Intermediate Range Order Research Articles

Short- and Intermediate-Range Order in Liquid Bi–BiBr3 Mixtures: Importance of Voids

The neutron diffraction (ND) measurements and the detailed structural modeling by reverse Monte Calro (RMC) simulation have been performed for l-BiBr 3 and l-45 and 55 mol % Bi mixtures. The partial structure factor S i j ( Q ), partial radial distribution function g i j ( r ), and snapshot produced from RMC configurations have been examined. The structure of l-BiBr 3 is dominated by the network of trigonal pyramidal units. The l-mixtures consist of the –Br–Bi–Br– chain fragments terminated by Br. A Voronoi–Delaunay analysis has been applied to characterize the voids within network or chain. We find that the voids are surrounded by six pyramids in l-BiBr 3 and by chains in l-mixture. The packing density is 0.59 for l-BiBr 3 and 0.45 for l-45 mol % Bi mixture which is substantially low compared with that of dense random packing. The void–void structure factor S ' V–V ( Q ) involving void centers have the pre-peak at ∼1.2 A -1 which is in common with the pre-peak in experimentally observed S ( Q ). It is co...

Similarities in 2- and 6-Line Ferrihydrite Based on Pair Distribution Function Analysis of X-ray Total Scattering

Iron(III) hydroxides are abundant in near-surface natural environments and play an important role in geochemical processes and the fate of contaminants. The issue of the structure of the common nanophase material ferrihydrite (ferric hydroxide) is controversial and has been debated in the literature for many years without definitive resolution. At least two types of ferrihydrite, the 2-line and 6-line forms, are conventionally recognized. It has been suggested that these forms possess different structures built up by different mixtures of distinct nanophase components. However, traditional crystallographic methods provide depictions of structure that are most sensitive either to short-range order (X-ray absorption) or long-range periodicity (X-ray diffraction or electron diffraction). We used high-energy X-ray total scattering for pair distribution function analysis to observe both the short- and intermediate-range ordering (exceeding ∼15 Å) of synthetic ferrihydrite with three distinct average domain sizes of 2, 3, and 6 nm. We show that there are no significant differences in the underlying structures of these materials and that the differences in the diffraction patterns can be entirely interpreted by variations in the average size of the coherent scattering domains. The average crystallite sizes inferred from the PDF analysis are in good agreement with direct observation by high-resolution transmission electron microscopy.

Structure of LiquidSiO2: A Measurement by High-Energy X-Ray Diffraction

The x-ray structure factor for liquid SiO2 has been measured by laser heating of an aerodynamically levitated droplet. The main structural changes of the melt compared to the room temperature glass are associated with an increase in the size of the SiO4 tetrahedra, indicating a small reduction in the average Si-O-Si bond torsion angle and an expansion of the network between 5 and 9 A. Strong directional bonds with little high temperature broadening and a high degree of intermediate range order are found to persist in the liquid state.

Microscopic origin of concentration fluctuations over intermediate range distances in network-forming disordered systems

Ab initio molecular dynamics simulations of liquid GeSe2 show that the first sharp diffraction peak in the concentration-concentration structure factor S-CC(k) is due to a sequence of connected fourfold rings. These subunits can be viewed as chains of edge-sharing tetrahedra. Ge atoms at the opposite tails of these chains are mostly miscoordinated and separated from each other by intermediate range distances. This result correlates a specific structural subunit to a subtle intermediate range order property, involving both the structural and chemical composition of the disordered network.

Observation of intermediate-range order in a nominally amorphous molecular semiconductor film

Synchrotron X-ray diffraction from a nominally amorphous molecular semiconductor film reveals both the presence of intermediate-range order (IRO) corresponding to crystalline domains with an average size of a few nanometres, and the growth of these domains upon exposure of the film to moisture.

