Pressure dependent structure of amorphous magnesium aluminosilicates: The effect of replacing magnesia by alumina at the enstatite composition

The effect of replacing magnesia by alumina on the pressure-dependent structure of amorphous enstatite was investigated by applying in situ high-pressure neutron diffraction with magnesium isotope substitution to glassy (MgO)0.375(Al2O3)0.125(SiO2)0.5. The replacement leads to a factor of 2.4 increase in the rate-of-change of the Mg-O coordination number with pressure, which increases from 4.76(4) at ambient pressure to 6.51(4) at 8.2 GPa, and accompanies a larger probability of magnesium finding bridging oxygen atoms as nearest-neighbors. The Al-O coordination number increases from 4.17(7) to 5.24(8) over the same pressure interval at a rate that increases when the pressure is above ∼3.5 GPa. On recovering the glass to ambient conditions, the Mg-O and Al-O coordination numbers reduce to 5.32(4) and 4.42(6), respectively. The Al-O value is in accordance with the results from solid-state 27Al nuclear magnetic resonance spectroscopy, which show the presence of six-coordinated aluminum species that are absent in the uncompressed material. These findings explain the appearance of distinct pressure-dependent structural transformation regimes in the preparation of permanently densified magnesium aluminosilicate glasses. They also indicate an anomalous minimum in the pressure dependence of the bulk modulus with an onset that suggests a pressure-dependent threshold for transitioning between scratch-resistant and crack-resistant material properties.

Iron coordination in liquid FeAl2O4

The structure of aerodynamically levitated liquid [Formula: see text] was measured by neutron diffraction with isotope substitution (NDIS). Classical and ab initio molecular dynamics simulations were performed and their results were found to be in close agreement with each other and the NDIS data. The results reveal that molten [Formula: see text] may be considered as an ionic liquid without any preference for particular short-range structural motifs. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Structure of amorphous materials in the NASICON system Na1+xTi2SixP3-xO12

The structure of glasses in the sodium (Na) super-ionic conductor (NASICON) system Na1+xTi₂Si_xP3-xO₁₂withx= 0.8 andx= 1.0 was explored by combining neutron and high-energy x-ray diffraction with²⁹Si,³¹P and²³Na solid-state nuclear magnetic resonance (NMR) spectroscopy. The²⁹Si magic angle spinning (MAS) NMR spectra reveal that the silica component remains fully polymerized in the form of Si⁴units, i.e. the silicon atoms are bound to four bridging oxygen atoms. The³¹P{²³Na} rotational echo adiabatic passage double resonance (REAPDOR) NMR data suggest that the³¹P MAS NMR line shape originates from four-coordinated Pⁿunits, wheren= 1, 2 or 3 is the number of bridging oxygen atoms per phosphorus atom. These sites differ in their³¹P-²³Na dipolar coupling strengths. The results support an intermediate range order scenario of a phosphosilicate mixed network-former glass in which the phosphate groups selectively attract the Na⁺modifier ions. Titanium takes a sub-octahedral coordination environment with a mean Ti-O coordination number of 5.17(4) forx= 0.8 and 4.86(4) forx= 1.0. A mismatch between the P-O and Si-O bond lengths of 8% is likely to inhibit the incorporation of silicon into the phosphorus sites of the NASICON crystal structure.

Effect of antifluorite layer on the magnetic order in Eu-based 1111 compounds, EuTAsF (T = Zn, Mn, and Fe)

The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu²⁺ in the [EuF]⁺ fluorite layers depending on the nature of the [TAs]^- antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at T_N ∼ 2.4-3 K due to an ordering of the Eu²⁺ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k₁ = (½ ½ 0) stripe order pattern with magnetic moments within the a-b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu²⁺ magnetic moments confined within the bc-plane. Additionally, the Mn²⁺ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c-direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J₃ between the adjacent [EuF]⁺ layers is comparable with in-plane J₁-J₂ interactions already in the case of semiconducting fluorite layers [TAs]^- (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs]^- ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.

