Hyperstoichiometric UO2.15 was characterized by neutron total scattering at high temperature in the single-phase UO2+x region of the U/O phase diagram. The diffraction data confirmed a single-phase fluorite structure at high temperature. Analysis of the short-range data showed that the same structural model does not fit the pair distribution functions well. Instead, structural models containing specific configurations of oxygen defect clusters best represent the local atomic arrangement. Prevalent defect clusters previously proposed were fit to the experimental data, and moderately distorted oxygen cuboctahedra hypothesized by recent molecular dynamics simulations fit the data most accurately.

Earth-abundant metal oxides are excellent candidates for photocatalytic applications due to their low cost and high stability in aqueous solutions. Materials that contain a combination of metal cations with an s2 electron lone pair and a d0 electronic configuration, such as BiVO4, possess favorable band gaps. BiVO4 has also been reported to possess noncentrosymmetric polar properties, such as flexoelectricity, piezo-photocatalysis, and an anomalous photovoltaic effect, despite its centrosymmetric crystal structure. Here, it is shown how centrosymmetric materials possessing s2 and d0 cations can display "hidden" local-scale features, often ignored by conventional crystallography, that influence their physical properties. Anomalous peak shapes are observed in the high-resolution synchrotron X-ray powder diffraction of BiVO4, and temperature-dependent local-scale distortions are revealed using neutron total scattering methods. Together, these suggest the polar properties of BiVO4 are related to local-scale distortions induced by the Bi3+ 6s2 electron lone pairs. This demonstrates the possibility of engineering specific interatomic distances between lone pair-bearing cations and the anion sublattice, creating new opportunities for photocatalytic and polar materials from compounds with long-range centrosymmetric structures.

LiNiO2 remains a critical archetypal material for high energy density Li-ion batteries, forming the basis of Ni-rich cathodes in use today. Nevertheless, there are still uncertainties surrounding the charging mechanism at high states of charge and the potential role of oxygen redox. We show that oxidation of O2- across the 4.2 V plateau forms O2 trapped in the particles and is accompanied by the formation of 8% Ni vacancies on the transition metal sites of previously fully dense transition metal layers. Such Ni vacancy formation on charging activates O-redox by generating non-bonding O 2p orbitals and is necessary to form vacancy clusters to accommodate O2 in the particles. Ni accumulates at and near the surface of the particles on charging, forming a Ni-rich shell approximately 5 nm thick, enhanced by loss of O2 from the surface, the resulting shell composition is Ni2.3+1.75O2. The overall Ni oxidation state of the particles measured by XAS in fluorescence yield mode after charging across the plateau to 4.3 V is approximately +3.8; however, taking account of the shell thickness and the shell Ni oxidation state of +2.3, this indicates a Ni oxidation state in the core closer to +4 for compositions beyond the plateau.

A detailed variable temperature neutron total scattering study of the potential nuclear waste matrix TlTcO4 was conducted. The long-range average structure of TlTcO4 undergoes an orthorhombic Pnma to tetragonal I41/amd phase transition below 600 K, consistent with previous synchrotron X-ray diffraction studies. However, several anomalies were observed in the Rietveld refinements to the neutron powder diffraction data, such as large atomic displacement parameters at low temperature and a shortening of the Tc-O bond distance upon heating. Modelling the short-range local structure of both the low- and high-temperature data required a lowering of symmetry to the monoclinic P21/c model due to the stereochemical activity of the Tl+ 6s2 lone pairs. Density functional theory calculations also verified this model to have a lower ground state energy than the corresponding long-range average structure. It is concluded that at low temperatures, the Tl+ 6s2 lone pairs are 'frozen' into the structure. Upon heating, the rigid TcO4 tetrahedra begin to rotate, as governed by the Γ3+ and M4+ modes. However, there is a disconnect between the two length scales, with the 6s2 lone pair electrons remaining stereochemically active on the local scale, as observed in the neutron pair distribution function fits. The orthorhombic Pnma to tetragonal I41/amd phase transition is seemingly the result of a change in the correlation length of the Tl+ 6s2 lone pairs, leading to a larger unit cell volume due to their uncorrelated displacements.

