugated system relative to linear analogues. The N-alkyldinaphthocarbazoles were characterized by UV-vis absorption and fluorescence spectroscopy as well as cyclic voltammetry. X-ray structure analysis based on synchrotron X-ray powder diffraction revealed that the N-dodecyl-substituted compound was oriented in an intimate herringbone packing motif, which allowed for p-type mobilities of 0.055 cm(2) V(-1) s(-1) from solution-processed organic field-effect transistors.

First-principles electronic structure calculations are presented on a variety of Au compounds and species--encompassing a wide range of formal oxidation states, coordination geometries, and chemical environments--in order to understand the potentially systematic behavior in the nature and energetics of d states that are implicated in catalytic activity. In particular, we monitor the position of the d-band center, which has been suggested to signal catalytic activity for reactions such as CO oxidation. We find a surprising absence of any kind of correlation between the formal oxidation state of Au and the position of the d-band center. Instead, we find that the center of the d band displays a nearly linear dependence on the degree of its filling, and this is a general relationship for Au irrespective of the chemistry or geometry of the particular Au compound. Across the compounds examined we find that even small calculated changes in the d-band filling result in a relatively large effect on the position of the d-band center. The results presented here have some important implications for the question of the catalytic activity of Au and indicate that the formal oxidation state is not a determining factor.

PbPdO₂, a ternary compound containing the lone pair active ion Pb²⁺ and the square planar d⁸Pd²⁺ ion, has attracted recent interest because of the suggestion that its electronic structure, calculated within density functional theory using either the local density or the generalized gradient approximation, displays zero-gap behavior. In light of the potential ease of doping magnetic ions in this structure, it has been suggested that the introduction of spin, in conjunction with zero band gap, can result in unusual magnetic ground states and unusual magnetotransport. It is known that most electronic structure calculations do not properly obtain a band gap even for the simple oxide PdO, and instead obtain a metal or a zero-gap semiconductor. Here we present density functional calculations employing a screened hybrid functional which correctly obtain a band gap for the electronic structure of PdO. When employed to calculate the electronic ground state of PbPdO₂, a band gap is again obtained, which is consistent with both the experimental data on this compound, as well as a consideration of valence states and of metal-oxygen connectivity in the crystal structure. We also present comparisons of the absolute positions (relative to the vacuum level) of the conduction band minima and the valence band maxima in α-PbO, PdO and PbPdO₂, which suggest ease of p-type doping in PbPdO₂, that has been observed even in nominally pure materials.

Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B-B bonding is of the 2-center 2-electron type. CrB(4) is metallic with a pseudogap at the Fermi level.

A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.

In many ostensibly crystalline materials, unit-cell-based descriptions do not always capture the complete physics of the system due to disruption in long-range order. In the series of cobalt hydroxides studied here, Co(OH)${}_{2\ensuremath{-}x}$(Cl)${}_{x}$(H${}_{2}$O)${}_{n}$, magnetic Bragg diffraction reveals a fully compensated N\'eel state, yet the materials show significant and open magnetization loops. A detailed analysis of the local structure defines the aperiodic arrangement of cobalt coordination polyhedra. Representation of the structure as a combination of distinct polyhedral motifs explains the existence of locally uncompensated moments and provides a quantitative agreement with bulk magnetic measurements and magnetic Bragg diffraction.

Structures of layered metal hydroxides are not well described by traditional crystallography. Total scattering from a synthesis-controlled subset of these materials, as described here, reveals that different cobalt coordination polyhedra cluster within each layer on short length scales, offering new insights and approaches for understanding the properties of these and related layered materials. Structures related to that of brucite [Mg(OH)(2)] are ubiquitous in the mineral world and offer a variety of useful functions ranging from catalysis and ion-exchange to sequestration and energy transduction, including applications in batteries. However, it has been difficult to resolve the atomic structure of these layered compounds because interlayer disorder disrupts the long-range periodicity necessary for diffraction-based structure determination. For this reason, traditional unit-cell-based descriptions have remained inaccurate. Here we apply, for the first time to such layered hydroxides, synchrotron X-ray total scattering methods-analyzing both the Bragg and diffuse components-to resolve the intralayer structure of three different alpha-cobalt hydroxides, revealing the nature and distribution of metal site coordination. The different compounds with incorporated chloride ions have been prepared with kinetic control of hydrolysis to yield different ratios of octahedrally and tetrahedrally coordinated cobalt ions within the layers, as confirmed by total scattering. Real-space analyses indicate local clustering of polyhedra within the layers, manifested in the weighted average of different ordered phases with fixed fractions of tetrahedrally coordinated cobalt sites. These results, hidden from an averaged unit-cell description, reveal new structural characteristics that are essential to understanding the origin of fundamental material properties such as color, anion exchange capacity, and magnetic behavior. Our results also provide further insights into the detailed mechanisms of aqueous hydrolysis chemistry of hydrated metal salts. We emphasize the power of the methods used here for establishing structure-property correlations in functional materials with related layered structures.