Temperature-dependent anisotropy in the bond lengths of UO2as a result of phonon-induced atomic correlations

Previous experiments on cubic UO₂have suggested that the temperature dependences of the nearest-neighbour U-O and U-U distances aredifferent. We have acquired total-scattering neutron diffraction patterns out toQ = 23.5 Å^-1for50<T<1023 K and produced via Fourier transform a pair-distribution functionPDF(r). ThePDF(r)shows quite clearly thatr(U-O), defined by the maximum of the U-O peak in thePDF(r), does in fact decrease with increasing temperature, whereasr(U-U)follows the lattice expansion as expected. We also observe that ther(U-O)contraction accelerates continuously aboveT ≈ 400 K, consistent with earlier experiments by others. Furthermore, by analysing the eigenvectors of the phonon modes, we show that theΔ5(TO1)phonon tends to separate the eight equivalent U-O distances into six shorter and two longer distances, where the longer pair contribute to a high-rtail observed in the U-O distance distribution becoming increasingly anisotropic at higherT. These results have significance for a wide range of materials in which heavy and light atoms are combined in a simple atomic structure.

Structure of diopside, enstatite, and magnesium aluminosilicate glasses: A joint approach using neutron and x-ray diffraction and solid-state NMR

Neutron diffraction with magnesium isotope substitution, high energy x-ray diffraction, and 29Si, 27Al, and 25Mg solid-state nuclear magnetic resonance (NMR) spectroscopy were used to measure the structure of glassy diopside (CaMgSi2O6), enstatite (MgSiO3), and four (MgO) x(Al2O3) y(SiO2)1−x−y glasses, with x = 0.375 or 0.25 along the 50 mol. % silica tie-line (1 − x − y = 0.5) or with x = 0.3 or 0.2 along the 60 mol. % silica tie-line (1 − x − y = 0.6). The bound coherent neutron scattering length of the isotope 25Mg was remeasured, and the value of 3.720(12) fm was obtained from a Rietveld refinement of the powder diffraction patterns measured for crystalline 25MgO. The diffraction results for the glasses show a broad asymmetric distribution of Mg–O nearest-neighbors with a coordination number of 4.40(4) and 4.46(4) for the diopside and enstatite glasses, respectively. As magnesia is replaced by alumina along a tie-line with 50 or 60 mol. % silica, the Mg–O coordination number increases with the weighted bond distance as less Mg2+ ions adopt a network-modifying role and more of these ions adopt a predominantly charge-compensating role. 25Mg magic angle spinning (MAS) NMR results could not resolve the different coordination environments of Mg2+ under the employed field strength (14.1 T) and spinning rate (20 kHz). The results emphasize the power of neutron diffraction with isotope substitution to provide unambiguous site-specific information on the coordination environment of magnesium in disordered materials.

Structure of molten NaCl and the decay of the pair-correlations

The structure of molten NaCl is investigated by combining neutron and x-ray diffraction with molecular dynamics simulations that employed interaction potentials with either rigid or polarizable ions. Special attention is paid to the asymptotic decay of the pair-correlation functions, which is related to the small-k behavior of the partial structure factors, where k denotes the magnitude of the scattering vector. The rigid-ion approach gives access to an effective restricted primitive model in which the anion and cation have equal but opposite charges and are otherwise identical. For this model, the decay of the pair-correlation functions is in qualitative agreement with simple theory. The polarizable ion approach gives a good account of the diffraction results and yields thermodynamic parameters (density, isothermal compressibility, Debye screening length, and heat capacity) in accord with experiment. The longest decay length for the partial pair-distribution functions is a factor of ≃2.5 times greater than the nearest-neighbor distance. The results are commensurate with the decay lengths found for the effective restricted primitive model, which are much shorter than those found in experiments on concentrated electrolytes or ionic liquids using surface force apparatus.