The magnetic properties of spinel nanoparticles can be controlled by synthesizing particles of a specific shape and size. The synthesized nanorods, nanodots and cubic nanoparticles have different crystal planes selectively exposed on the surface. The surface effects on the static magnetic properties are well documented, while their influence on spin waves dispersion is still being debated. Our ability to manipulate spin waves using surface and defect engineering in magnetic nanoparticles is the key to designing magnonic devices. We synthesized cubic and spherical nanoparticles of a classical antiferromagnetic material Co3O4 to study the shape and size effects on their static and dynamic magnetic proprieties. Using a combination of experimental methods, we probed the magnetic and crystal structures of our samples and directly measured spin wave dispersions using inelastic neutron scattering. We found a weak, but unquestionable, increase in exchange interactions for the cubic nanoparticles as compared to spherical nanoparticle and bulk powder reference samples. Interestingly, the exchange interactions in spherical nanoparticles have bulk-like properties, despite a ferromagnetic contribution from canted surface spins.

The perovskite structured oxides of composition ABO3 3 are considered strong candidates for solid-state electrolytes in all-solid-state batteries due to their chemical and structural flexibility. However, further improvements must be made before they become commercially viable, and this requires a clear understanding of the structure- property relationships. In this study, the local structure of the perovskite sodium-ion solid electrolyte series Na 1/2 _ x La 1/2 _ x Sr 2x ZrO 3 (NLSZ, x = 1 4 , 1 6 , 1 8 , 1 16 ) was investigated via neutron total scattering. Small-box modelling against the neutron pair distribution function with the orthorhombic Pbnm structure showed local-scale features that deviate from the average structure. Big-box modelling revealed significant differences between the bonding configurations of the different A-site cations, which impacts the ionic conductivity of the material. This study demonstrates how understanding local-scale disorder is important for tuning the structure-property relationships of inorganic solid-state electrolyte materials in sustainable battery technologies.

The local atomic structure of SnSe was characterized across its orthorhombic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn displacements with a dipolar character persist in the high-symmetry high-temperature phase, albeit with a symmetry different from that of the ordered displacements below the transition. The analysis implies that the transition is neither order-disorder nor displacive but rather a complex crossover. Robust ferrocoupled SnSe intralayer distortions suggest a ferroelectriclike instability as the driving force. These local symmetry-lowering Sn displacements are likely integral to the ultralow lattice thermal conductivity mechanism in SnSe.

The atomic structures of the lanthanide tantalates, Ln3TaO7, series (Ln = Pr, Tb, Dy, Ho, Tm, Yb) were systematically investigated using total scattering techniques.

The local atomic structure of SnSe was characterized across its orthorhmbic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn off-centering distortions persist in the high symmetry high temperature phase, with symmetry different from that of ordered distortions below the transition. The analysis implies that the transition is neither order-disorder nor displacive, but rather a complex crossover where the character of coupling changes from 3D-like at low temperature to 2D-like at high temperature. Robust ferro-coupled SnSe intra-layer distortions suggest a ferroelectric-like instability as the driving force. Complex local Sn off-centering is integral to the ultra-low lattice thermal conductivity mechanism in SnSe.

