X-ray Pair Distribution Function Research Articles

The structure and crystal chemistry of vernadite in ferromanganese crusts.

The structure and crystal chemistry of vernadite in ferromanganese crusts from the Magellan Seamount in the north-west Pacific Ocean have been investigated using synchrotron X-ray diffraction (XRD), X-ray pair distribution function (PDF) and high-resolution transmission electron microscopy (TEM). XRD patterns of vernadite mainly show two strong diffraction peaks at 2.42-2.43 Å and 1.41 Å without or with a broad (001) diffraction peak, indicating thin layer nanophases along the c-direction. TEM images show flat and curved sheet-like nanocrystals with (001) layer thickness of ∼7.2 Å and ∼9.6 Å, and their interstratified structure. PDF patterns of the vernadite are similar to those from synthetic δ-MnO2 and defective birnessite, suggesting a phyllomanganate framework. Combined XRD/PDF patterns suggest that vernadite in the outer part is associated with a higher density interlayer species at triple-edge sharing sites. The proportion of the 10 Å phase increases from the outer (young) part to the inner (old) part of the Mn crusts due to aging and sorption of Mn, Co and Ni from ambient seawater. This study suggests that this combined method of synchrotron radiation XRD/PDF and high-resolution TEM is a powerful tool to determine atomic structures of poorly crystallized nano-minerals. The mixture model of vernadite structure will help to understand the partitioning and distribution of trace elements in the ferromanganese crusts.

Photoinduced dynamics of nematic order parameter in FeSe

Formation of electronic nematicity is a common thread of unconventional superconductors. In iron-based materials, the long-range nematic order is revealed by the orthorhombic lattice distortion, which importance is a highly controversial topic due the small magnitude of the distortion. Here, we study the local crystal structure of FeSe and its interaction with electronic degrees of freedom using ultrafast electron diffraction, x-ray pair distribution function analysis, and transmission electron microscopy and find a significant lattice response to local nematicity. The study demonstrates how local lattice distortions, which exist even at temperatures above the nematic phase transition, can be released by photoexcitation, leading to an enhancement of the crystalline order. The observed local atomic structures and their out-of-equilibrium behavior unravel a sophisticated coupling between the lattice and nematic order parameter in FeSe.

Operando Observation of Cobalt Catalysts for Water Splitting By X-Ray and Infrared Absorption Spectroscopy

Recently, electrochemical hydrogen production by water splitting has been receiving attention with regard to achieving a sustainable society. For commercial applications, improving overall water splitting efficiency has been required for the development of highly active oxygen evolution electrocatalysts. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. In this situation, we have developed many operando observation techniques for XAFS and infrared absorption spectroscopy using attenuated total reflection mode (ATR-IR) to observe the oxygen evolution electrocatalysts. For example, hard X-ray XAFS can observe the electronic states and local structures of metal species in the catalysts. On the other hand, tender and soft X-ray XAFS and ATR-IR can reveal the chemical states of light elements in the catalysts. Therefore, we investigated the reaction mechanism of Co-KPi and similar catalysts for water splitting using various kinds of operando XAS and ATR-IR spectroscopy (Figure 1). At first, we prepared for Co-KPi and some similar cobalt catalysts in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution by electrodeposition method. It was confirmed that these cobalt catalysts are composed of the edge-sharing CoO6 octahedral structure (CoOOH) by Co K-edge XAFS using hard X-ray. Next, operando O K-edge XAFS spectra for cobalt catalysts were taken using soft X-ray under electrochemical control. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). The CoO2 peak was obviously observed for Co-KPi of highly active OER electrocatalyst, compared with Co-KOAc and Co-KCl. Finally, operando P K-edge XAFS and ATR-IR revealed the phosphate ion is bounded on the cobalt clusters to stabilize the CoO2 structure. Therefore, we revealed that Co-KPi can function as highly active OER electrocatalyst due to the presence of CoO2 strucutre stabilized by phosphate ion. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). Figure 1

Synthesis, Crystal Structure, and Selected Properties of [Au(S2 CNH2 )2 ]SCN: A Precursor for Gold Macro-Needles Consisting of Gold Nanoparticles Glued by Graphitic Carbon Nitride.

