Networks under pressure: the development of in situ high-pressure neutron diffraction for glassy and liquid materials

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.

Recovering local structure information from high-pressure total scattering experiments

High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. High-pressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressure-transmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydro-static compression. The method applies a molecular-dynamics-informed empirical correction and a non-negative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected high-pressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of α-quartz under hydro-static pressure is presented, extracting compression behaviour of the real-space structure.

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.

Liquid structure under extreme conditions: high-pressure x-ray diffraction studies

Under extreme conditions of high pressure and temperature, liquids can undergo substantial structural transformations as their atoms rearrange to minimise energy within a more confined volume. Understanding the structural response of liquids under extreme conditions is important across a variety of disciplines, from fundamental physics and exotic chemistry to materials and planetary science.In situexperiments and atomistic simulations can provide crucial insight into the nature of liquid-liquid phase transitions and the complex phase diagrams and melting relations of high-pressure materials. Structural changes in natural magmas at the high-pressures experienced in deep planetary interiors can have a profound impact on their physical properties, knowledge of which is important to inform geochemical models of magmatic processes. Generating the extreme conditions required to melt samples at high-pressure, whilst simultaneously measuring their liquid structure, is a considerable challenge. The measurement, analysis, and interpretation of structural data is further complicated by the inherent disordered nature of liquids at the atomic-scale. However, recent advances in high-pressure technology mean that liquid diffraction measurements are becoming more routinely feasible at synchrotron facilities around the world. This topical review examines methods for high pressure synchrotron x-ray diffraction of liquids and the wide variety of systems which have been studied by them, from simple liquid metals and their remarkable complex behaviour at high-pressure, to molecular-polymeric liquid-liquid transitions in pnicogen and chalcogen liquids, and density-driven structural transformations in water and silicate melts.

In situ high pressure neutron diffraction and Raman spectroscopy of 20BaO-80TeO2 glass

The short-range structure of 20BaO–80TeO₂ glass was studied in situ by high pressure neutron diffraction and high pressure Raman spectroscopy. Neutron diffraction measurements were performed at the PEARL instrument of the ISIS spallation neutron source up to a maximum pressure of 9.0 ± 0.5 GPa. The diffraction data was analysed via reverse Monte Carlo simulations and the changes in the glass short-range structural properties, Ba–O, Te–O and O–O bond lengths and speciation were studied as a function of pressure. Te–O co-ordination increases from 3.51 ± 0.05 to 3.73 ± 0.05, Ba–O coordination from 6.24 ± 0.19 to 6.99 ± 0.34 and O–O coordination from 6.00 ± 0.05 to 6.69 ± 0.06 with an increase in pressure from ambient to 9.0 GPa. In situ high pressure Raman studies found that the ratio of intensities of the two bands at 668 cm⁻¹ and 724 cm⁻¹ increases from 0.99 to 1.18 on applying pressure up to 19.28 ± 0.01 GPa, and that these changes are due to the conversion of TeO₃ into TeO₄ structural units in the tellurite network. It is found that pressure causes densification of the tellurite network by the enhancement of co-ordination of cations, and an increase in distribution of Te–O and Ba–O bond lengths. The original glass structure is restored upon the release of pressure.

The short-range structure of 20BaO–80TeO₂ glass was studied in situ by high pressure neutron diffraction and high pressure Raman spectroscopy.

Domain Structural Transition and Structural Heterogeneity in GeO2 Glass Under Densification

The domain structural transition and structural heterogeneity (SH) in GeO₂ glass at 300 K and pressures up to 100 GPa are studied by means of molecular dynamics (MD) simulation. The results demonstrate that the structure of GeO₂ glass comprises domain D4, domain D5, or domain D6, which depends strongly on pressure, where domain Dx (x = 4, 5, or 6) is a cluster of connected GeO _x units, in which all Ge atoms possess the same coordination number of x. In the range of 9-18 GPa, GeO₂ glass undergoes a structural transformation from domain D4 to domain D6 via domain D5. Under densification, structural evolution occurs along with the O _xx → O _xy atom variation, which comprises the processes of both merging and splitting of domain Dx and the exchange of domain-boundary (DB) atoms. The densification leads to a decrease of the Voronoi polygon (VP) volume of atoms. We found that the coexistence of separate domain structures is the origin of spatial SH in GeO₂ glass. Pressure-dependent structural heterogeneity in GeO₂ glass is also discussed in detail.

