Pair Distribution Function Measurements Research Articles

Correlated defect nanoregions in a metal–organic framework

Throughout much of condensed matter science, correlated disorder is key to material function. While structural and compositional defects are known to exist within a variety of metal–organic frameworks, the prevailing understanding is that these defects are only ever included in a random manner. Here we show—using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering, and pair distribution function measurements—that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate metal–organic framework. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some metal–organic frameworks might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in metal–organic framework design.

GeO2–SnCoC Composite Anode Material for Lithium-Ion Batteries

Current methods for extending the cycle life of volume-expanded anode materials for lithium-ion batteries mainly focus on development of nanosize three-dimensional structures and composite materials. We propose a novel anode material of GeO2–Sn30Co30C40 that is synthesized by high energy ball milling (SPEX). This material depends on the nanosized and composite concept, which combines the advantageous properties of Sn–Co–C (long cycle life) and GeO2 (high capacity). The composite anode shows a reversible capacity over 800 mAh/g with good capacity retention. Furthermore, the first-cycle Coulombic efficiency is 80%, much higher than the 34.6% obtained for pure GeO2. Pair distribution function measurements indicated the reversible reaction of GeO2 and SnO2, which is the key factor in the improved Coulombic efficiency. This reversibility can be explained by the catalytic role of Co3Ge2 phase, which facilities the conversion reactions of metal oxides and acts as an electronic conductive component for the compos...

Resolving Precursor Deligation, Surface Species Evolution, and Nanoparticle Nucleation during Palladium Atomic Layer Deposition

The synthesis of highly dispersed palladium nanoparticles on TiO2 surfaces from palladium hexafluoroacetylacetonate (Pd(hfac)2) was investigated using in situ infrared (IR) spectroscopy, in situ X-ray absorption spectroscopy (XAS), and in situ pair distribution function (PDF) measurements under practical atomic layer deposition conditions. Residual surface chlorine was found to directly participate in the transformation of organometallic compounds to nanoparticles. Deligation of the Pd(hfac)2, evolution of the surface species, and nucleation of the Pd nanoparticles were precisely resolved. This knowledge can help direct the future design of advanced heterogeneous catalysts from organometallic compounds.

Metastability and Structural Polymorphism in Noble Metals: The Role of Composition and Metal Atom Coordination in Mono- and Bimetallic Nanoclusters

This study examines structural variations found in the atomic ordering of different transition metal nanoparticles synthesized via a common, kinetically controlled protocol: reduction of an aqueous solution of metal precursor salt(s) with NaBH₄ at 273 K in the presence of a capping polymer ligand. These noble metal nanoparticles were characterized at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy (C(s)-STEM). It was found for monometallic samples that the third row, face-centered-cubic (fcc), transition metal [(3M)-Ir, Pt, and Au] particles exhibited more coherently ordered geometries than their second row, fcc, transition metal [(2M)-Rh, Pd, and Ag] analogues. The former exhibit growth habits favoring crystalline phases with specific facet structures while the latter samples are dominated by more disordered atomic arrangements that include complex systems of facets and twinning. Atomic pair distribution function (PDF) measurements further confirmed these observations, establishing that the 3M clusters exhibit longer ranged ordering than their 2M counterparts. The assembly of intracolumn bimetallic nanoparticles (Au-Ag, Pt-Pd, and Ir-Rh) using the same experimental conditions showed a strong tendency for the 3M atoms to template long-ranged, crystalline growth of 2M metal atoms extending up to over 8 nm beyond the 3M core.

Absolute x-ray energy calibration over a wide energy range using a diffraction-based iterative method

In this paper, we report a method of precise and fast absolute x-ray energy calibration over a wide energy range using an iterative x-ray diffraction based method. Although accurate x-ray energy calibration is indispensable for x-ray energy-sensitive scattering and diffraction experiments, there is still a lack of effective methods to precisely calibrate energy over a wide range, especially when normal transmission monitoring is not an option and complicated micro-focusing optics are fixed in place. It is found that by using an iterative algorithm the x-ray energy is only tied to the relative offset of sample-to-detector distance, which can be readily varied with high precision of the order of 10(-5) -10(-6) spatial resolution using gauge blocks. Even starting with arbitrary initial values of 0.1 Å, 0.3 Å, and 0.4 Å, the iteration process converges to a value within 3.5 eV for 31.122 keV x-rays after three iterations. Different common diffraction standards CeO(2), Au, and Si show an energy deviation of 14 eV. As an application, the proposed method has been applied to determine the energy-sensitive first sharp diffraction peak of network forming GeO(2) glass at high pressure, exhibiting a distinct behavior in the pressure range of 2-4 GPa. Another application presented is pair distribution function measurement using calibrated high-energy x-rays at 82.273 keV. Unlike the traditional x-ray absorption-based calibration method, the proposed approach does not rely on any edges of specific elements, and is applicable to the hard x-ray region where no appropriate absorption edge is available.

Size-Dependent Amorphization of NanoscaleY2O3at High Pressure

Y2O3 with particle sizes ranging from 5 nm to 1 μm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y2O3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.