From SmOF to SmH0.78OF0.22: H/F Substitution in Oxide Fluorides as a Synthesis Route to Heteroanionic Compounds

Mixed anionic hydrides of the rare earths are a fascinating class of compounds as potential functional materials, especially in luminescence, as photochromic thin films and for ion conduction. For exploratory studies, the effectiveness of various synthesis methods must be investigated, which is done here for metathesis reactions. The reaction of Sm₂O₃ with PTFE yields SmOF (P2₁/c, a = 5.60133(19) Å, b = 5.65567(19) Å, c = 5.6282(2) Å, β = 90.169(5)°, V = 178.295(11) ų, and Z = 4) in a new, probably metastable, polymorph of the baddeleyite-type structure. Metathesis reactions of SmOF with LiH, NaH, or CaH₂ led to a samarium hydride oxide fluoride, SmH_xOF_1-x; i.e., incomplete H/F exchange occurs. X-ray diffraction and neutron diffraction on a compound with x = 0.78 obtained via NaH reveal hydride, oxide, and fluoride ions to be partially ordered. SmH_0.78OF_0.22 (Ia3̅, a = 10.947(2) Å, V = 1311.7(4) ų, Z = 32) crystallizes in an anti-Li₃AlN₂-type structure with distorted cubic anion coordination for samarium atoms (site symmetry 3̅ and 2) and distorted tetrahedral arrangement of samarium atoms around the anions (site symmetry 1 and 3). It is a fully structurally characterized hydride oxide fluoride and shows a rare crystal chemical feature─the occupation of a crystallographic site by three different anions (0.188 H + 0.667 O + 0.145 F). Interatomic distances between samarium and hydrogen and samarium and the mixed hydrogen/oxygen/fluorine site range from 2.45 to 2.48 Å and 2.29 to 2.42 Å, respectively, and are similar to those in samarium hydride, samarium oxide, and samarium fluoride. Fluoride extraction by reaction with alkali and alkaline earth hydrides has thus proven to be a useful synthesis route to hydride oxides and also hydride oxide halogenides, which might be further exploited in exploratory research on heteroanionic metal hydrides.

Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures

The structure of a concentrated solution of NaCl in D₂O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl-O and O-O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D-O or O⋯D-O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion-water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl⁰ units, and coexist with solvent-separated Na⁺-Na⁺ and Cl^--Cl^- ion pairs. The exchange of water molecules with Na⁺ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes.

Combined specular and off-specular reflectometry: elucidating the complex structure of soft buried interfaces

Neutron specular reflectometry (SR) and off-specular scattering (OSS) are nondestructive techniques which, through deuteration, give a high contrast even among chemically identical species and are therefore highly suitable for investigations of soft-matter thin films. Through a combination of these two techniques, the former yielding a density profile in the direction normal to the sample surface and the latter yielding a depth-resolved in-plane lateral structure, one can obtain quite detailed information on buried morphology on length scales ranging from the order of ångströms to ∼10 µm. This is illustrated via quantitative evaluation of data on SR and OSS collected in time-of-flight (ToF) measurements of a set of films composed of immiscible polymer layers, protonated poly(methyl methacrylate) and deuterated polystyrene, undergoing a decomposition process upon annealing. Joint SR and OSS data analysis was performed by the use of a quick and robust originally developed algorithm including a common absolute-scale normalization of both types of scattering, which are intricately linked, constraining the model to a high degree. This, particularly, makes it possible to distinguish readily between different dewetting scenarios driven either by the nucleation and growth of defects (holes, protrusions etc.) or by thermal fluctuations in the buried interface between layers. Finally, the 2D OSS maps of particular cases are presented in different spaces and qualitative differences are explained, allowing also the qualitative differentiation of the in-plane structure of long-range order, the correlated roughness and bulk defects by a simple inspection of the scattering maps prior to quantitative fits.

Molecular Dynamics and Neutron Scattering Studies of Potassium Chloride in Aqueous Solution

Neutron diffraction with isotopic substitution (NDIS) experiments were done on both natural abundance potassium and isotopically labeled ⁴¹KCl heavy water solutions to characterize the solvent structuring around the potassium ion in water. Preliminary measurements suggested that the literature value for the coherent neutron scattering length (2.69 fm) for ⁴¹K was significantly in error. This value was remeasured using a neutron powder diffractometer and found to be 2.40 fm. This revision increases significantly the contrast between the natural abundance K and ⁴¹K by about 30% (from 1.0 to 1.3 fm). The experimentally determined structure factor of the potassium ion was then compared to that calculated from molecular dynamics (MD) simulations. Previous neutron scattering measurements of potassium gave a solvation number of 5.5 (see below). In this study, the NDIS and MD results are in good agreement and allowed us to derive a coordination number of 6.1 for water molecules and 0.8 for chloride ions around each K⁺ ion in 4 molal aqueous KCl solution.