A new preparation route is developed for the synthesis of needle-like crystals of [Au(S2 CNH2 )2 ]SCN, which avoids disproportionation of the AuI salt used as a starting material. In the crystal structure, the two crystallographically independent AuIII centers are in a square-planar environment of two S2 CNH2 ligands. The Hirshfeld surface analysis reveals the presence of noncovalent intermolecular S⋅⋅⋅S interactions, which are essential for the spatial arrangement of the molecules. Density functional theory (DFT) calculations including dispersion and damping corrections result in a unit cell volume very close to the value determined experimentally. Thermal decomposition in an inert atmosphere generates black needles with lengths of up to 500 μm. X-ray powder diffraction and pair distribution function analyses demonstrate that the needles are composed of nanosized crystals with a volume-weighted average domain size of 20(1) nm. According to results of X-ray photoemission experiments, the black needles are covered by a nitrogen-rich carbon nitride with composition near (CN)2 N. 13 C solid-state NMR investigations indicate that two different carbon species are present, with signals corresponding well to heptazine units as in melon and triazine units as in poly(triazin imide) type compounds. Scanning transmission electron microscopy tomography evidences that the needles are composed of slightly elongated nanoparticles.

Deciphering charge-storage mechanisms in 3D MnOx@carbon electrode nanoarchitectures for rechargeable zinc-ion cells

We previously demonstrated that electrode architectures comprising nanoscale birnessite-like MnO x affixed to three-dimensional carbon nanofoam (CNF) scaffolds offer performance advantages when used as cathodes in rechargeable zinc-ion cells. To discern chemical and physical changes at the MnO x @CNF electrode upon deep charge/discharge in aqueous Zn 2+ -containing electrolytes, we deploy electroanalytical methods and ex situ characterization by microscopy, elemental analysis, x-ray photoelectron spectroscopy, x-ray diffraction, and x-ray pair distribution function analyses. Our findings verify that redox processes at the MnO x are accompanied by reversible precipitation/dissolution of crystalline zinc hydroxide sulfate (Zn 4 (OH) 6 (SO 4 )· x H 2 O), mediated by the more uniformly reactive electrode structure inherent to the CNF scaffold.

Structural characterisation of amorphous solid dispersions via metropolis matrix factorisation of pair distribution function data.

We measure the X-ray pair distribution functions (PDFs) of a series of felodipine:copovidone amorphous solid dispersions. Using a newly-developed Metropolis Matrix Factorisation (MMF) algorithm we extract from these data the PDF of the amorphous felodipine component in isolation. Our MMF analysis allows quantification of the degree of drug crystallinity in each sample, and structural characterisation of the amorphous drug via its PDF. Comparison with atomistic simulations reveals that the (in)accessibility of conformational rotamers distinguishes amorphous and crystalline felodipine, in turn suggesting design routes for stabilising the amorphous form. We discuss the conceptual importance of our results in the context of characterising not only amorphous pharmaceuticals, but complex mixtures in general.

Ubiquitous Short-Range Distortion of Hybrid Perovskites and Hydrogen-Bonding Role: the MAPbCl3 Case

The local structure of MAPbCl3 has been investigated through the combined use of X-ray and neutron pair distribution function (PDF) analysis and computational modeling. The results indicate the presence of a significant level of distortion at the short-range, irrespective of the average structure found by diffraction, and this suggests a possible correlation between the type of distortion and the features of the strongest H–X bond. The distortion of the octahedra decreases by reducing temperature, but the overall tilting of the octahedral system increases, and such tilting depends on the size of the anion. The comparison of the short-range structural data available on the MAPbX3 family strongly indicates a common pattern of the local structural distortion in these systems.