NMR studies of short and intermediate range ordering of amorphous Si–B–N–C–H pre-ceramic at the pyrolysis stage of 600 °C

Inorganic random networks of a pre-ceramic stage of the amorphous ceramic SiBN3C, derived from the single-source precursor Cl3Si(NH)BCl2 (TADB), have been synthesized with isotope-labeled materials. This allowed characterization at an atomic level of the development of the pre-ceramic polymer at a crucial pyrolysis stage of 600 °C by solid-state NMR. Using several techniques including 13C{15N}REDOR, 11B{13C}REDOR, 11B{15N}REDOR and 13C-2D-RFDR, the atomic network of the 13C, 15N enriched sample was investigated. These combined experiments have lead to the important result that carbon forms unsaturated bonds at this stage of the pyrolysis but remains bonded to nitrogen, maintaining a homogeneous distribution of the elements. Carbon atoms seem to be incorporated into the network by using nitrogen atoms as bridges, similar to boron and silicon. The coordinations of boron and silicon are quite similar to those in the final ceramic SiBN3C after pyrolysis at 1400 °C.

Local Structure around Pd Atoms in Pd<SUB>42.5</SUB>Ni<SUB>7.5</SUB>Cu<SUB>30</SUB>P<SUB>20</SUB> Excellent Glass-Former Studied by Anomalous X-ray Scattering

In order to study local structure around the Pd atoms in Pd42.5Ni7.5Cu30P20 excellent metallic glass-former, an anomalous X-ray scattering (AXS) experiment was performed at energies close to the Pd K absorption edge at the beamline BM02 of the European Synchrotron Radiation Facility. The differential structure factor, ΔPdS(Q), was obtained with a good statistical quality, which demonstrates that a pre-shoulder at about 20 nm−1, indicating the existence of an intermediate-range order, originates from the Pd-Pd atomic correlation. The first peak in the differential pair correlation function, ΔPdg(r), shows a longer inter-atomic length around the Pd atoms than the average value. The local structure around the Pd atoms is discussed in detail by comparing to the previous experiments of AXS and electronic structure.

Comparative study of ion conducting pathways in borate glasses

The conduction pathways in metal-halide doped silver, lithium, and sodium diborate glasses have been examined by bond valence analysis of reverse Monte Carlo (RMC) produced structural models of the glasses. Although all glass compositions have basically the same short-range structure of the boron-oxygen network, it is evident that the intermediate-range structure is strongly dependent on the type of mobile ion. The topography of the pathways and the coordination of the pathway sites differ distinctly between the three glass systems. The mobile silver ions in the AgI-doped glass tend to be mainly iodine-coordinated and travel in homogeneously distributed pathways located in salt-rich channels of the borate network. In the NaCl-doped glass, there is an inhomogeneous spatial distribution of pathways that reflects the inhomogeneous introduction of salt ions into the glass. However, since the salt clusters are not connected, no long-range conduction pathways are formed without including also oxygen-rich regions. The pathways in the LiCl-doped glass are slightly more evenly distributed compared to the NaCl-doped glass (but not as ordered as in the AgI-doped glass), and the regions of mainly oxygen-coordinated pathway sites are of higher importance for the long-range migration. In order to more accurately investigate how these differences in themore » intermediate-range order of the glasses affect the ionic conductivity, we have compared the realistic structure models to more or less randomized structures. An important conclusion from this comparison is that we find no evidence that a pronounced intermediate-range order in the atomic structure or in the network of conduction pathways, as in the AgI-doped glass, is beneficial for the dc conductivity.« less

Local dimensionality and intermediate range ordering of ion conduction pathways in borate glasses

Conduction pathways in metal halide doped silver, lithium and sodium diborate glasses have been examined by bond valence analysis of Reverse Monte Carlo (RMC) produced structural models of the glasses. Although all glass compositions have basically the same short-range structure of the boron–oxygen network it is evident that the intermediate-range structure is strongly dependent on the type of mobile ion. The differences are particularly large for the highly doped glasses, where the AgI salt is homogenously introduced to the glass causing an expansion of the B–O network with the formation of pronounced conduction pathways between neighboring borate segments. In contrast, the dopant salts LiCl and NaCl are considerably more inhomogeneously introduced forming microscopic clusters of salt ions, but also regions containing almost no salt ions. The network of the conduction pathways is directly related to the real (atomic) structure, and therefore reflects its features. The characterization of the conduction pathways is discussed in relation to the experimentally observed ionic conductivity.