Adjustable Magnetic Phase Transition Inducing Unusual Zero Thermal Expansion in Cubic RCo2-Based Intermetallic Compounds (R = Rare Earth)

Metallic materials that exhibit negligible thermal expansion or zero thermal expansion (ZTE) have great merit for practical applications, but these materials are rare and their thermal expansions are difficult to control. Here, we successfully tailored the thermal expansion behaviors from strongly but abruptly negative to zero over wide temperature ranges in a series of (Gd,R)(Co,Fe)₂ (R = Dy, Ho, Er) intermetallic compounds by tuning the composition to bring the first-order magnetic phase transition to second-order. Interestingly, an unusual isotropic ZTE property with a coefficient of thermal expansion of α _l = 0.16(0) × 10^-6 K^-1 was achieved in cubic Gd_0.25Dy_0.75Co_1.93Fe_0.07 (GDCF) in the temperature range of 10-275 K. The short-wavelength neutron powder diffraction, synchrotron X-ray diffraction, and magnetic measurement studies evidence that this ZTE behavior was ascribed to the rare-earth-moment-dominated spontaneous volume magnetostriction, which can be controlled by an adjustable magnetic phase transition. The present work extends the scope of the ZTE family and provides an effective approach to exploring ZTE materials, such as by adjusting the magnetism or ferroelectricity-related phase transition in the family of functional materials.

A case of multifunctional intermetallic compounds: negative thermal expansion coupling with magnetocaloric effect in (Gd,Ho)(Co,Fe)2

The negative thermal expansion (NTE) and magnetocaloric effect (MCE) are highly correlated in (Gd,Ho)(Co,Fe)₂, both of which are governed by the rare earth moment, whereas the MCE is further reinforced through an intermediate contribution from NTE.

Structure and dynamics of high-temperature strontium aluminosilicate melts

We report the study of high-temperature melts (1600-2300 °C) and related glasses in the SrO-Al2O3-SiO2 phase diagram considering three series: (i) depolymerized ([SrO]/[Al2O3] = 3); (ii) fully polymerized ([SrO]/[Al2O3] = 1); and (iii) per-aluminous ([SrO]/[Al2O3] < 1). By considering the results from high-temperature 27Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO4 units during cooling for high-silica compositions. A few five-fold coordinated VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from 27Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al-(OSi)p(OAl)(4-p) units.

Structure of Strontium Aluminosilicate Glasses from Molecular Dynamics Simulation, Neutron Diffraction, and Nuclear Magnetic Resonance Studies

The structure of strontium glasses with the composition (SiO₂)_{1-2 x}(Al₂O₃) _x(SrO) _x ( R = [SrO]/[Al₂O₃] = 1) and (SiO₂)_{1-4 x}(Al₂O₃) _x(SrO)_{3 x} ( R = 3) has been explored experimentally over both short- and intermediate-length scales using neutron diffraction, ²⁷Al and ²⁹Si nuclear magnetic resonance, and classical molecular dynamics simulations in model systems containing around 10 000 atoms. We aim at understanding the structural role of aluminum and strontium as a function of the chemical composition of these glasses. The short- and medium-range structure such as aluminum coordination, bond angle distribution, Q^{( n)} distribution, and oxygen speciation have been systematically studied. Two potential forms of the repulsive short-range interactions have been investigated, namely, the Buckingham and Morse forms. The comparison of these forms allows us to derive general trends independent of the particular choice of the potential form. In both cases, it is found that aluminum ions are mainly fourfold coordinated and mix with the silicon network favoring the Al/Si mixing in terms of Al-O-Si linkages. For the R = 1 glass series, despite the full charge compensation ([SrO] = [Al₂O₃]), a small fraction of fivefold aluminum is observed both experimentally and in MD simulations, whereas the concentration of sixfold aluminum is negligible. MD shows that the fivefold aluminum units AlO₅ preferentially adopt a small ring configuration and link to tricoordinated oxygen atoms whose population increases with the aluminum content and are mainly found in OAl₃ and OAl₂Si configurations. The modeled Sr speciation mainly involves SrO₇ and SrO₈ polyhedra, giving a range of average Sr²⁺ coordination numbers between 7 and 8 slightly dependent on the short-range repulsive potential form. A detailed statistical analysis of T-O-T' (T, T' = Al,Si), accounting for the population of the various oxygen speciations, reveals that both potentials predict a nearly identical Al/Si mixing.

