Pair Distribution Function Technique Research Articles

Synthesis of new perovskite-type oxyfluorides, BaInO2F and comparison of the structure among perovskite-type oxyfluorides

Most perovskite-type oxyfluorides have a cubic structure even though the tolerance factors of these compounds are not unity. In this study, new perovskite-type oxyfluorides, BaInO2F, with a tolerance factor less than unity was synthesized and its crystal structure was compared with other perovskite-type oxyfluorides. In addition, its local structure was investigated using the atomic pair distribution function (PDF) technique. BaInO2F was synthesized by a low-temperature fluorination method, and its structure was cubic with an abnormally large displacement factor of anions. By comparing the structures of various perovskite-type oxyfluorides, we found that the ion species showing abnormally large displacement factor varied with the tolerance factors. In the case of BaFeO2F, the abnormally large displacement factor of the B-site ion, Fe3+, originated from the displacement of the Fe3+ ion from the center of a BX6 octahedron along <110> direction. According to the PDF analysis, the local structure of BaInO2F was not cubic but a triclinic system with tilted and distorted BX6 octahedra. On the basis of these results, we considred that the structure of oxyfluorides with a tolerance factor less than unity was relaxed by the tilt of the octahedra, similar to perovskite-type oxides. The long-range order of the anion displacements, however, was suppressed by the structural disorder arising from the random distribution of oxide and fluoride ions, therefore the average structure observed by diffraction methods was predicted to be a cubic structure.

Short- and middle-range order structures of KNbO3 nanocrystals

The atomic-scale structure of KNbO3 nanocrystals with the particle size of 115 ∼165 Å has been studied by combining Rietveld refinement and the atomic pair-distribution function (PDF) technique. We performed high-energy synchrotron X-ray scattering to extract short- and middle-range order structures. The unit cell size of the KNbO3 nanocrystals expanded as the particle size was reduced. PDF analysis of the whole nanocrystals in the initial phase of the crystal growth showed that it is close to a rhombohedral structure at the short-range order, tetragonal structure at the middle-range order, and cubic structure at the long-range order. The local structure is asymptotic to the average structure as the fitting range is elongated. The size of the region in which the local structure is same as that of the bulk, we named this the coherent length, is closely related to the average structure.

Conductive Nanosized Magnéli-Phase Ti4O7 with a Core@Shell Structure.

Magnéli-phase Ti4O7, known for its high electrical conductivity and corrosion resistance, is typically prepared by hydrogen reduction at high temperatures (∼1000 °C), leading to large particles. Nanosized Ti4O7 have been explored for application toward high specific surface area electrode materials and electrocatalyst supports; nonetheless, the particle size of Ti4O7 is still insufficient for utilization as a support. In this study, we have pursued a novel synthetic approach for nanosized Ti4O7 platelets with a length of 10-80 nm and thickness of 3-10 nm even under high-temperature conditions. We herein describe the use of SiO2 beads as a core to obtain a SiO2 core coated with multilayers of TiO2 nanosheets exfoliated from layered H2Ti4O7 which is subsequently subjected to high-temperature reduction to prepare a SiO2-core@Ti4O7-shell structure. The pair distribution function technique has proven that the shell is transformed to single-phase Ti4O7. The electrical double layer capacitance of SiO2-core@Ti4O7-shell was much larger than that of conventionally synthesized Ti4O7 particles with a micrometer size. The results show the beneficial effects of the SiO2-core@Ti4O7-shell structure, and it is the first example of the synthesis for conductive Ti4O7 with a high specific surface area even under conditions of high-temperature synthesis.

The rise of the X-ray atomic pair distribution function method: a series of fortunate events.

The atomic pair distribution function (PDF) technique is a powerful approach to gain quantitative insight into the structure of materials where the structural coherence extends only over a few nanometres. In this paper, I focus on PDF from synchrotron X-rays and describe what is the PDF and where it came from, as well as key moments on the journey that have contributed to its enormous recent growth and expanding impact in materials science today. Synchrotron X-ray sources played a starring role in this story. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Physical design of multipurpose physics neutron diffractometer for the CSNS

The Multipurpose Physics neutron diffractometer (MP) will be built at the China Spallation Neutron Source (CSNS) in the coming years. It is a time of flight diffractometer dedicated to the study of complex crystalline materials through the total scattering or pair distribution function (PDF) technique. The MP diffractometer will be able to determine the PDF from the atomic to the nanometer scale. We present a validation of the main physical parameters of the MP diffractometer through Monte Carlo simulation. In particular the moderator choice, guide system and placement of the chopper system is investigated. The best momentum transfer resolution δQ/Q is reached in backscattering and is expected to be in the order of 0.2 %, the neutron flux at the sample is of the order of 107 n/s/cm2 and the momentum transfer range in the order of 0.08–100 Å −1.

