Diamond Anvil Cell Experiments Research Articles

Raman frequencies of diamond under non-hydrostatic pressure

The Raman frequencies of diamond subjected to non-hydrostatic uniaxial stress along the [001] and [111] crystallographic axes have been calculated with density-functional-theory methods and the results fitted to a simple analytical form. The data are analyzed in the context of the recently proposed use of the shift of the high-frequency edge of the Raman band of diamond as a pressure scale in diamond-anvil cell experiments. Combining theoretical and experimental data, we are able to determine the stress state of the diamond anvil in ultra-high-pressure experiments. We find that shear stresses close to the tip of the anvil can reach values exceeding 1 Mbar.

High-pressure thermal conductivity and compressional velocity of NaCl in B1 and B2 phase

Sodium chloride (NaCl) is an important, commonly used pressure medium and pressure calibrant in diamond-anvil cell (DAC) experiments. Its thermal conductivity at high pressure–temperature (P–T) conditions is a critical parameter to model heat conduction and temperature distribution within an NaCl-loaded DAC. Here we couple ultrafast optical pump-probe methods with the DAC to study thermal conductivity and compressional velocity of NaCl in B1 and B2 phase to 66 GPa at room temperature. Using an externally-heated DAC, we further show that thermal conductivity of NaCl-B1 phase follows a typical T−1 dependence. The high P–T thermal conductivity of NaCl enables us to confirm the validity of Leibfried-Schlömann equation, a commonly used model for the P–T dependence of thermal conductivity, over a large compression range (~ 35% volume compression in NaCl-B1 phase, followed by ~ 20% compression in the polymorphic B2 phase). The compressional velocities of NaCl-B1 and B2 phase both scale approximately linearly with density, indicating the applicability of Birch’s law to NaCl within the density range we study. Our findings offer critical insights into the dominant physical mechanism of phonon transport in NaCl, as well as important data that significantly enhance the accuracy of modeling the spatiotemporal evolution of temperature within an NaCl-loaded DAC.

In situ high-pressure nuclear magnetic resonance crystallography in one and two dimensions

Recent developments in in situ nuclear magnetic resonance (NMR) spectroscopy under extreme conditions have led to the observation of a wide variety of physical phenomena that are not accessible with standard high-pressure experimental probes. However, inherent di- or quadrupolar line broadening in diamond anvil cell (DAC)-based NMR experiments often limits detailed investigation of local atomic structures, especially if different phases or local environments coexist. Here, we describe our progress in the development of high-resolution NMR experiments in DACs using one- and two-dimensional homonuclear decoupling experiments at pressures up to the megabar regime. Using this technique, spectral resolutions of the order of 1 ppm and below have been achieved, enabling high-pressure structural analysis. Several examples are presented that demonstrate the wide applicability of this method for extreme conditions research.

Cinema:Snap: Real-time tools for analysis of dynamic diamond anvil cell experiment data.

We developed tools and a workflow for real-time analysis of data from dynamic diamond anvil cell experiments performed at user light sources. These tools allow users to determine the phases of matter observed during the compression of materials in order to make decisions during an experiment to improve the quality of experimental results and maximize the use of scarce experimental facility time. The tools fill a gap in dynamic compression data analysis tools that are real-time, are flexible to the needs of high-pressure scientists, connect to automated processing of results, can be easily incorporated into workflows with existing tools and data formats, and support remote experimental data analysis workflows. Specific analytics developed include novel automated two-peak analysis for overlapping peaks and multiple phases, coordinated views of pressure and temperature values, full-compression contour plots, and configurable views of integrated x-ray diffraction. We present an experimental use case to show how the tools produce real-time analytics that help the scientists revise parameters for the next compression.