Structure of semiconducting versus fast-ion conducting glasses in the Ag-Ge-Se system

The transition from a semiconductor to a fast-ion conductor with increasing silver content along the Ag x (Ge0.25Se0.75)(100-x) tie line (0≤x≤25) was investigated on multiple length scales by employing a combination of electric force microscopy, X-ray diffraction, and neutron diffraction. The microscopy results show separation into silver-rich and silver-poor phases, where the Ag-rich phase percolates at the onset of fast-ion conductivity. The method of neutron diffraction with Ag isotope substitution was applied to the x=5 and x=25 compositions, and the results indicate an evolution in structure of the Ag-rich phase with change of composition. The Ag-Se nearest-neighbours are distributed about a distance of 2.64(1) Å, and the Ag-Se coordination number increases from 2.6(3) at x=5 to 3.3(2) at x=25. For x=25, the measured Ag-Ag partial pair-distribution function gives 1.9(2) Ag-Ag nearest-neighbours at a distance of 3.02(2) Å. The results show breakage of Se-Se homopolar bonds as silver is added to the Ge0.25Se0.75 base glass, and the limit of glass-formation at x≃28 coincides with an elimination of these bonds. A model is proposed for tracking the breakage of Se-Se homopolar bonds as silver is added to the base glass.

What Is the Actual Local Crystalline Structure of Uranium Dioxide, UO2? A New Perspective for the Most Used Nuclear Fuel

Up to now, uranium dioxide, the most used nuclear fuel, was said to have a Fm3̅m crystalline structure from 30 to 3000 K, and its behavior was modeled under this assumption. However, recently X-ray diffraction experiments provided atomic pair-distribution functions of UO₂, in which UO distance was shorter than the expected value for the Fm3̅m space group. Here we show neutron diffraction results that confirm this shorter UO bond, and we also modeled the corresponding pair-distribution function showing that UO₂ has a local Pa3̅ symmetry. The existence of a local lower symmetry in UO₂ could explain some unexpected properties of UO₂ that would justify UO₂ modeling to be reassessed. It also deserves more study from an academic point of view because of its good thermoelectric properties that may originate from its particular crystalline structure.

Optimizing the counting times for sample-in-container scattering experiments

A method is given for choosing the relative counting times for the sample-in-container and empty container parts of a fixed-duration scattering experiment in order to minimize the statistical error on the container-corrected intensity. The method is applied to angular-dispersive diffraction experiments, and the effect on the fractional error of mis-estimating the relative run times is considered.

Structural Changes in the Local Environment of Uranium Atoms in the Three Phases of U4O9

The crystal structure of U4O9 remains an enigma because of its differences with U(4+) and U(5+) coordination polyhedral mixtures, as shown in the XANES experimental results. To better understand this crystal structure, its diffraction pattern was measured at seven different temperatures using neutron diffraction before being independently refined by Rietveld's method and pair distribution function analysis. The O cuboctahedron-a structural element consisting of 13 oxygen atoms-is a specific feature of the U4O9 crystal structure. The volume of the cuboctahedron decreases when the temperature increases, whereas the overall volume of the crystal cell increases. This feature can be correlated with the two U4O9 phase transitions that induce sharp changes in the cuboctahedron geometry, suggesting that this structural element has internal dynamics. In particular, these structural modifications in the γ phase suggest that the high-temperature phase can be described as a mixture of U(4+) and U(5+) coordination polyhedra, the latter having U-O distances shorter than 2.2 Å, that are absent in the former. These changes in uranium polyhedra as a function of temperature are tentatively interpreted using steric arguments. They also raise the question of charge localization on the different U ion sites in the low-temperature phases of U4O9.