A melting-like process and the local structures during the anatase-to-rutile transition

Phase transition of titanium oxide from anatase to rutile was investigated by using transmitted electron microscope (TEM), scanning electron microscope (SEM) and X-ray diffraction (XRD), in combination with the atomic pair distribution function (PDF) technique. The transition range is about 528–805 °C and the critical size is ~73 nm for the sol-gel samples calcined for 6 h. A melting-like process was evidently observed in the transition samples by SEM and it will speed up the phase transition. PDF reveals that anatase crystallites are packed by neighbouring TiO6 octahedra in a cis-coordinated way. Anatase crystallites larger than the critical size will be unstable and transform into rutile in a trans-coordinated way by torsion and rotation of part of octahedra and it will rapidly spread the entire crystallites driven by a melting-like process. We suggest the maximum size of anatase will be more technically useful for the description of the phase stability from anatase to rutile because unstable small anatase crystals are consumed during longer calcination process to grow the larger anatase crystals.

Stabilization of Polar Nanoregions in Pb-free Ferroelectrics.

The formation of polar nanoregions through solid-solution additions is known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nanoregions (PNR), understanding their real-space atomic structure and dynamics of their formation remains a considerable challenge. Here, using the method of dynamic pair distribution function, we provide direct insights into the role of solid-solution additions towards the stabilization of polar nanoregions in the Pb-free ferroelectric of Ba(Zr,Ti)O_{3}. It is shown that for an optimum level of substitution of Ti by larger Zr ions, the dynamics of atomic displacements for ferroelectric polarization are slowed sufficiently below THz frequencies, which leads to increased local correlation among dipoles within PNRs. The dynamic pair distribution function technique demonstrates a unique capability to obtain insights into locally correlated atomic dynamics in disordered materials, including new Pb-free ferroelectrics, which is necessary to understand and control their functional properties.

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo2F7 and NaSrCo2F7 with magnetic pair distribution function analysis

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

Recent advances in the magnetic pair distribution function technique

Recent advances in the magnetic pair distribution function technique

Lithiation Thermodynamics and Kinetics of the TiO2 (B) Nanoparticles.

TiO2 (B) has attracted considerable attention in recent years because it exhibits the largest capacity among all studied titania polymorphs, with high rate performance for Li intercalation being achieved when this material is nanostructured. However, due to the complex nature of its lithiation mechanism and practical challenges in probing Li structure in nanostructured materials, a definitive understanding of the lithiation thermodynamics has yet to be established. A comprehensive mechanistic investigation of the TiO2 (B) nanoparticles is therefore presented using a combination of in situ/operando X-ray pair distribution function (PDF) and electrochemical techniques. The discharge begins with surface reactions in parallel with Li insertion into the subsurface of the nanoparticles. The Li bulk insertion starts with a single-phase reaction into the A2 site, a position adjacent to the b-channel. A change of the Li diffusion pathway from that along this open channel to that along the c-direction is likely to occur at the composition of Li0.25TiO2 until Li0.5TiO2 is attained, leading to a two-step A2-site incorporation with one step kinetically distinct from the other. Subsequent Li insertion involves the C' site, a position situated inside the channel, and follows a rapid two-phase reaction to form Li0.75TiO2. Due to the high diffusion barrier associated with the further lithiation, Li insertion into the A1 site, another position adjacent to the channel neighboring the A2 sites, is kinetically restricted. This study not only explores the lithiation reaction thermodynamics and mechanisms of nanoparticulate TiO2 (B) but also serves as a strong reference for future studies of the bulk phase, and for future calculations to study the Li transport properties of TiO2 (B).