Operando Studies Reveal Structural Evolution with Electrochemical Cycling in Li–CoS2

The drive toward high energy density alternatives to Li-ion batteries has led to great interest in energy storage materials not inherently constrained by the capacity limits of the currently employed intercalation electrode materials. Among the alternatives under consideration are electrode materials with theoretical capacities many times greater than intercalation electrodes that store charge through so-called conversion reactions. However, the significant structural changes that enable the high theoretical capacity of conversion systems contribute to issues of poor efficiency and short cycle life. To better understand cycling issues in conversion systems, we study the local structure evolution of CoS2 during Li storage. Being metallic and potentially capable of redox on both anion and cation sites, CoS2 would be expected to display promise as a cathode material. Through combined ex situ X-ray absorption near-edge spectroscopy and pair distribution function analysis from operando X-ray total scattering, ...

Competing Structural Influences in the Li Superionic Conducting Argyrodites Li6PS5- xSe xBr (0 ≤ x ≤ 1) upon Se Substitution.

Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure-property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li6PS5- xSe xBr (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li6PS5- xSe xBr transport behavior was found to be largely unaffected by the incorporation of Se2- due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimizing the ionic conductivity of solid ion conductors, local structural influences cannot be ignored and the idea of "the softer the lattice, the better" does not always hold true.

Local structure study of the iron-based systems of BaFe2As2 and LiFeAs by X-ray PDF and XAFS analyses

X-ray pair distribution function (PDF) and Extended X-ray absorption fine structure (EXAFS) measurements were carried out on two Fe-based systems BaFe2As2 and LiFeAs to study mainly on the local structures of their FeAs planes. For the former system exhibiting the tetragonal (space group I4/mmm)-orthorhombic (Fmmm) structural phase transition at temperature TS ∼140 K, we show the T dependent characteristics of the local distortion from room temperature to the low temperature Fmmm phase, which support the existence of another local orthorhombic phase (I2mm) [Cantoni et al.: Adv. Mater. 27 (2015) 2715] at 300 K. We show the magnitudes of the local Fe atom shifts from their average Fe sites in these phases as the important information on what roles the lattice distortion has in the arguments on various physical properties of FeAs planes. For LiFeAs with the superconducting transition at ∼18 K but no structural transition, a small distortion has been found below 200 K.

Nanodomains and local structure in ternary alkaline-earth hexaborides

The local structures of ternary alkaline-earth hexaborides (MB6, M = Ca0.5Sr0.5, Ca0.5Ba0.5 and Sr0.5Ba0.5) have been analysed using X-ray pair distribution function (PDF) analysis, Raman spectroscopy and transmission electron microscopy (TEM). The results show significant local deviations from the average cubic structure within the boron sub-lattice and support the conclusion that rapid synthesis processes lead to the formation of coherent nanodomains over length scales of about 10 nm. Reverse Monte Carlo fitting of the PDFs allows for quantification of the displacement disorder within the boron sub-lattice as a function of sample composition. Detailed Raman spectroscopy studies and high-resolution TEM support the models derived from X-ray scattering. The average magnitude of the static displacement disorder varies by sample composition and positively correlates with the cation radius ratios across the three compositions. The new models form a foundation for future computational and experimental studies aimed at understanding and predicting properties of hexaborides.

Accumulators for the Capture of Heavy Metals in Thermal Conversion Systems

A clay ceramic with organic additives (biomass and biochar) was investigated for the development of highly porous accumulators to capture heavy metals in thermal conversion systems. The structure was characterized using X-ray pair distribution function analysis, differential scanning calorimetry, and scanning electron microscopy. It was found that the organic additives transformed into porosity during firing. The morphology of the pores also corresponded to the morphology of the organic additives. Hence, the clay ceramic with a 15-wt% addition of biochar had a porosity of 46 vol% with 20-μm interconnected pores after firing. The resulting accumulator was found to capture cadmium (a model for heavy metals with high volatility) via condensation of the cadmium vapor as 2-μm beads in the pores. The cadmium capture efficiency reached up to 57% using a 15-wt% addition of biochar. Furthermore, cadmium was captured at higher temperatures than the condensation temperature in the atmosphere. This means that heavy metals may be captured before they condense in fly ash to promote the recycling of this material.