From atomic structure to excess entropy: a neutron diffraction and density functional theory study of CaO-Al₂O₃-SiO₂ melts

Calcium aluminosilicate CaO-Al2O3-SiO2 (CAS) melts with compositions (CaO-SiO2)(x)(Al2O3)(1-x) for x  <  0.5 and (Al2O3)(x)(SiO2)(1-x) for x ≥ 0.5 are studied using neutron diffraction with aerodynamic levitation and density functional theory molecular dynamics modelling. Simulated structure factors are found to be in good agreement with experimental structure factors. Local atomic structures from simulations reveal the role of calcium cations as a network modifier, and aluminium cations as a non-tetrahedral network former. Distributions of tetrahedral order show that an increasing concentration of the network former Al increases entropy, while an increasing concentration of the network modifier Ca decreases entropy. This trend is opposite to the conventional understanding that increasing amounts of network former should increase order in the network liquid, and so decrease entropy. The two-body correlation entropy S2 is found to not correlate with the excess entropy values obtained from thermochemical databases, while entropies including higher-order correlations such as tetrahedral order, O-M-O or M-O-M bond angles and Q(N) environments show a clear linear correlation between computed entropy and database excess entropy. The possible relationship between atomic structures and excess entropy is discussed.

The atomic scale structure of graphene powder studied by neutron and X-ray diffraction

The structure of graphene obtained by chemical exfoliation of graphiteviathe oxidation/reduction procedure has been determined using wide-angle scattering of neutrons and X-rays combined with computer simulations based on classical molecular dynamics (MD). A comparison of results obtained from wide-angle neutron scattering (WANS) with the D4 neutron diffractometer dedicated for liquids and amorphous materials (Institute Laue–Langevin in Grenoble) and from wide-angle X-ray scattering (WAXS) with the laboratory Rigaku-Denki D/MAX RAPID II diffractometer has shown that both techniques provide data of a good quality that can be used to derive precise and valuable structural information about graphene. To obtain detailed structural information, the paracrystal formalism has been used along with MD simulations. The MD simulations were performed at 300 K with second-generation reactive empirical bond order potential for atoms lying in the same layer and the Lennard–Jones potential for interlayer interactions. The proposed models consist of three-layered systems, 36 Å in diameter, in which mono-vacancy, di-vacancy and Stone–Thrower–Wales types of defects are introduced. The reported results show that the WANS and WAXS methods together with the MD simulations contribute to a detailed description of the graphene materials, including the presence of topological defects, which is important as their structure at the atomic scale dramatically affects their electrical and mechanical properties.

High-pressure transformation of SiO₂ glass from a tetrahedral to an octahedral network: a joint approach using neutron diffraction and molecular dynamics

A combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and molecular dynamics simulations employing a many-body interatomic potential model is used to investigate the structure of cold-compressed silica glass. The simulations give a good account of the neutron diffraction results and of existing x-ray diffraction results at pressures up to ~60  GPa. On the basis of the molecular dynamics results, an atomistic model for densification is proposed in which rings are "zipped" by a pairing of five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence of the mean primitive ring size ⟨n⟩ on the mean Si-O coordination number, thereby linking a parameter that is sensitive to ordering on multiple length scales to a readily measurable parameter that describes the local coordination environment.

Magnetic structure of the metallic triangular antiferromagnet Ag2NiO2

The magnetic structure of the metallic antiferromagnet Ag2NiO2 with the Néel temperature TN = 56 K has been investigated by means of a neutron diffraction technique using a powder sample in the temperature range between 5 and 65 K. The antiferromagnetic (AF) diffraction peaks are clearly observed below TN and can be indexed with the propagation vector [Formula: see text]. Based on the results of both a representational analysis and a Rietveld refinement of the magnetic peaks, the AF spin structure is determined as an A-type AF structure with ml = m0cos(2πk ⋅l), where ml is the moment at the lth Ni(3+) site and m0 = (0.31,0,0.65) μB at 5 K.

