Development of chemical and topological structure in aluminosilicate liquids and glasses at high pressure

Transformations to the aluminum coordination environment and network polymerization in amorphous aluminosilicates under pressure

The structure of calcium aluminosilicate glasses (CaO)x(Al2O3)y(SiO2)1-x-y with the near tectosilicate compositions x ≃ 0.19 and 1 - x - y ≃ 0.61 or x ≃ 0.26 and 1 - x - y ≃ 0.49 was investigated by in situ high-pressure neutron diffraction and 27Al nuclear magnetic resonance (NMR) spectroscopy. The results show three distinct pressure regimes for the transformation of the aluminum coordination environment from tetrahedral to octahedral, which map onto the deformations observed in the production of permanently densified materials. The oxygen packing fraction serves as a marker for signaling a change to the coordination number of the network forming motifs. For a wide variety of permanently densified aluminosilicates, the aluminum speciation shares a common dependence on the reduced density ρ' = ρ/ρ0, where ρ is the density and ρ0 is its value for the uncompressed material. The observed increase in the Al-O coordination number with ρ' originates primarily from the formation of six-coordinated aluminum Al(VI) species, the fraction of which increases rapidly beyond a threshold ρthr'∼ 1.1. The findings are combined to produce a self-consistent model for pressure-induced structural change. Provided the glass network is depolymerized, one-coordinated non-bridging oxygen atoms are consumed to produce two-coordinated bridging oxygen atoms, thus increasing the network connectivity in accordance with the results from 17O NMR experiments. Otherwise, three-coordinated oxygen atoms or triclusters appear, and their fraction is quantified by reference to the mean coordination number of the silicon plus aluminum species. The impact of treating Al(VI) as a network modifier is discussed.

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)'.

Coordination Changes in Densified Aluminate Glass upon Compression up to 65 GPa: A View from Solid-State Nuclear Magnetic Resonance

Deciphering the structural evolution in irreversibly densified oxide glasses is crucial for fabricating functional glasses with tunable properties and elucidating the nature of pressure-induced anomalous plastic deformation in glasses. High-resolution NMR spectroscopy quantifies atomic-level structural information on densified glasses; however, its application is limited to the low-pressure range due to technical challenges. Here, we report the first high-resolution NMR spectra of oxide glass compressed by diamond anvil cells at room temperature, extending the pressure record of such studies from 24 to 65 GPa. The results constrain the densification path through coordination transformation of Al cations. Based on a statistical thermodynamic model, the stepwise changes in the Al fractions of oxide glasses and the effects of network polymerization on the densification paths are quantified. These results extend the knowledge on densification of the previously unattainable pressure conditions and contribute to understanding the origin of mechanical strengthening of the glasses.

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.

Laser-induced structural modification in calcium aluminosilicate glasses using molecular dynamic simulations

Glass structures of multicomponent oxide systems (CaO–Al₂O₃–SiO₂) are studied using a simulated pulsed laser with molecular dynamics. The short- and intermediate-range order structures revealed a direct correlation between the transformation of Al^(IV) to Al^(V), regions of increased density following laser processing, inherent reduction in the average T–O–T (T = Al, Si) angle, and associated elongation of the T–O bonding distance. Variable laser pulse energies were simulated across calcium aluminosilicate glasses with high silica content (50–80%) to identify densification trends attributed to composition and laser energy. High-intensity pulsed laser effects on fictive temperature and shockwave promotion are discussed in detail for their role in glass densification. Laser-induced structural changes are found to be highly dependent on pulse energy and glass chemistry.

Structural dynamics of basaltic melt at mantle conditions with implications for magma oceans and superplumes

Transport properties like diffusivity and viscosity of melts dictated the evolution of the Earth’s early magma oceans. We report the structure, density, diffusivity, electrical conductivity and viscosity of a model basaltic (Ca₁₁Mg₇Al₈Si₂₂O₇₄) melt from first-principles molecular dynamics calculations at temperatures of 2200 K (0 to 82 GPa) and 3000 K (40–70 GPa). A key finding is that, although the density and coordination numbers around Si and Al increase with pressure, the Si–O and Al–O bonds become more ionic and weaker. The temporal atomic interactions at high pressure are fluxional and fragile, making the atoms more mobile and reversing the trend in transport properties at pressures near 50 GPa. The reversed melt viscosity under lower mantle conditions allows new constraints on the timescales of the early Earth’s magma oceans and also provides the first tantalizing explanation for the horizontal deflections of superplumes at ~1000 km below the Earth’s surface.