Local structure and phase transitions of KNbO3

Bond length distributions of nearest- and higher-neighbor distances in KNbO3, which is the parent compound of lead-free piezoelectric materials, have been obtained by X-ray pair-distribution function (PDF) analysis. The Rietveld analysis of high-energy powder X-ray diffraction data as a function of temperature from 15 to 1073 K over four phases shows the variation in structure from rhombohedral to cubic structures. The refinements on the local structure by the PDF method indicate that only the rhombohedral structure can describe the observed bond distributions within the unit cell. The rhombohedral distortion is maintained locally in all four phases.

Time-of-flight neutron total scattering with applied electric fields: Ex situ and in situ studies of ferroelectric materials.

Characterizing the structural response of functional materials (e.g., piezoelectrics and ferroelectrics) to electric fields is key for the creation of structure-property relationships. Here, we present a new sample environment and data reduction routines which allow the measurement of time-of-flight neutron total scattering during the in situ or ex situ application of high voltage (<10 kV) to a sample. Instead of utilizing the entire detector space of the diffractometer, only selected regions of detector pixels with scattering at the desired angle to the sample electric field are interrogated, which allows the generation of orientation-dependent reciprocal space patterns and real-space pair distribution functions (PDFs). We demonstrate the method using the relaxor ferroelectric Na1/2Bi1/2TiO3 and observe lattice expansion parallel and contraction perpendicular to the electric field for both in situ and ex situ experiments, revealing the irreversible nature of the local scale structural changes to this composition. Additionally, changes in the distributions of nearest neighbor metal-oxygen bond lengths are observed, which have been difficult to observe in previously measured analogous orientation-dependent X-ray PDFs. Considerations related to sample positioning and background subtraction are discussed, and future research directions are suggested.

Barium titanate nanoparticles: Short-range lattice distortions with long-range cubic order

Barium titanate (BTO) nanoparticles (sizes 10--500 nm) exhibit a displacement of the Ti atom from the center of the perovskite unit cell as inferred from synchrotron x-ray diffraction patterns (XRD) analyzed using atomic pair distribution functions (PDFs). Fits to PDFs acquired at temperatures of 20 \ifmmode^\circ\else\textdegree\fi{}C--220 \ifmmode^\circ\else\textdegree\fi{}C indicate that these Ti displacements (\ensuremath{\sim}0.1 \AA{}) are comparable to or even greater than those in the bulk material. Moreover, these displacements persist at temperatures well above 120 \ifmmode^\circ\else\textdegree\fi{}C, where the tetragonal-to-pseudocubic phase transition occurs in the bulk. Tetragonal Raman spectral lines were observed for all sizes of these BTO nanoparticles and confirm a distorted unit cell up to 120 \ifmmode^\circ\else\textdegree\fi{}C. Above 120 \ifmmode^\circ\else\textdegree\fi{}C, the small BTO nanoparticles (10, 50, 100 nm) continue to display tetragonal Raman lines, though with slowly decreasing amplitudes as the temperature rises. In contrast, the tetragonal Raman lines of large BTO nanoparticles (300, 400, 500 nm) disappear abruptly above 120 \ifmmode^\circ\else\textdegree\fi{}C, suggestive of bulk material. Indeed, fits to large-particle x-ray PDFs over the range 20--60 \AA{} reveal a sharp, long-range structural change toward a cubic lattice at 120 \ifmmode^\circ\else\textdegree\fi{}C, again consistent with bulk material. This sharp, long-range structural change is absent in the small particles. In fact, laboratory XRD Bragg peak profiles for the small BTO particles appear to be singlets at 20 \ifmmode^\circ\else\textdegree\fi{}C, indicating that significant long-range cubic order already exists at room temperature. As temperature rises, this long-range cubic order is gradually reinforced, as inferred from long-range fits of the small particle PDFs. By combining information from x-ray PDFs, Raman spectra, and Bragg peak profiles, we conclude that small BTO nanoparticles exhibit both short-range (unit-cell) distortion and long-range (mesoscale) cubic order from 20 \ifmmode^\circ\else\textdegree\fi{}C to 220 \ifmmode^\circ\else\textdegree\fi{}C, while the large nanoparticles behave as bulk material, differing from small particles only by exhibiting long-range tetragonal order below 120 \ifmmode^\circ\else\textdegree\fi{}C and a mesoscale structural phase change at 120 \ifmmode^\circ\else\textdegree\fi{}C.