(Invited) Resolving Sites for Catalysis in Amorphous Metal Oxide and Molecular Water-Oxidation Catalysts Using in-Situ X-Ray Techniques

A key challenge for the development of materials and technologies for solar-to-fuels energy conversion lies in resolving structure, structural dynamics, and mechanisms underlying solar fuels catalysis, and measuring these structural parameters during functional conditions. This presentation will discuss the resolution of sites for solar-driven catalysis in amorphous cobalt oxide and molecular water-oxidation catalysts using a combination of in-situ high energy X-ray (60 keV) scattering with atomic pair distribution function (PDF) analysis, cobalt L3 edge X-ray absorption spectroscopy, and anomalous wide angle X-ray scattering (AWAXS), with a goal of resolving structure, "one electron at a time", following successive, single-electron, photo-initiated electron transfers. We have extended the in-situ PDF technique by developing 3-D porous electrode architectures that enable structural characterization of interfacial thin films during photo-electrochemistry with 0.2 Å spatial resolution. PDF-electrochemistry measurements for the cobalt borate OEC film, CoBi, poised across the potentials from 0.5 V and 1.4 V vs NHE show lattice contraction of the domains as a result of bond shortening by the accumulation of metal centers in high valence oxidation states. The structural change is associated with increased metal-oxo covalency and charge delocalization. Comparable in-situ PDF measurements for the cobalt phosphate, CoPi, OEC show that charge accumulation leads to similar domain lattice contraction. In addition, for CoPi the CoIICoIII to CoIIICoIII transition is found to be coupled to Co-O peak broadening and amplitude decreases for pairs involving edge O atoms, identifying these as the sites for redox activity. Corresponding changes are seen in the P-O peak. These results provide experimental evidence for a Pi edge-associated model with bond length disorder. The PDF data shows that Pi remains edge-bound at the onset of water oxidation, suggesting the possible positioning of Pi for function in proton-coupled electron transfer. The cobalt OEC are also distinguished by X-ray absorption at the L3 edge, with CoPi showing both octahedral and tetrahedral Co(II) assigned to lattice and edge or extra-domain connecting sites, respectively. Finally, we demonstrate new AWAXS measurements for structure resolution in molecular and mixed metal OEC.

Bi4TaO8Cl Nano-Photocatalyst: Influence of Local, Average, and Band Structure

The average structure, local structure, and band structure of nanoparticles of photocatalyst Bi4TaO8Cl, an Aurivillius-Sillen layered material, has been studied by powder neutron Rietveld refinement, neutron pair distribution function technique, Raman scattering, and density functional theory calculations. A significant local structural deviation of nano-Bi4TaO8Cl was established in contrast to the local structure of bulk-Bi4TaO8Cl. Local structure was further supported by Raman scattering measurements. Through DFT calculations, we identify specific features in the electronic band structure that correlate lower secondary structural distortions in nano-Bi4TaO8Cl. Increased distortion of TaO6, decreased Ta-O-Ta bond angle, and increased octahedral tilt in the local structure of nano-Bi4TaO8Cl influence the band structure and the electron hole pair migration. Therefore, in addition to morphology and size, the local structure of a nanomaterial contributes to the photocatalytic performance. Trapping experiments confirm the role of superoxide radical in the photocatalysis mechanism of this material. Such studies help in developing new functional materials with better photocatalytic efficiency to address energy and environmental issues.

Amorphous Transition Metal Polysulfides As Electrode Materials for Li-Ion Batteries

Sulfur cathodes in conversion reaction batteries operate by a different mechanism than that of intercalation materials, and they possess larger theoretical capacities. However, sulfur-based electrode materials suffer from parasitic polysulfide shuttling, which contribute to decreased capacity retention and cyclability. We demonstrate that porous transition metal polysulfide chalcogels achieve high gravimetric capacity as electrode materials for lithium-ion batteries. Transition metal polysulfide chalcogels are amorphous, and comprise polysulfide chains connected by inorganic linkers. The linkers appear to act as a “glue" in the electrode to prevent polysulfide shuttling. In particular, we examined 3d and 4d transition metal polysulfide chalcogels (M’M”Sx in which M is a transition metal) and their role in maintaining structural stability in the electrode. In the MoS3.4 case study, the Mo polysulfide chalcogels function as electrodes in carbonate– as well as ether–based electrolytes and achieve an initial gravimetric capacity of 600 mAh/g. We employ X-ray absorption spectroscopy and operando pair distribution function techniques to elucidate the structural evolution of the electrode. Raman and X-ray photoelectron spectroscopy track the chemical moieties that arise during the anion-redox-driven processes. We find the redox state of Mo remains unchanged across the electrochemical cycling even to 1 V and correspondingly, the redox is anion–driven. Porous chalcogels offer a new class of electrode materials for achieving high-capacity Li-ion batteries.