Natural gas hydrates (NGH) form on the ocean floor and in sub-surface permafrost deposits in high-pressure, low temperature environments.Research on these deposits is driven by their potential as an energy source.Naturally occurring CH4 hydrates primarily crystallize in the sI clathrate structure.The lattice is composed of hydrogen bonded water cages (the host), each of which occlude one gas molecule (the guest).Though the sI framework can host other molecules, this research focuses on CH4-CO2 hydrates to support current explorations in which CH4 may be harvested from hydrate deposits via exchange with CO2.CO2 replacement in the hydrate structure is energetically preferred, facilitating CO2 byproduct sequestration while providing CH4 as a fuel source.Equilibrium models predict that a mixed hydrate solid solution in which CO2 replaces some CH4 is stable at higher temperature and lower pressure compared to pure CH4 hydrate, and thermodynamic stability increases as the CO2 fraction increases.The impact of varying the type of guest molecules and mixed guest systems is a relevant topic to explore due to concerns about the stability of NGH under changing environmental conditions.A detailed understanding of the guest-host interactions in gas hydrates is necessary for the advancement of emerging technologies and processes which will utilize NGH deposits.The gas hydrate crystal has a high degree of disorder at all temperatures due to the motion of the occluded gas molecules and their interactions with the H2O lattice.Molecular dynamics (MD) simulations show that this disorder is not described by long-range crystallographic models.In situ neutron total scattering experiments and pair distribution function (PDF) analysis are used to characterize the short-range order in CH4-CO2 hydrates.Extraction of detailed information from PDF data of complex systems requires methods such as MD simulations combined with Reverse Monte Carlo (RMC) fitting.MD models of CH4-CO2 hydrates demonstrate the benefit of neutron PDF experiments by providing simulated PDFs, visualization of molecular motion, and analysis of thermodynamic interaction energies throughout the CH4-CO2 guest composition while providing large-box structural models for PDF data analysis.Variable temperature neutron PDF data of CH4, CO2, and mixed CH4-CO2 hydrate were collected on the NOMAD beamline at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory.RMCProfile was used to fit the data to large-box models produced with MD simulations.PDF experiments performed in situ provide structural characterization, while the variable temperature measurements lead to inferred dynamics.This analysis shows that when CH4 and CO2 co-occupy the hydrate, the host is more strongly distorted than in either pure CH4 or pure CO2 hydrates, but this becomes less defined with increasing temperature.The presence of CO2 in mixed hydrate increases the stability range and creates a barrier for CH4 to completely leave the structure.

Neutron diffraction and total scattering are combined to investigate a series of single-phase 10-component compositionally complex fluorite-based oxides, [(Pr0.375Nd0.375Yb0.25)2(Ti0.5Hf0.25Zr0.25)2O7]1-x[(DyHoErNb) O7]x, denoted as 10CCFBOxNb. A long-range order-disorder transition (ODT) occurs at x = 0.81 & PLUSMN; 0.01 from the ordered pyrochlore to disordered defect fluorite. In contrast to ternary oxides, this ODT occurs abruptly without an observable two-phase region; moreover, the phase stability in 10CCFBOs deviates from the well-established criteria for simpler oxides. Rietveld refinements of neutron diffraction patterns suggest that this ODT occurs via the migration of oxygen anions from the position 48f to 8a, with a small final jump at the ODT; however, the 8a oxygen occupancy changes gradually (without an observable discontinuous jump). We further discover diffuse scattering in Nb-rich compositions, which suggests the presence of short-range order. Using small-box modelling, four compositions near ODT (x = 0.75, 0.8, 0.85, and 1) can be better fitted by C2221 weberite ordering for the local polyhedral structure at nanoscale. Interestingly, 10CCFBO0.75Nb and 10CCFBO0.8Nb possess both long-range pyrochlore order and short-range weberite-type order, which can be understood from severe local distortion of the pyrochlore polyhedral structure. Thus, weberite-type short-range order emerges before the ODT, coexisting and interacting with long-range pyrochlore order. After the ODT, the long-range pyrochlore order vanishes but the short-range weberite-type order persists in the long-range disordered defect fluorite structure. Notably, a drop in the thermal conductivity coincides with emergence of the short-range order, instead of the long-range ODT.

Here, we provide an in-depth structural characterization of the amorphous ionic glasses LiPON and LiSiPON with high Li content. Based on ab initio molecular dynamics simulations, the structure of these materials is an inverted structure with either isolated polyanion tetrahedra or polyanion dimers in a Li+ matrix. Based on neutron scattering data, this type of inverted structure leads to a significant amount of medium-range ordering in the structure, as demonstrated by two sharp diffraction peaks and a periodic structural oscillation in the density function G(r). While this medium-range ordering is commonly observed in liquids and metallic glasses, it has not previously been observed in oxides. On a local scale, adding N and Si increases the number of anion bridges and polyanion dimer structures, leading to higher ionic conductivity. In the medium-range ordering, the addition of Si leads to more disorder in the polyanion substructure but a significant increase in the ordering of the O substructure. Finally, we demonstrate that this inverted structure with medium-range ordering results in a glassy material that is both mechanically stiff and ductile on the nanoscale.