The bound coherent neutron scattering lengths of the oxygen isotopes

The technique of neutron interferometry was used to measure the bound coherent neutron scattering length b(coh) of the oxygen isotopes (17)O and (18)O. From the measured difference in optical path between two water samples, either H(2)(17)O or H(2)(18)O versus H(2)(nat)O, where nat denotes the natural isotopic composition, we obtain b(coh,(17)O) = 5.867(4) fm and b(coh,(18)O) = 6.009(5) fm, based on the accurately known value of b(coh,(nat)O) = 5.805(4) fm which is equal to b(coh,(16)O) within the experimental uncertainty. Our results for b(coh,(17)O) and b(coh,(18)O) differ appreciably from the standard tabulated values of 5.6(5) fm and 5.84(7) fm, respectively. In particular, our measured scattering-length contrast of 0.204(3) fm between (18)O and (nat)O is nearly a factor of 6 greater than the tabulated value, which renders feasible neutron diffraction experiments using (18)O isotope substitution and thereby offers new possibilities for measuring the partial structure factors of oxygen-containing compounds, such as water.

Mechanisms of network collapse in GeO2 glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO(2) is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density ρ/ρ(0) > or approximately equal to 1.16, where ρ(0) is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO(5) units versus octahedral GeO(6) units as they replace tetrahedral GeO(4) units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when ρ/ρ(0) = 1 and fourfold rings when ρ/ρ(0) = 1.64.

Structural transformations on vitrification in the fragile glass-forming system CaAl2O4

The structure of the fragile glass-forming material CaAl(2)O(4) was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO(5) polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO(4) tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass.

Isotope effects in water as investigated by neutron diffraction and path integral molecular dynamics

The structures of heavy and light water at 300 K were investigated by using a joint approach in which the method of neutron diffraction with oxygen isotope substitution was complemented by path integral molecular dynamics simulations. The diffraction results, which give intra-molecular O-D and O-H bond distances of 0.985(5) and 0.990(5) Å, were found to be in best agreement with those obtained by using the flexible anharmonic TTM3-F water model. Both techniques show a difference of  ≃ 0.5% between the O-D and O-H intra-molecular bond lengths, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions. Further consideration of the O-O correlations is needed in order to improve agreement with experiment.

Oxygen as a site specific probe of the structure of water and oxide materials

The method of oxygen isotope substitution in neutron diffraction is introduced as a site specific structural probe. It is employed to measure the structure of light versus heavy water, thus circumventing the assumption of isomorphism between H and D as used in more traditional neutron diffraction methods. The intramolecular and intermolecular O-H and O-D pair correlations are in excellent agreement with path integral molecular dynamics simulations, both techniques showing a difference of ≃0.5% between the O-H and O-D intramolecular bond distances. The results support the validity of a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intramolecular and intermolecular quantum contributions.

The structure of liquid calcium aluminates as investigated using neutron and high energy x-ray diffraction in combination with molecular dynamics simulation methods

The structure of laser heated aerodynamically levitated (CaO)(x)(Al₂O₃)(1-x) high temperature liquids, with x = 0.33, 0.5, 0.75, was measured by using neutron and high energy x-ray diffraction. The partial structure factors for the liquids at 2500 K were also determined using molecular dynamics computer simulations. The simulation results are in excellent agreement with the diffraction measurements. The results show a predominant tetrahedral Al coordination with approximately 20% of fivefold coordinated Al at x = 0.33 which reduces with increasing CaO concentration. The Ca atoms occupy a broad range of coordination environments but with a predominance of sixfold distorted octahedra. The simulations confirm the presence of 13, 7 and 0.6% OAl₃ triclusters connecting AlO₄ tetrahedra in the structure of CA2 (x = 0.33), CA (x = 0.5) and C3A (x = 0.75) liquids, respectively.

Liquid-liquid phase transition in supercooled yttria-alumina

The structure and thermal characteristics of aerodynamically levitated samples of yttria-alumina in the liquid, supercooled liquid and solid phases were explored in an extensive series of high energy x-ray diffraction, small angle neutron scattering, and pyrometric cooling measurements. Particular focus was placed on the compound (Y2O3)(x)(Al2O3)(1-x) with x = 0.2 for which a liquid-liquid phase transition at a temperature of 1788 K has recently been reported. No structural or thermal signature in support of this metastable phase transition could be found.

