68 research outputs found
Deciphering diffuse scattering with machine learning and the equivariant foundation model: The case of molten FeO
Bridging the gap between diffuse x-ray or neutron scattering measurements and
predicted structures derived from atom-atom pair potentials in disordered
materials, has been a longstanding challenge in condensed matter physics. This
perspective gives a brief overview of the traditional approaches employed over
the past several decades. Namely, the use of approximate interatomic pair
potentials that relate 3-dimensional structural models to the measured
structure factor and its associated pair distribution function. The use of
machine learned interatomic potentials has grown in the past few years, and has
been particularly successful in the cases of ionic and oxide systems. Recent
advances in large scale sampling, along with a direct integration of scattering
measurements into the model development, has provided improved agreement
between experiments and large-scale models calculated with quantum mechanical
accuracy. However, details of local polyhedral bonding and connectivity in
meta-stable disordered systems still require improvement. Here we leverage
MACE-MP-0; a newly introduced equivariant foundation model and validate the
results against high-quality experimental scattering data for the case of
molten iron(II) oxide (FeO). These preliminary results suggest that the
emerging foundation model has the potential to surpass the traditional
limitations of classical interatomic potentials.Comment: 9 pages, 5 figure
Structure of lanthanum and cerium phosphate glasses by the method of isomorphic substitution in neutron diffraction
Neutron diffraction was used to measure the total structure factors for several rare-earth ion R3+ (La3+ or Ce3+) phosphate glasses with composition close to RAl0.35P3.24O10.12. By assuming isomorphic structures, difference function methods were employed to separate, essentially, those correlations involving R3+ from the remainder. A self-consistent model of the glass structure was thereby developed in which the Al correlations were taken into explicit account. The glass network was found to be made from interlinked PO4 tetrahedra having 2.2(1) terminal oxygen atoms, OT, at 1.51(1) Angstrom, and 1.8(1) bridging oxygen atoms, OB, at 1.60(1) Angstrom. Rare-earth cations bonded to an average of 7.5(2) OT nearest neighbors in a broad and asymmetric distribution. The Al3+ ion acted as a network modifier and formed OT-A1-OT linkages that helped strengthen the glass. The connectivity of the R-centered coordination polyhedra was quantified in terms of a parameter f(s) and used to develop a model for the dependence on composition of the A1-OT coordination number in R-A1-P-O glasses. By using recent 17 A1 nuclear-magnetic-resonance data, it was shown that this connectivity decreases monotonically with increasing Al content. The chemical durability of the glasses appeared to be at a maximum when the connectivity of the R-centered coordination polyhedra was at a minimum. The relation of f(s) to the glass transition temperature, Tg, was discussed
Exploring the Structure of High Temperature, Iron-bearing Liquids
This paper describes the direct measurements of the structure of iron-bearing liquids using a combination of containerless techniques and in-situ high energy x-ray diffraction.These capabilities provide data that is important to help model and optimize processes such as smelting, steel making, and controlling slag chemistry. A successful programme of liquid studies has been undertaken and the Advanced Photon Source using these combined techniques which include the provision of gas mixing and the control of pO2and the changing influence of mixed valance elements. It is possible to combine rapid image acquisition with quenching of liquids to obtain the full diffraction patterns of deeply supercooled liquids and the metastable supercooled liquid regime, where the liquid structures and viscosity change most dramatically, can also be explored
The Structure of Liquid and Amorphous Hafnia.
Understanding the atomic structure of amorphous solids is important in predicting and tuning their macroscopic behavior. Here, we use a combination of high-energy X-ray diffraction, neutron diffraction, and molecular dynamics simulations to benchmark the atomic interactions in the high temperature stable liquid and low-density amorphous solid states of hafnia. The diffraction results reveal an average Hf-O coordination number of ~7 exists in both the liquid and amorphous nanoparticle forms studied. The measured pair distribution functions are compared to those generated from several simulation models in the literature. We have also performed ab initio and classical molecular dynamics simulations that show density has a strong effect on the polyhedral connectivity. The liquid shows a broad distribution of Hf-Hf interactions, while the formation of low-density amorphous nanoclusters can reproduce the sharp split peak in the Hf-Hf partial pair distribution function observed in experiment. The agglomeration of amorphous nanoparticles condensed from the gas phase is associated with the formation of both edge-sharing and corner-sharing HfO6,7 polyhedra resembling that observed in the monoclinic phase
The First Direct Detection of Kirkwood Transitions in Concentrated Aqueous Electrolytes using Small Angle X-ray Scattering
Ion-ion correlations, screening, and equilibrium bulk structure in various
concentrated electrolytes are investigated using synchrotron small angle X-ray
scattering (SAXS), theory, and molecular simulation. Utilizing SAXS
measurements we provide estimates of the Kirkwood Transition (KT) for a variety
of aqueous electrolytes (NaCl, CaCl, SrCl, and ErCl). The KT may be
defined as the concentration above which the ion-ion correlations cease to
decay exponentially with a single length scale given by the Debye length
and develop an additional length scale, that
reflects the formation of local domains of charge. Theoretical models of the KT
have been known for decades for highly idealized models of electrolytes, but
experimental verification of KT in real electrolytes has yet to be confirmed.
Herein, we provide consistent theoretical and experimental estimates of both
the inverse screening lengths and inverse domain size, for the
aforementioned electrolyte systems. Taken together, and are known
descriptors of the KT and provide a view into the complexity of ion-ion
interaction beyond the well-accepted Debye-H\"{u}ckel limit. Our findings
suggest a picture of interaction for real electrolytes that is more general
than that found in idealized models that is manifest in the precise form of the
non-local response function that we estimate through the interpretation of the
experimental SAXS signal. Importantly, the additional complexity of describing
ion-ion interaction of real electrolytes will implicate the short-range ion-ion
interactions that can only be computed via molecular simulation and provide a
quantitative approach to describe electrolyte phenomena beyond Debye-H\"{u}ckel
theory.Comment: 3
The local ordering of polar solvents around crystalline carbon nitride nanosheets in solution
The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60–125 nm in diameter and Li+ and Br− ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4–6 Å
−1
range, with real space differences persisting to at least 10 Ã…. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br
−
close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution.
This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'
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