An abundant source of CH4 can be found in natural hydrate deposits. Recent demonstration of CH4 recovery from hydrates via CO2 exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH4, CO2, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO2. Molecular vibrations are examined in CH4, CO2, and mixed CH4-CO2 hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH4, as it is extremely sensitive to 1H. The presence of CO2 in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH4 in mixed CH4-CO2 hydrate at 5 K. At 190 K, a large broadening of the CH4 librational modes indicates disorder in the structure leading to dissociation.

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H2O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO4 is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl+ 6s2 lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl+ 6s2 in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO4 occur due to changes in the correlation length of the Tl+ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl+ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.

Integration of renewable feedstocks into the current energy infrastructure will require the development of catalysts and sorbents that can maintain high surface area and catalytic activity under challenging thermal/hydrothermal environments and acid gas (SOx, NOx and H2S) exposure.The design of new stable and acid-gas-resistant catalysts requires a deep understanding of sintering and acid gas interaction with active sites.Addressing these challenges will require advanced operando characterization of materials to effectively guide materials discovery.Studying the time-resolved structural evolution of materials under gas flow conditions is key to understanding catalytic performance under real-world operating conditions with the end goal of extracting design strategies for industrially relevant catalysts.Total scattering, including both Bragg and diffuse scattering signals, enables the study of structural evolution in catalysts and can provide key insights into how long-range, nanoscale, and local atomic structure motifs differ and deliver unique properties.Synchrotron X-ray scattering provides active metal site sensitivity and unprecedented temporal resolution, while neutron scattering offers light atom sensitivity and superior penetration of sample environments.We present in-situ studies following multiple length scales of interest in two very different catalytic material systems with these probes: (1) the oxidation and reduction behaviors of ceria nanorods at elevated temperatures, specifically following the nature of oxygen vacancies; and (2) sinter resistance, degradation, and regeneration behavior of novel high-entropy fluorite catalyst supports under acid gas exposure.Further, we will discuss the design and development of the hazardous gas handling system (HGHS) system under construction at the Nanoscale Ordered Materials Diffractometer (NOMAD) at the Spallation Neutron Source at Oak Ridge National Lab.The HGHS will deliver in-situ exposure to industrially-relevant acid gas at NOMAD, enabling investigations of acid gas interactions with sorbents and catalysts, which will be a unique capability among neutron sources in the world and will aid in the design of new materials and processes with higher energy efficiency and a smaller emissions footprint.

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and “diffusons” give rise to the observed amorphous-like thermal conductivity.

We describe the local structural properties of the iron oxychalcogenides, La$_2$O$_2$Fe$_2$O$M_2$ ($M$ = S, Se), by using pair distribution function (PDF) analysis applied to total scattering data. Our results of neutron powder diffraction show that $M$ = S and Se possess similar nuclear structure at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe$_2$O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1-2 nm, we observed short-range orthorhombic-like structure suggestive of nematic fluctuations in both of these materials.

Nematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high- T c superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La 2 O 2 Fe 2 OM 2 M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C 2 fluctuations while neutron diffraction indicates that global C 4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity.

LiNiO2 (LNO) represents the end member in the compositional space of the LiNi1-x-yMnxCoyO2 (as x and y approach zero) cathode system. Despite its high theoretical specific capacity (275 mAh/g), LNO suffers from phase transitions with large volume change and unfavorable reactions upon electrochemical cycling, which restricts its practical use in the application of lithium-ion batteries. While the contributing factor to the structural instability is commonly linked to the undesired volume collapse associated with the H2-H3 phase transition, detailed analysis of structural evolution following the entire route of phase transitions (H1-M-H2-H3) in real time under battery operating conditions remains a challenging task. In this work, we employ operando neutron diffraction to study the structural changes (crystal lattice, Li/Ni-O bond length, O-Ni-O bond angles, and LiO2/NiO2 layer thickness) of LNO cathode in a home-built LixNiO2||graphite full cell during Li+ de-/intercalation. In particular, the anomalous increase(decrease) of Ni-O(Li-O) bond length at high SOC (>similar to 85 %) in the H3 phase is discussed in the context of O2- (2p)-> Ni4+(3d) negative charge transfer.