Structure and Liquid Fragility in Sodium Carbonate

The relationship between local structure and dynamics is explored for molten sodium carbonate. A flexible fluctuating-charge model, which allows for changes in the shape and charge distribution of the carbonate molecular anion, is developed. The system shows the evolution of highly temperature-dependent complex low-dimensional structures which control the dynamics (and hence the liquid fragility). By varying the molecular anion charge distribution, the key interactions responsible for the formation of these structures can be identified and rationalized. An increase in the mean charge separation within the carbonate ions increases the connectivity of the emerging structures and leads to an increase in the system fragility.

The structure of liquid alkali nitrates and nitrites

State of the art high energy X-ray diffraction experiments and simulation models (employing a description of charge transfer) are applied to pure molten alkali nitrates and nitrites and uncover significant emerging structure.

Topological ordering in liquid UO2

A molecular dynamics model of liquid UO2 that is in good agreement with recent high-energy x-ray diffraction data has been analyzed using the Bhatia-Thornton formalism. A pre-peak appears in the topological structure factor S NN(Q) at Q  =  1.85(1)Å(-1) which is not present in the more common, element specific Faber-Ziman partial structure factors. A radical Voronoi tessellation of the 3D molecular dynamics model shows the presence of a wide distribution of clusters, consistent with presence of highly mobile oxygen atoms. However, 4-fold Voronoi polyhedra (n 4) are found to dominate the structure and the majority of clusters can be described by the distribution n 3  ⩽  n 4  ⩾  n 5. It is argued that an open network of 4-fold Voronoi polyhedra could explain the origin of the pre-peak in S NN(Q) and the topological ordering observed in liquid UO2.

The structure of water around the compressibility minimum

Here we present diffraction data that yield the oxygen-oxygen pair distribution function, g(OO)(r) over the range 254.2-365.9 K. The running O-O coordination number, which represents the integral of the pair distribution function as a function of radial distance, is found to exhibit an isosbestic point at 3.30(5) Å. The probability of finding an oxygen atom surrounding another oxygen at this distance is therefore shown to be independent of temperature and corresponds to an O-O coordination number of 4.3(2). Moreover, the experimental data also show a continuous transition associated with the second peak position in g(OO)(r) concomitant with the compressibility minimum at 319 K.

Molten uranium dioxide structure and dynamics

Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts.

Low cation coordination in oxide melts

The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on Y2O3, and Ho2O3 melts at 2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline Y2O3 (or Ho2O3). Investigation of La2O3, ZrO2, and Al2O3 melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as SiO2. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.

Examples of Hydrothermal Titration and Real Time X-Ray Diffraction in the Synthesis of Open Frameworks

Hydrothermal titration (HTT) techniques represent a new synthetic strategy for the production of open frameworks. As an example, initial digestion of amorphous GeS2 in the presence of tetraethyl ammonium hydroxide (TEAOH) at 100°C is followed by injection of Ag+ into this mixture to condense the [Ge4S10]4− clusters and form a framework consisting of 8-ring channels bounded by alternating GeS-clusters and Ag+. The rational use of this apparatus depends upon determining the timing of injection. Insight into the changes occurring during reactant digestion is provided by real time synchrotron X-ray powder diffraction. Changes in the the system dimethylamine-Sb-S have been followed in real time at different temperatures. The transformation from mixtures of DMA, Sb and S to the known open phase DMA-SbS-SB8 was followed by observing monochromatic X-ray scattering detected on an imaging plates over a 8 hr period with one continuous exposure.

Elasticity of agr-Cristobalite: A Silicon Dioxide with a Negative Poisson's Ratio

Laser Brillouin spectroscopy was used to determine the adiabatic single-crystal elastic stiffness coefficients of silicon dioxide (SiO(2)) in the alpha-cristobalite structure. This SiO(2) polymorph, unlike other silicas and silicates, exhibits a negative Poisson's ratio; alpha-cristobalite contracts laterally when compressed and expands laterally when stretched. Tensorial analysis of the elastic coefficients shows that Poisson's ratio reaches a maximum value of -0.5 in some directions, whereas averaged values for the single-phased aggregate yield a Poisson's ratio of -0.16.

Sr3Ga4O9 – a strontium gallate with a new type of tetrahedral layer structure

Synthetic strontium gallate Sr3Ga4O9 is triclinic, space group P1̅ (a = 6.7490(9)Å, b = 8.0494(12)Å, c = 9.7295(15)Å, α = 85.01(2)º, β = 72.71(2)º, γ = 74.24(2)º, V = 485.7(3)Å3, Z = 2, D calc = 4.69 g·cm-3). The structure was solved by direct methods using 2656 unique reflections and refined to a residual of R1 = 0.042 (146 parameters). The compound belongs to the group of tetrahedral single layer structures. Individual sheets are parallel to (100), thus producing a prominent cleavage. The stepped layers can be described as being built by the condensation of six tetrahedra-wide bands. The bands in turn consist of two open branched dreier single chains connected via common corners and they contain tertiary and quaternary tetrahedra Q3 and Q4 in the ratio 1:1. The stacking of the layers containing four-, six- and eight-membered rings results in a three-dimensional structure in which the strontium cations reside in the voids between the layers and are coordinated by six to seven oxygen neighbors.