Transport properties of melts in the deep Earth have dictated the evolution of the early Earth’s magma oceans and also govern many modern dynamic processes, such as plate tectonics. Here, the authors find there is a reversal in the trends of transport properties of basaltic melts at pressures near 50 GPa, with implications for the timescales of early Earth’s magma oceans.

A Practical Review of the Laser-Heated Diamond Anvil Cell for University Laboratories and Synchrotron Applications

In the past couple of decades, the laser-heated diamond anvil cell (combined with in situ techniques) has become an extensively used tool for studying pressure-temperature-induced evolution of various physical (and chemical) properties of materials. In this review, the general challenges associated with the use of the laser-heated diamond anvil cells are discussed together with the recent progress in the use of this tool combined with synchrotron X-ray diffraction and absorption spectroscopy.

Structural Ordering in Liquid Gallium under Extreme Conditions

The atomic-scale structure, melting curve, and equation of state of liquid gallium has been measured to high pressure (p) and high temperature (T) up to 26 GPa and 900 K by in situ synchrotron x-ray diffraction. Ab initio molecular dynamics simulations up to 33.4 GPa and 1000 K are in excellent agreement with the experimental measurements, providing detailed insight at the level of pair distribution functions. The results reveal an absence of dimeric bonding in the liquid state and a continuous increase in average coordination number n[over ¯]_{Ga}^{Ga} from 10.4(2) at 0.1 GPa approaching ∼12 by 25 GPa. Topological cluster analysis of the simulation trajectories finds increasing fractions of fivefold symmetric and crystalline motifs at high p-T. Although the liquid progressively resembles a hard-sphere structure towards the melting curve, the deviation from this simple description remains large (≥40%) across all p-T space, with specific motifs of different geometries strongly correlating with low local two-body excess entropy at high p-T.

The HXD95: a modified Bassett-type hydrothermal diamond-anvil cell for in situ XRD experiments up to 5 GPa and 1300 K

A new diamond-anvil cell apparatus for in situ synchrotron X-ray diffraction measurements of liquids and glasses, at pressures from ambient to 5 GPa and temperatures from ambient to 1300 K, is reported. This portable setup enables in situ monitoring of the melting of complex compounds and the determination of the structure and properties of melts under moderately high pressure and high temperature conditions relevant to industrial processes and magmatic processes in the Earth's crust and shallow mantle. The device was constructed according to a modified Bassett-type hydrothermal diamond-anvil cell design with a large angular opening (θ = 95°). This paper reports the successful application of this device to record in situ synchrotron X-ray diffraction of liquid Ga and synthetic PbSiO3 glass to 1100 K and 3 GPa.

CO3+1 network formation in ultra-high pressure carbonate liquids

Carbonate liquids are an important class of molten salts, not just for industrial applications, but also in geological processes. Carbonates are generally expected to be simple liquids, in terms of ionic interactions between the molecular carbonate anions and metal cations, and therefore relatively structureless compared to more “polymerized” silicate melts. But there is increasing evidence from phase relations, metal solubility, glass spectroscopy and simulations to suggest the emergence of carbonate “networks” at length scales longer than the component molecular anions. The stability of these emergent structures are known to be sensitive to temperature, but are also predicted to be favoured by pressure. This is important as a recent study suggests that subducted surface carbonate may melt near the Earth’s transition zone (~44 km), representing a barrier to the deep carbon cycle depending on the buoyancy and viscosity of these liquids. In this study we demonstrate a major advance in our understanding of carbonate liquids by combining simulations and high pressure measurements on a carbonate glass, (K₂CO₃-MgCO₃) to pressures in excess of 40 GPa, far higher than any previous in situ study. We show the clear formation of extended low-dimensional carbonate networks of close CO₃²⁻ pairs and the emergence of a “three plus one” local coordination environment, producing an unexpected increase in viscosity with pressure. Although carbonate melts may still be buoyant in the lower mantle, an increased viscosity by at least three orders of magnitude will restrict the upward mobility, possibly resulting in entrainment by the down-going slab.