Modelling and validation of particle size distributions of supported nanoparticles using the pair distribution function technique

The particle size of supported catalysts is a key characteristic for determining structure–property relationships. It is a challenge to obtain this information accurately and in situ using crystallographic methods owing to the small size of such particles (&lt;5 nm) and the fact that they are supported. In this work, the pair distribution function (PDF) technique was used to obtain the particle size distribution of supported Pt catalysts as they grow under typical synthesis conditions. The PDF of Pt nanoparticles grown on zeolite X was isolated and refined using two models: a monodisperse spherical model (single particle size) and a lognormal size distribution. The results were compared and validated using scanning transmission electron microscopy (STEM) results. Both models describe the same trends in average particle size with temperature, but the results of the number-weighted lognormal size distributions can also accurately describe the mean size and the width of the size distributions obtained from STEM. Since the PDF yields crystallite sizes, these results suggest that the grown Pt nanoparticles are monocrystalline. This work shows that refinement of the PDF of small supported monocrystalline nanoparticles can yield accurate mean particle sizes and distributions.

Molybdenum Polysulfide Chalcogels as High-Capacity, Anion-Redox-Driven Electrode Materials for Li-Ion Batteries

Sulfur cathodes in conversion reaction batteries offer high gravimetric capacity but suffer from parasitic polysulfide shuttling. We demonstrate here that transition metal chalcogels of approximate formula MoS3.4 achieve a high gravimetric capacity close to 600 mAh g–1 (close to 1000 mAh g–1 on a sulfur basis) as electrode materials for lithium-ion batteries. Transition metal chalcogels are amorphous and comprise polysulfide chains connected by inorganic linkers. The linkers appear to act as a “glue” in the electrode to prevent polysulfide shuttling. The Mo chalcogels function as electrodes in carbonate- and ether-based electrolytes, which further provides evidence of polysulfide solubility not being a limiting issue. We employ X-ray spectroscopy and operando pair distribution function techniques to elucidate the structural evolution of the electrode. Raman and X-ray photoelectron spectroscopy track the chemical moieties that arise during the anion-redox-driven processes. We find the redox state of Mo rem...

Size of Au-Nanoparticles Supported on Mesostructural Cellular Foams Studied by the Pair Distribution Function Technique

Mesostructural cellular foam (MCF) materials that were modified by Zr, Nb, and Mo incorporation, followed by APTMS (3-aminopropyl-trimethoxysilane) grafting and gold loading were studied using the pair distribution function (PDF) technique. Measurements were focused on changes in gold crystallite sizes and on local geometry changes in the supports. Initially, ex situ prepared samples were investigated at different stages of synthesis and after catalytic oxidation of carbon monoxide. The crystallization and agglomeration of gold species as well as carbon monoxide oxidation were then tracked by in situ high energy diffraction measurements. The influence of metal type (Nb or Mo) and incorporation method in the MCF material on the agglomeration of metallic gold particles during increasing calcination temperature was determined. The structure of MCF materials was preserved during calcination and oxidation of CO and local symmetry of gold particles is not changed under CO oxidation conditions. In samples oxidiz...

Local structure analysis of NaNbO3 and AgNbO3 modified by Li substitution

We analyzed the local structures of NaNbO3 and AgNbO3 by combining the X-ray absorption fine structure (XAFS) and atomic pair-distribution function (PDF) techniques. NaNbO3 is known to be an antiferroelectric material at room temperature. It also undergoes a diffuse phase transition, in which orthorhombic and rhombohedral phases coexist over a wide temperature range. We found a disordered feature in the nearest-neighbor bond distance corresponding to the Nb–O bonds. The disordered bond distribution disappeared when Li was substituted for Na. A similar disorder feature was found in AgNbO3. The disordered site can be specified by combining XAFS and PDF techniques. The sequences of disordered and complex phase transitions are attributable to the competition between the tolerance of the AO12 cage and the tilt of NbO6 octahedra.