La(4)LiAuO(8) and La(2)BaPdO(5), two previously known oxides, are presented as model compounds for examining the role of isolated and immobilized Au(3+) and Pd(2+) ions in heterogeneous catalysis. Structural characterization, stability, surface composition, and electronic structure of these compounds are presented. These are examined in studies ranging from synchrotron X-ray scattering, including pair distribution function (PDF) and maximum entropy method (MEM) analysis, to density functional calculations of the electronic structures. The exceptional stability displayed by these compounds as verified by thermogravimetric analysis can be attributed to the presence of covalent Au-O and Pd-O interactions revealed in MEM studies, which suggests a criterion for stabilizing these highly oxophobic transition metals in oxide environments. Catalytic testing of the two compounds as heterogeneous catalysts in the oxidation of CO to CO(2) are presented. La(2)BaPdO(5) appears to be an effective catalyst for CO oxidation, despite the low surface area of the oxide being used. This is the first time that a fully ordered (rather than doped) Pd(2+) oxide had been used to catalyze CO oxidation. La(4)LiAuO(8) on the other hand, is much less effective at catalyzing CO oxidation. Differences in the reactivities of the two compounds are discussed with respect to differences in their density functional electronic structures.

Two new Dion-Jacobson type layered perovskite solid solutions, RbCa2-xSrxM3O10 (M = Nb, Ta; 0 ≤ x ≤ 2), were prepared and studied by X-ray powder diffraction, neutron powder diffraction, and Raman spectroscopy. X-ray powder diffraction confirmed single-phase solid solution formation with continuous expansion of the idealized primitive tetragonal unit cell with increasing strontium content. Neutron powder diffraction studies of selected samples revealed lower symmetries and larger unit cells, as necessitated by octahedral tilting within the perovskite slabs, compared to the idealized primitive cell. As the average size of the A-cation in the perovskite slab is varied from Sr2+ to Ca2+, more extensive octahedral tilting is introduced. Vibrational modes of the perovskite slab observed using Raman spectroscopy show subtle changes as a function of calcium/strontium content and more intriguing differences between the isostructural niobates and tantalates.

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr_2O_4 and ZnCr_2O_4 is accompanied by a structural change that helps relieve the frustration. Analysis of high-resolution synchrotron X-ray scattering reveals that the low-temperature structures are well described by a two-phase model of tetragonal I4_1/amd and orthorhombic Fddd symmetries. The Cr_4 tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I4_1/amd phase. The spin-Jahn-Teller distortion is approximately one order of magnitude smaller than is observed in first-order Jahn-Teller spinels such as NiCr_2O_4 and CuCr_2O_4. In analogy with NiCr_2O_4 and CuCr_2O_4, we further suggest that the precise nature of magnetic ordering can itself provide a second driving force for structural change.

The platinum group metals (PGMs) are widely employed as catalysts, especially for the mitigation of automotive exhaust pollutants. The low natural abundance of PGMs and increasing demand from the expanding automotive sector necessitates strategies to improve the efficiency of PGM use. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime. An appealing strategy for reducing metal loading is the substitution of PGM ions into oxide hosts: the use of single atoms (ions) as catalytic active sites represents a highly atom-efficient alternative to the use of nanoparticles. This review addresses the crystal chemistry and reactivity of oxide compounds of precious metals that are, or could be relevant to developing an understanding of the role of precious metal ions in heterogeneous catalysis. We review the chemical conditions that facilitate stabilization of the notoriously oxophobic precious metals in oxide environments, and survey complex oxide hosts that have proven to be amenable to reversible redox cycling of PGMs.

PbPdO$_2$, a ternary compound containing the lone-pair active ion Pb$^{2+}$ and the square-planar d$^8$ Pd$^{2+}$ ion, has attracted recent interest because of the suggestion that its electronic structure, calculated within density functional theory using either the local density or the generalized gradient approximations, displays zero-gap behavior. In light of the potential ease of doping magnetic ions in this structure, it has been suggested that the introduction of spin, in conjunction with zero band gap, can result in unusual magnetic ground states and unusual magnetotransport. It is known that most electronic structure calculations do not properly obtain a band gap even for the simple oxide PdO, and instead obtain a metal or a zero-gap semiconductor. Here we present density functional calculations employing a screened hybrid functional (HSE) which correctly obtain a band gap for the electronic structure of PdO. When employed to calculate the electronic ground state of PbPdO$_2$, a band gap is again obtained, which is consistent both with the experimental data on this compound, as well as a consideration of valence states and of metal-oxygen connectivity in the crystal structure. We also present comparisons of the absolute positions (relative to the vacuum level) of the conduction band minima and the valence band maxima in {\alpha}-PbO, PdO and PbPdO$_2$, which suggest ease of $p$-type doping in PbPdO$_2$ that has been observed even in nominally pure materials.