Optimizing high-pressure pair distribution function measurements in diamond anvil cells

Pair distribution function (PDF) methods have great potential for the study of diverse high-pressure phenomena. However, the measurement of high-quality, high-resolution X-ray PDF data (toQmax > 20 Å−1) remains a technical challenge. An optimized approach to measuring high-pressure total scattering data for samples contained within a diamond anvil cell (DAC) is presented here. This method takes into account the coupled influences of instrument parameters (photon energy, detector type and positioning, beam size/shape, focusing), pressure-cell parameters (target pressure range, DAC type, diamonds, pressure-transmitting media, backing plates, pressure calibration) and data reduction on the resulting PDF. The efficacy of our approach is demonstrated by the high-quality, high-pressure PDFs obtained for representative materials spanning strongly and weakly scattering systems, and crystalline and amorphous samples. These are the highest-resolution high-pressure PDFs reported to date and include those for α-alumina (toQmax = 20 Å−1), BaTiO3(toQmax= 30 Å−1) and pressure-amorphized zeolite (toQmax = 20 Å−1).

Synchrotron applications of an amorphous silicon flat-panel detector

A GE Revolution 41RT flat-panel detector (GE 41RT) from GE Healthcare (GE) has been in operation at the Advanced Photon Source for over two years. The detector has an active area of 41 cm x 41 cm with 200 microm x 200 microm pixel size. The nominal working photon energy is around 80 keV. The physical set-up and utility software of the detector system are discussed in this article. The linearity of the detector response was measured at 80.7 keV. The memory effect of the detector element, called lag, was also measured at different exposure times and gain settings. The modulation transfer function was measured in terms of the line-spread function using a 25 microm x 1 cm tungsten slit. The background (dark) signal, the signal that the detector will carry without exposure to X-rays, was measured at three different gain settings and with exposure times of 1 ms to 15 s. The radial geometric flatness of the sensor panel was measured using the diffraction pattern from a CeO(2) powder standard. The large active area and fast data-capturing rate, i.e. 8 frames s(-1) in radiography mode, 30 frames s(-1) in fluoroscopy mode, make the GE 41RT one of a kind and very versatile in synchrotron diffraction. The loading behavior of a Cu/Nb multilayer material is used to demonstrate the use of the detector in a strain-stress experiment. Data from the measurement of various samples, amorphous SiO(2) in particular, are presented to show the detector effectiveness in pair distribution function measurements.

High Pressure Pair Distribution Function Studies of Green River Oil Shale

The compression behavior of a silicate-rich oil shale from the Green River formation in the pressure range 0.0−2.4 GPa was studied using in situ high pressure X-ray pair distribution function (PDF) measurements for the sample contained within a Paris−Edinburgh cell. The real-space local structural information in the PDF, G(r), was used to evaluate the compressibility of the oil shale. Specifically, the pressure-induced reduction in the medium- to long-range atom distances (∼6−20 A) yielded an average sample compressibility corresponding to a bulk modulus of ca. 61−67 GPa. A structural model consisting of a three phase mixture of the principal crystalline oil shale components (quartz, albite and Illite) provided a good fit to the ambient pressure PDF data (R ∼ 30.7%). Indeed the features in the PDF beyond ∼6 A, were similarly well fit by a single phase model of the highest symmetry, highly crystalline quartz component. The factors influencing the observed compression behavior are discussed.

Characterization and application of a GE amorphous silicon flat panel detector in a synchrotron light source

Characterization, in the language of synchrotron radiation, was performed on a GE Revolution 41RT flat panel detector using the X-ray light source at the Advanced Photon Source (APS). The detector has an active area of 41×41 cm 2 with 200×200 μm 2 pixel size. The nominal working photon energy is around 80 keV. Modulation transfer function (MTF) was measured in terms of line spread function (LSF) using a 25 μm×1 cm tungsten slit. Memory effects of the detector elements, called lag, were also measured. The large area and fast data capturing rate −8 fps in unbinned mode, 30 fps in binned or region of interest (ROI) mode—make the GE flat panel detector a unique and very versatile detector for synchrotron experiments. In particular, we present data from pair distribution function (PDF) measurements to demonstrate the special features of this detector.

Applications of an amorphous silicon-based area detector for high-resolution, high-sensitivity and fast time-resolved pair distribution function measurements

The application of a large-area (41 × 41 cm, 2048 × 2048 or 1024 × 1024 pixel) high-sensitivity (detective quantum efficiency > 65%) fast-readout (up to 7.5 or 30 Hz) flat-panel detector based on an amorphous silicon array system to the collection of high-energy X-ray scattering data for quantitative pair distribution function (PDF) analysis is evaluated and discussed. Data were collected over a range of exposure times (0.13 s–7 min) for benchmark PDF samples: crystalline nickel metal and amorphous silica (SiO2). The high real-space resolution of the resultant PDFs (withQmaxup to ∼40 Å−1) and the high quality of fits to data [RNi(0.13s)= 10.5%,RNi(1.3s)= 6.3%] obtained in short measurement times indicate that this detector is well suited to studies of materials disorder. Further applications of the detector to locate weakly scattering H2molecules within the porous Prussian blue system, {\rm Mn}^{\rm II}_{\,3}[CoIII(CN)6]2·xH2, and to follow thein situreduction of PtIVO2to Pt0at 30 Hz, confirm the high sensitivity of the detector and demonstrate a new potential for fast time-resolved studies.

Oxidation state of copper in RE 2CuO 4 compounds determined via XANES and bond valence methods

The copper oxidation state was determined from bond valence sums and X-ray absorption near edge structure for the simple T' structure. There is a similarity in the overall dependence of copper valence with respect to the lattice constant, a, for both XANES as well as BVS. The XANES data confirm electron diffraction and pair distribution function measurements which show that the local structure is significantly distorted from the average T' structure.