Establishing the structure of GeS(2) at high pressures and temperatures: a combined approach using x-ray and neutron diffraction

The change in structure of glassy GeS(2) with pressure increasing to [Formula: see text] at ambient temperature was explored by using in situ neutron and x-ray diffraction. Under ambient conditions, the glass structure is made from a mixture of corner- and edge-sharing Ge(S(1/2))(4) tetrahedra where 47(5)% of the Ge atoms are involved in edge-sharing configurations. The network formed by these tetrahedra orders on an intermediate range as manifested by the appearance of a pronounced first sharp diffraction peak in the measured total structure factors at a scattering vector k = 1.02(2) Å(-1) which has a large contribution from Ge-Ge correlations. The intermediate range order breaks down when the pressure on the glass increases above ≈2 GPa but there does not appear to be a significant alteration of the Ge-S coordination number or corresponding bond length with increasing density. The results for the glass are consistent with a densification mechanism in which there is a replacement of edge-sharing by corner-sharing Ge centred tetrahedral motifs and/or a reduction in the Ge-[Formula: see text]-Ge bond angle between corner-sharing tetrahedral motifs with increasing pressure. The change in structure with increasing temperature at a pressure of [Formula: see text] was also investigated by means of in situ x-ray diffraction as the glass crystallized and then liquefied. At 5.2(1) GPa and 828(50) K the system forms a tetragonal crystal, with space group [Formula: see text] and cell parameters a = b = 4.97704(12) and c = 9.5355(4) Å, wherein corner-sharing Ge(S(1/2))(4) tetrahedra pack to form a dense three-dimensional network. A method is described for correcting x-ray diffraction data taken in situ under high pressure, high temperature conditions for a cylindrical sample, container and gasket geometry with a parallel incident beam and with a scattered beam that is defined using an oscillating radial collimator. A method is also outlined for obtaining coordination numbers from direct integration of the peaks in measured x-ray total pair distribution functions.

Magnetic critical scattering in solid Co80Pd20

Using small-angle neutron scattering combined with a containerless aerodynamic levitation technique for high temperatures, we have measured the temperature dependence of the correlation length ξ of near-critical magnetic fluctuations in the solid phase of the completely miscible fcc alloy Co₈₀Pd₂₀. A fit to our data yields a critical exponent ν = 0.76 ± 0.05 for the divergence of ξ(T) above the ferromagnetic transition temperature T_c. This value of ν is consistent with the prediction of the three-dimensional Heisenberg model for magnetic critical scattering.

Structure of molten yttrium aluminates: a neutron diffraction study

We used the aerodynamic levitation technique combined with CO₂ laser heating to study the structure of liquid yttrium aluminates above their melting point with neutron diffraction. For various yttria contents, we determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. In particular, we derived Al-O and Y-O bond distances and coordination numbers. Experimental data are compared with ab initio molecular dynamics, carried out using the VASP code where the interatomic forces are obtained from density functional theory. In particular, partial pair correlation functions have been calculated and are in relatively good agreement with the experimental observations.

Structure and dynamics of levitated liquid materials

Aerodynamic levitation is a simple way to suspend samples which can be heated with CO2 lasers. The advantages of this technique are the simplicity and compactness of the device, making it possible to integrate the device easily into different kinds of experiments. In addition, all types of sample can be used, including metals and oxides. The integration of this technique at synchrotron and neutron sources provides powerful tools to study molten materials.

Identification of the relative distribution of rare-earth ions in phosphate glasses

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl(0.30)P(3.05)O(9.62) was measured by employing the method of isomorphic substitution in neutron diffraction. It is found that 7.9(7) R-R nearest neighbors reside at 5.62(6) A in a network made from interlinked PO4 tetrahedra. Provided that the role of Al is explicitly considered, a self-consistent account of the local matrix atom correlations can be developed in which there are 1.68(9) bridging and 2.32(9) terminal oxygen atoms per phosphorus.