Investigation of Structural Evolution in 3d-Transition Metal Ferrites MFe2O4 (M = Fe, Co, Ni and Cu) As Conversion Type Electrodes for Li-Ion Batterie

Growing demand of Li-ion batteries initiates the on-going researches on new electrode materials with improved performance. Conversion-type anode materials have been a topic of interest ever since the beginning of the 21st century due to their intrinsically high capacity, which can enhance significantly the energy density of future lithium-ion batteries [1]. For metal oxides, operating via conversion, the initial discharge involves the reduction of the transition metals to a composite material consisting of metallic nanoparticles (2-8 nm) dispersed in an amorphous Li2O matrix. Upon following charge, the metal particles are oxidized, however, the structure and composition of thus formed nanostructured metal oxide may be different comparing to the pristine oxide. Nanocrystalline transition metal Iron oxides MFe2O4 (M = Fe, Co, Ni and Cu) are especially attractive because of low cost and environmental compatibility.In this work, a comparative study of different metal ferrites as conversion type model system for Li-ion batteries, to elucidate the influence of partial substitution of Fe in the spinel structure with different 3d-cations, are reported and also a comparative study of their electrochemical eevolution mechanism is investigated using in situ diffraction and pair distribution function analysis. In order to investigate the electrochemical mechanism in detail, the structural evolution of the materials in the 1st cycle was tracked using in situ synchrotron diffraction and ex situ PDF techniques. It can be observed that the electrochemical mechanism in the first discharge cycle could occur via two different processes. Either by the direct reduction of initial material into respective binary oxide or by a Li-intercalation process in the spinel structure during the initial discharge, which is further transformed to metal nanoparticles dispersed in Li2O matrix. Due to low crystallite size and the formation of the amorphous (poorly diffracting) phases, laboratory and synchrotron conventional X-ray diffraction methods can provide only limited insight into the processes occurring upon cycling conversion-type materials. Pair Distribution Function (PDF) analysis can be used for probing local as well as the long-range structure of the material is more informative for such systems containing both crystalline and amorphous components [2]. The initial electrochemical lithiation of Fe3O4 anode operating via conversion mechanism is investigated by PDF analysis. The ex-situ PDF measurements have been performed on the samples with different lithium content, prepared by discharging in the electrochemical cell voltage range of 3.0 to 0.1 V vs. Li+/Li (See figure 1). The details of the electrochemical mechanism of transition metal ferrites as negative electrode material for Li-ion batteries will be discussed. Acknowledgement: The financial support from DFG within the Research Priority Program SPP 1473, “Materials with new Design for improved Li ion batteries-WeNDeLIB” is gratefully acknowledged. This work has benefited from the beamtime allocated by the P02.1 beamline, PETRA, Hamburg. [1] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacín, Advanced Energy Materials, 22, E170-E192, 2010. [2] T. Proffen and H. Kim, Journal of Materials Chemistry, 19, 5078, 2009. Figure 1

Total scattering atomic pair distribution function: new methodology for nanostructure determination

ABSTRACTThe conventional X-ray diffraction (XRD) methods probe for the presence of long-range order towards a solution of the average crystal structure. Experimentally, structural information about long-range, periodic atomic ordering is reflected in the Bragg scattering while local atomic structural deviations from the average structure mainly affect the diffuse scattering intensities. In order to obtain structural information about both average and local atomic structures, a technique that takes in account both Bragg and diffuse scattering needs to be employed, such as the total scattering atomic pair distribution function (PDF) technique. This article introduces a PDF-based methodology that can be applied to extract precise structural information about nanoparticles such as the size of the crystalline core region, the degree of crystallinity, the atomic structure of the core region, local bonding, and the degree of the internal disorder, as a function of the nanoparticle diameter. This article sheds light on a new PDF-based methodology that can yield precise quantitative structural information about small nanocrystals from XRD data, and also describes the essential aspects of this proposed methodology as well as its great potential. This method is generally applicable to the characterisation of the nanoscale solid, many of which may exhibit complex disordered structure.

Thermodynamics, Kinetics and Structural Evolution of ε-LiVOPO4 over Multiple Lithium Intercalation

In this work, we demonstrate the stable cycling of more than one Li in solid-state-synthesized e-LiVOPO4 over more than 20 cycles for the first time. Using a combination of density functional theory (DFT) calculations, X-ray pair distribution function (PDF) analysis and X-ray absorption near edge structure (XANES) measurements, we present a comprehensive analysis of the thermodynamics, kinetics, and structural evolution of e-LixVOPO4 over the entire lithiation range. We identify two intermediate phases at x = 1.5 and 1.75 in the low-voltage regime using DFT calculations, and the computed and electrochemical voltage profiles are in excellent agreement. Operando PDF and EXAFS techniques show a reversible hysteretic change in the short ( 2.4 A) during low-voltage cycling. Hydrogen intercalation from electrolyte decomposition is a possible explanation for the ∼2.4 A V—O bond and its irreversible extension. Finally, we show tha...