Local structure and ageing of basaluminite at different pH values and sulphate concentrations

Basaluminite is a poorly crystalline aluminium sulphate oxyhydroxide that precipitates in acid mine drainage waters and acid sulphate soils when pH increases above 4.5 approximately. The pH increase can be due to mixing with natural waters or by the addition of an alkaline reagent, a method commonly used in remediation treatments. Basaluminite is a scavenger of elements such as Se, As, Sb, Mo, P and rare earth elements (REEs) and acts as a vector for their transport in the environment. The retention of these elements has important environmental and economic impact due to their toxicity and to the potential extraction of REEs for technological applications. Despite the long history of basaluminite, initially described as a mineral in the late 40s, little is known about its stability and its eventual transformation in other less metastable phases. Here, the stability of synthetic basaluminite in aqueous solutions with different pH values and sulphate concentrations was studied for 81 days. Chemical analyses of the aqueous phase combined with High-Energy X-ray Diffraction and Pair Distribution Function (PDF) analyses of the solids were used to characterize the ageing process. In contact with water, basaluminite incongruently dissolves, precipitating amorphous Al-hydroxide and releasing sulphate to the solution. This process and the recrystallization of amorphous Al-hydroxide into nanoboehmite were enhanced with increasing pH. However, for high SO4 concentrations, such as those found in acid mine waters, the transformation to nanoboehmite was partially inhibited. The ageing processes highlighted here point to a potential pathway for the desorption of trace elements upon neutralization of acid mine drainage. Furthermore, PDF analyses of basaluminite show a high similarity with the local structure of a synthetic amorphous Al-hydroxide.

New developments of X-ray diffraction and pair distribution function beamlines 11-ID and 17-BM at Advanced Photon Source

New developments of X-ray diffraction and pair distribution function beamlines 11-ID and 17-BM at Advanced Photon Source

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts.

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

Stability, Composition, and Core–Shell Particle Structure of Uranium(IV)-Silicate Colloids

Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide(IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilize U(IV) colloids. Here the formation, composition, and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates <220 nm. X-ray absorption spectroscopy, ultrafiltration, and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO2-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)3(H3SiO4)32- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of conditions relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.

Local structures and temperature-driven polarization rotation in Zr-rich PbZr1-xTixO3

PbZr1-xTixO3, which has abundant structural variations in the corresponding physical properties, has been used in a large variety of applications. To understand the effect of the structure on its high-performance piezoelectric properties, its local and average structures are studied. Total scattering data have been obtained from high-energy synchrotron powder diffraction experiments at 20 K and 300 K. Using the reverse Monte Carlo method, information on cation displacements has been extracted from X-ray Pair Distribution Function data. This suggests that the local disorder of the B cations is mainly driven by thermal motion, while the local disorder of Pb is most likely caused by more complex factors, such as displacive disorder. Both rhombohedral and monoclinic local polarizations are observed in Zr-rich PZT, whose directions depend on temperature.