Dimeric Molecular Structure of Molten Gallium Trichloride and a Hidden Evolution toward a Possible Liquid-Liquid Transition

Group 13 trihalides MY₃ (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric M₂Y₆ molecular structure in the solid state and a mixture of M₂Y₆ dimers and MY₃ monomers in the vapor phase are potential candidates for entropy-driven liquid-liquid transition M₂Y₆ ⇄ 2MY₃ at elevated temperatures. Using pulsed neutron diffraction and high-energy X-ray scattering supported by structural modeling, we show a dimer molecular structure of liquid GaCl₃ above the melting point at 351 K and midway between the boiling point (474 K) and the critical temperature (694 K) with almost hidden characteristic evolution toward a possible liquid-liquid transition. In contrast to edge-sharing (ES) dimers in solid and vapor of D_2h symmetry, the ES Ga₂Cl₆ molecules in the melt have a puckered structure of the central four-membered ring with shorter Cl-Cl (2.90-3.09 Å) and longer Ga-Ga (3.20-3.26 Å) second-neighbor correlations. The elongation of Ga-Ga intramolecular distances with increasing temperature simultaneously with diminished Cl-Cl nearest neighbor contacts destabilizes the ES dimers, indicating the first step toward dimer dissociation.

In-situ Raman spectroscopic measurements of the deformation region in indented glasses

This paper describes the design and integration of a custom-built optical instrument for in-situ Raman microscopy suitable for collecting high-quality spectroscopic data during the indentation of glass materials. It will further show that the reported experimental setup enables meaningful in-situ spectroscopic observations during indentation of fused silica at forces in the millinewton range. The aim of the paper is to demonstrate the vital importance of matching the analysis volume of the Raman microscope with the indentation-induced deformation volume to capture the full extent of the related spectral alterations by minimizing spectral contributions from the unperturbed bulk material (in-situ and ex-situ) and indenter probe (in-situ only). In this context, the paper will also touch upon possible pitfalls in ex-situ and in-situ Raman measurements on indented glasses in cases where the analysis and deformation volumes are not well matched and describe the misinterpretations that may result.

The high pressure gas capabilities at Oak Ridge National Laboratory's neutron facilities

The study of samples subjected to high pressure gas is an important asset in materials research and has consequently been a priority of the sample environment development at the Oak Ridge National Laboratory's (ORNL) neutron program. Such effort has resulted in the availability of an extensive combination of pressure cells and gas intensifiers (both commercially available and custom made). These resources are available across both neutron facilities at ORNL: the Spallation Neutron Source and the High Flux Isotope Reactor. Current capabilities include, for example, in situ measurements up to 6 kbar and a 3 kbar hydrogen-capable intensifier with a gas recovery feature. In this communication, we will review the existing suite of high pressure gas capabilities, with special emphasis on recent in-house developments. A number of examples will be presented to illustrate how such capabilities are being deployed on neutron beamlines to enable frontier science.

Amorphous boron oxide at megabar pressures via inelastic X-ray scattering

Structural transition in amorphous oxides, including glasses, under extreme compression above megabar pressures (>1 million atmospheric pressure, 100 GPa) results in unique densification paths that differ from those in crystals. Experimentally verifying the atomistic origins of such densifications beyond 100 GPa remains unknown. Progress in inelastic X-ray scattering (IXS) provided insights into the pressure-induced bonding changes in oxide glasses; however, IXS has a signal intensity several orders of magnitude smaller than that of elastic X-rays, posing challenges for probing glass structures above 100 GPa near the Earth's core-mantle boundary. Here, we report megabar IXS spectra for prototypical B₂O₃ glasses at high pressure up to ∼120 GPa, where it is found that only four-coordinated boron (^[4]B) is prevalent. The reduction in the ^[4]B-O length up to 120 GPa is minor, indicating the extended stability of sp³-bonded ^[4]B. In contrast, a substantial decrease in the average O-O distance upon compression is revealed, suggesting that the densification in B₂O₃ glasses is primarily due to O-O distance reduction without the formation of ^[5]B. Together with earlier results with other archetypal oxide glasses, such as SiO₂ and GeO₂, the current results confirm that the transition pressure of the formation of highly coordinated framework cations systematically increases with the decreasing atomic radius of the cations. These observations highlight a new opportunity to study the structure of oxide glass above megabar pressures, yielding the atomistic origins of densification in melts at the Earth's core-mantle boundary.