Intermediate-range structure of amorphous GeSe 2 alloy

Neutron and high-energy X-ray diffraction measurements have been carried out at KENS-KEK and SPring-8, in order to investigate detailed structures of amorphous GeSe 2 prepared by the rapid quenching method. Considerable intermediate-range ordering is evident from the presence of a first sharp diffraction peak at low magnetides of the scattering vector. Well-defined first and second peaks are found at around 2.36 and 3.90 Å in the pair distribution function g( r). A small shoulder related to the existence of edge-sharing tetrahedral units is observed at a shorter r side of the second peak in g( r). Partial distribution functions, partial structure factors and bond-angle distributions can be obtained by Reverse Monte Carlo simulation. Results suggest that there are homo-polar bonds in tetrahedral structural units, and then the glass network is constructed by not only corner-sharing but also edge-sharing tetrahedra.

Adding a Length Scale to the Polyamorphic Ice Debate

X-ray scattering and molecular dynamics simulations have been used to correlate the short range oxygen-oxygen structure with the intermediate range ordering (IRO) upon annealing very high density amorphous ice. While it is clear that the IRO that defines the network structure breaks down continuously to a minimum level, where there are weakened correlations extending beyond 7 Angstrom, at this point the local structure (O-O-O angles) is observed to change abruptly, allowing a continuous reemergence of a new IRO network. This is very different from a classic first order transition and helps reconcile previous data.

Temperature dependence of the viscosity through the glass transition in metaphosphate glasses and polystyrene

We measured the temperature dependence of the viscosity of metaphosphate glasses and polystyrene in the liquid and glass states using a method developed by authors. The results show the Vogel–Tammann–Fulcher equation on the viscosity–temperature relation above the glass transition temperature T g and the Arrhenius one below T g. In order to explain this viscosity change, the intermediate range order (IRO) is introduced. The effect of composition on the viscosity of metaphosphate glass and that of thermal and mechanical treatments on the viscosity of polystyrene were measured. The kinetic (viscous) transition was discovered at a temperature lower by 20 K than T g determined by differential scanning calorimetry. We propose that the glass transition is a self-organization of the IRO of nanometer orders in size.

Compositional dependence of the first sharp diffraction peaks in alkali silicate glasses: A molecular dynamics study

The compositional dependence of the first sharp diffraction peaks (FSDPs) of lithium, sodium and potassium silicate glasses were studied by calculating the neutron static structure factors. The advantage of using molecular dynamics simulations is that the contributions of partial structure factors to the FSDPs can be easily determined and provides the basis for detailed analysis. Examination of the correlations of the FSDP with the short and medium range structure reveal that the position and the shape of FSDP strongly depend on the type and concentration of alkali oxide in alkali silicate glasses. The characteristic repeating distances and the characteristic correlation lengths both decrease with increasing alkali oxide concentration indicating a decrease in the intermediate range order. In the lithium silicate glasses, the characteristic correlation length increases with lithium oxide concentration that is anomalous in that the trend is opposite to the other alkali silicate glasses. This anomaly is explained by the high field strength of lithium ions that increases the intermediate range order of the silicon oxygen network.

On low-temperature polyamorphous transformations

A theory of polyamorphous transformations in glasses is constructed in the framework of a model of heterophase fluctuations with allowance for the fact that a glass inherits the short- and intermediate-range order from the liquid. A multicomponent order parameter describing the concentration of fluctuons with different types of short-range order is introduced, along with the concepts of isoconfigurational and non-isoconfigurational transitions in the glass. Taking the nonergodicity, nonequilibrium, and multiplicity of structural states of a glass into account leads to a kinetic criterion of observability of polyamorphism of a glass. As an example, a theory is constructed for the low-temperature first-order phase transition in an orientational glass based on doped fullerite. The relaxation processes of this system are described, including the subsystem of tunneling states. The possibility of a hierarchy of polyamorphous transformations in a glass is discussed.

Monte Carlo simulations of the NIMROD diffractometer

The near and intermediate range order diffractometer (NIMROD) has been selected as a day one instrument on the second target station at ISIS. Uniquely, NIMROD will provide continuous access to particle separations ranging from the interatomic ( < 1 Å ) to the mesoscopic ( < 300 Å ) . This instrument is mainly designed for structural investigations, although the possibility of putting a Fermi chopper (and corresponding NIMONIC chopper) in the incident beam line, will potentially allow the performance of low resolution inelastic scattering measurements. The performance characteristics of the TOF diffractometer have been simulated by means of a series of Monte Carlo calculations. In particular, the flux as a function of the transferred momentum Q as well as the resolution in Q and transferred energy have been estimated. Moreover, the possibility of including a honeycomb collimator in order to achieve better resolution has been tested. Here, we want to present the design of this diffractometer that will bridge the gap between wide- and small-angle neutron scattering experiments.