Characterization of the stuffed framework structures BaAlGaO4 and BaFeGaO4

The crystal structures of the compounds BaMGaO4 (M = Al, Fe) have been investigated using conventional single-crystal X-ray diffraction data for M = Fe and with synchrotron radiation data for M = Al. BaAlGaO4 (space group P6322, a = 5.2788(4) Å and c = 8.7835(7) Å, V = 211.97(4) Å3, Z = 2, D calc. = 4.67 g cm–3, wR2 = 0.089 for 242 independent reflections with I > 2σ(I)) is isostructural with BaAl2O4-I. Main building units are layers of six-membered rings of (Al,Ga)O4-tetrahedra perpendicular to [001]. The sequence of directedness of the tetrahedral apices in all single six membered rings (S6R) is UDUDUD (U: up, D: down). Stacking of layers parallel to c results in a three-dimensional framework containing tunnels, where the Ba atoms are located. They are coordinated by nine oxygen ligands in form of distorted tricapped trigonal antiprisms. BaFeGaO4 also belongs to the hexagonal crystal system (space group P63, a = 10.8377(6) Å and c = 8.6865(6) Å, V = 883.59(9) Å3, Z = 8, D calc. = 4.92 g cm–3, wR2 = 0.099 for 1042 independent observed reflections). The crystal showed twinning by merohedry which was accounted for in the refinement calculations. Within a single tetrahedral layer of the framework structure two different types of ditrigonal-shaped rings are distinguishable: one fourth of the rings have an UDUDUD topology, whereas the sequence of the remaining rings is UUUDDD. The Fe3+- and Ga3+-cations show an ordered distribution among the four symmetrically independent T-sites: two positions contain either Fe or Ga, whereas on the remaining two sites there is a definite preference for one of the two cation species. The Ba atoms in the cavities of the framework are coordinated by eight to nine oxygen neighbors. The observed twinning in BaFeGaO4 can be attributed to the existence of pseudo twofold axis within the tetrahedral layers, running parallel [100] and the corresponding symmetrically equivalent directions.

Structural changes in nanocrystalline mackinawite (FeS) at high pressure

The high-pressure behavior of nanocrystalline mackinawite (FeS) with particle sizes of 6, 7 and 8 nm has been investigated by high-energy X-ray total scattering and pair distribution function analysis. An irreversible first-order structural phase transition from tetragonal mackinawite to orthorhombic FeS-II was observed at about 3 GPa. The transition is induced by the closure of the van der Waals gap in the layered mackinawite structure. A grain size effect on the transition pressure and the compressibility was observed.

Observation of cation reordering during the olivine-spinel transition in fayalite by in situ synchrotron X-ray diffraction at high pressure and temperature

The olivine-spinel phase transition in fayalite at high pressure and temperature has been studied using time-resolved x-ray diffraction. Structure refinements show a delay of cation reordering relative to anions during the phase transformation and an increase in the cell volume while the cations reorder into their sites. A significant stress drop in the sample is observed. This experiment, for the first time, quantitatively demonstrates a pseudomartensitic transformation: a diffusionless anion sublattice transition coupled with short-range diffusional cation reordering.

K5.76Ga5.76Si10.24O32.3.4H2O, a gallosilicate with the zeolite gismondine topology

The title compound, K-GaSi-GIS, potassium gallium silicon oxide hydrate, was synthesized hydrothermally and its crystal structure was determined from data collected on a single crystal of dimensions 10 x 10 x 8 microm at a synchrotron X-ray source. The compound, which has the aluminosilicate (AlSi) zeolite gismondine (GIS) topology, Ca(4)[Al(8)Si(8)O(32)].16H(2)O, crystallizes in the tetragonal space group I4(1)/a. A disordered distribution of the framework Si/Ga sites leads to higher symmetry of the GIS-type network compared with the usual monoclinic symmetry in AlSi-GIS. Framework Ga substitution for Al in AlSi-GIS leads to substantial distortion of the crankshaft chains, reducing the effective pore dimensions and suggesting the possibility of pore-dimension control via partial framework-cation substitution.

IN SITU STUDIES OF THE PROPERTIES OF MATERIALS UNDER HIGH-PRESSURE AND TEMPERATURE CONDITIONS USING MULTI-ANVIL APPARATUS AND SYNCHROTRON X-RAYS

▪ Abstract  Increased access to multi-anvil high-pressure devices interfaced to synchrotron X-ray radiation sources has led to a new class of experiments. These new capabilities include (a) high-precision crystal structure determination and refinement from powder X-ray diffraction data; (b) the determination of kinetic parameters and structure from time-resolved diffraction data; (c) the determination of absolute pressures by the combined use of ultrasonic techniques at high pressures and temperatures with simultaneous monitoring of X-ray diffraction; and (d) the determination of the strength and rheological properties of materials through the monitoring of the relaxation of broadened diffraction peak widths in the presence of a well-characterized deviatoric stress field generated in the multi-anvil high-pressure apparatus.

An Antimony Sulfide with a Two-Dimensional, Intersecting System of Channels

A novel open antimony sulfide framework of composition [Sb(3)S(5)](-) has been synthesized hydrothermally, in the presence of tetramethyl ammonium ions. Determination of the crystal structure demonstrates that the (Me(4)N)(+) molecules (Me, methyl) occlude a system of intersecting channels at 90 degrees to one another. The three-dimensional framework is assembled from a new building unit, flat s-shaped buckled sheets of edge-linked 4-rings containing Sb(2)S(2) moieties. These are cross-linked in two directions to form the system of channels.