Revisiting local structural changes in GeO2 glass at high pressure

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO₂ glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a marked decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number  >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO₂ glass.

Neutron total scattering of crystalline materials in the gigapascal regime

Neutron total scattering of disordered crystalline materials provides direct experimental access to the local (short-range) structure. The ways in which this local structure agrees (or disagrees) with the long-range crystal structure can provide important insight into structure–property relationships. High-pressure neutron diffraction using a Paris–Edinburgh (P–E) pressure cell allows experimenters to explore the ways in which materials are affected by pressure, can reveal new synthetic routes to novel functional materials and has important applications in many areas, including geology, engineering and planetary science. However, the combination of these two experimental techniques poses unique challenges for both data collection and analysis. In this paper it is shown that, with only minor modifications to the standard P–E press setup, high-quality total scattering data can be obtained from crystalline materials in the gigapascal pressure regime on the PEARL diffractometer at ISIS. The quality of the data is assessed through the calculation of coordination numbers and the use of reverse Monte Carlo refinements. The time required to collect data of sufficient quality for detailed analysis is assessed and is found to be of the order of 8 h for a quartz sample. Finally, data from the perovskite LaCo0.35Mn0.65O3 are presented and reveal that PEARL total scattering data offer the potential of extracting local structural information from complex materials at high pressure.

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.

Universal amorphous-amorphous transition in GexSe100-x glasses under pressure

Pressure induced structural modifications in vitreous Ge_xSe_100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa. The Ge-Se interatomic distances extracted from XAS data show a two-step response to the applied pressure; a gradual decrease followed by an increase at around 15–20 GPa, depending on the composition. This increase is attributed to the metallization event that can be traced with the red shift in Ge K edge energy which is also identified by the principal peak position of the structure factor. The densification mechanisms are studied in details by means of AIMD simulations and compared to the experimental results. The evolution of bond angle distributions, interatomic distances and coordination numbers are examined and lead to similar pressure-induced structural changes for any composition.

Direct Volumetric Study of High-Pressure Driven Polyamorphism and Relaxation in the Glassy Germanium Chalcogenides

High precision measurements were taken of the specific volume of glassy germanium chalcogenides GeSe2, GeS2, Ge17Se83, and Ge8Se92 under hydrostatic pressure to 8.5 GPa. For GeSe2 and GeS2 glasses in the pressure range to 3 GPa the behavior is an elastic one with bulk modulus softening at pressures above 2 GPa. At higher pressures the relaxation processes begin that have logarithmic kinetics. The relaxation rate for GeSe2 glasses has a clearly pronounced maximum at 3.5-4.5 GPa, which is indicative of the existence of several mechanisms of structural transformations. For nonstoichiometric glasses inelastic behavior is observed at pressures above 1-1.5 GPa, the relaxation rate being much less than that for stoichiometric ones. For all the glasses we observe the "loss of memory" about the prehistory: A pressure rising after relaxation causes the return of values of the specific volume to the curve of compression without relaxation. After depressurization the residual densification makes up nearly 7% in stoichiometric glasses and 1.5% in Ge17Se83 glasses. The values of the effective bulk modulus for nonstoichiometric glasses coincide upon pressure lowering with the values after isobaric relaxations during pressure increase, whereas for GeSe2 the moduli during the decompression exceed substantially the values after isobaric relaxations at compression path. The results obtained demonstrate high capacity of the volumetric measurements to reveal the nature of the transformations in glassy germanium chalcogenides under compression.