Structure and dynamics on superionic conducting phosphate glasses by neutron scattering

Neutron diffraction and inelastic scattering experiments have been performed on superionic conducting phosphate glasses, AgI–AgPO 3 and its undoped glass AgPO 3. In general, the static, elastic and dynamic structure factors S( Q), S( Q, ω=0), S( Q, ω), respectively, exhibit a prepeak at very low Q∼0.7 Å −1 related to the intermediate range order ∼10 Å, but not for the AgPO 3. The low-energy excitation, or Boson peak (BP), at around 2–3 meV appears in all undoped and doped glasses.The amplitude of the BP increases and the position shifts to lower energy with addition of AgI. The inelastic contribution can be expressed in terms of sound waves, Δ S( Q, ω) α Q 2 S( Q), which is consistent with an acoustic mode. The expansion of the phosphate network plays a dominant role on increasing the mobility of the Ag ions, the ionic conductivity, the prepeak and the BP intensities.

Material structure and metastability of hydrogenated nanocrystalline silicon solar cells

We find that the volume fraction of amorphous component in hydrogenated nanocrystalline silicon intrinsic layers is not necessarily the determining factor for the light-induced metastability ofn-i-p solar cells. Small grains and/or intermediate range order may play an important role in improving the stability. The distribution of nanocrystallites along the growth direction is also important. Based on the findings, we have optimized the hydrogen dilution profiling for controlling the structural evolution and have reduced the light-induced degradation of solar cells. As a result, we have achieved initial and stable active-area efficiencies of 14.1% and 13.2%, respectively, using ana-Si:H/nc-Si:H/nc-Si:H triple-junction structure.

Partial structure analysis of glassy As2Se3 using anomalous X-ray scattering

Anomalous X-ray scattering experiments have been performed on glassy As2Se3 at room temperature in order to clarify the role of each constituent for short- and intermediate-range order. The experiments were performed using intense X-rays from a third-generation synchrotron facility, ESRF. Differential structure factors, ΔiS(Q), close to the As and Se K edges were obtained from a detailed analysis. The prepeak around Q=12nm−1 in the total structure factor, S(Q), indicating evidence of intermediate-range order, is dominated by the As–As correlation around 12nm−1 and the Se–Se correlation around 14.5nm−1. The origin of the prepeak is carefully discussed.

Influence of growth temperature on order within silicon films grown by ultrahigh-vacuum evaporation on silica

We study the role that the growth temperature plays in determining the amount of order present within silicon films deposited on fused silica substrates through ultrahigh-vacuum evaporation at growth temperatures ranging from 98 to 572°C. Through measurements of the Raman and optical absorption spectra, we quantitatively determine how the growth temperature influences the order present within 11 such films. We employ three disparate measures of order for the purposes of this study: the breadth of the transverse-optic phonon Raman peak, this being related to the amount of short-range order present; the area under the transverse-acoustic Raman peak divided by the area under the corresponding transverse-optic peak, this being related to the amount of intermediate-range order present; and the breadth of the optical absorption tail, which is a general measure of the overall amount of order present. All three measures of order indicate a dramatic increase in the amount of order present for growth temperatures above 400°C.

Intermediate-range order in molten network-forming systems

The structural origin of intermediate-range order in two archetypal network-forming liquids, $\mathrm{Ge}{\mathrm{Se}}_{2}$ and $\mathrm{Zn}{\mathrm{Cl}}_{2}$, is investigated via molecular dynamics computer simulation. Relatively simple ionic models, in which a proper description of ion polarization effectively controls the network connectivity, are shown to be capable of showing a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration structure factor. This feature is shown to arise from the presence of percolating edge-sharing polyhedral units which act to disrupt a corner-sharing network and introduce addition cation density fluctuations on both short- and intermediate-range length scales.