Experimental thermal equation of state of B2−KCl

The alkali halides are often used as optically-transparent pressure-transmitting media and thermal insulators in laser-heated diamond anvil cell experiments. High P--T equations of state for these materials would allow them to be used simultaneously as sensitive in situ pressure markers, making sample preparation and data analysis simpler. KCl is especially useful for this application because its high melting point, crystallographic simplicity, and low electron density are ideal for use in high-temperature x-ray diffraction experiments. However, the high-temperature static equation of state data for this material is limited in pressure to 8 GPa. As experiments routinely exceed 100 GPa in pressure and thousands of Kelvin, it is necessary for the equation of state of KCl to be determined experimentally at these conditions if it is to be used as an equation of state calibrant in the future. This paper combines new high-pressure, high-temperature data of the B2 phase of KCl (up to 167 GPa and 2400 K) with the previously published room-temperature data of Dewaele et al. [Phys. Rev. B 85, 214105 (2012)] to produce a Mie-Gr\"uneisen-Debye thermal equation of state for this material. The room-temperature equation of state parameters are similar to those reported previously: ${V}_{0}=32.0(3) {\mathrm{cm}}^{3}/\mathrm{mole}, {K}_{0}=24(1)$ GPa, and ${K}_{0}^{\ensuremath{'}}=4.56(5)$. The thermal parameters, ${\ensuremath{\gamma}}_{0}$ and $q$ are 2.9(4) and 1.0(1), respectively. While a $q$ of 1 is expected, the ${\ensuremath{\gamma}}_{0}$ is higher than expected from previous calculations, lower pressure experimental data, and the available shockwave data. Thus, previously-reported equations of state underestimate the pressure of KCl at high temperatures.

Melting of Tantalum at Multimegabar Pressures on the Nanosecond Timescale

Tantalum was once thought to be the canonical bcc metal, but is now predicted to transition to the Pnma phase at the high pressures and temperatures expected along the principal Hugoniot. Furthermore, there remains a significant discrepancy between a number of static diamond anvil cell experiments and gas gun experiments in the measured melt temperatures at high pressures. Our insitu x-ray diffraction experiments on shock compressed tantalum show that it does not transition to the Pnma phase or other candidate phases at high pressure. We observe incipient melting at approximately 254±15 GPa and complete melting by 317±10 GPa. These transition pressures from the nanosecond experiments presented here are consistent with what can be inferred from microsecond gas gun sound velocity measurements. Furthermore, the observation of a coexistence region on the Hugoniot implies the lack of significant kinetically controlled deviation from equilibrium behavior. Consequently, we find that kinetics of phase transitions cannot be used to explain the discrepancy between static and dynamic measurements of the tantalum melt curve. Using available high pressure thermodynamic data for tantalum and our measurements of the incipient and complete melting transition pressures, we are able to infer a melting temperature 8070_{-750}^{+1250} K at 254±15 GPa, which is consistent with ambient and a recent static high pressure melt curve measurement.

High‐Pressure Phase Stability and Thermoelastic Properties of Iron Carbonitrides and Nitrogen in the Deep Earth

AbstractIron‐dominant metallic phases are likely the primary hosts for nitrogen in the reduced deep Earth, hence the storage of nitrogen in the lower mantle and the core is governed by the behavior of the Fe‐N‐C system at high temperatures and pressures. In this study, phase transitions and thermoelastic properties of iron carbonitrides were investigated at high pressure‐temperature conditions by diamond anvil cell experiments and first‐principles calculations. Experimental data revealed no phase transition in ε‐type Fe4 (N0.6C0.4) or Fe7 (N0.75C0.25)3 up to 60 GPa at room temperature. At high temperature, Fe7 (N0.75C0.25)3 transforms into the Fe3C‐type phase at ∼27 GPa, and then into the Fe7C3‐type phase at ∼45 GPa, which is also corroborated by our theoretical calculations. We found that the phase stability of iron carbonitrides mainly depends on the N/C ratio, and the elastic properties of iron carbonitrides are dominantly affected by the Fe/(N+C) ratio. Iron carbonitrides with diverse structures may be the main host for nitrogen in the deep mantle. Some iron carbonitride inclusions in lower mantle diamonds could be the residue of the primordial mantle or originate from subducted nitrogen‐bearing materials, rather than iron‐enriched phases of the outer core. In addition, our experiments confirmed the existence of Fe7C3‐type Fe7C3‐Fe7N3 solid solutions above 40 GPa. Fe7C3‐type Fe7(C, N)3 has comparable density and thermoelastic properties to its isostructural endmembers and may be a promising candidate constituent of the Earth's inner core.

Reconciling metal\u2013silicate partitioning and late accretion in the Earth

Highly siderophile elements (HSE), including platinum, provide powerful geochemical tools for studying planet formation. Late accretion of chondritic components to Earth after core formation has been invoked as the main source of mantle HSE. However, core formation could also have contributed to the mantle’s HSE content. Here we present measurements of platinum metal-silicate partitioning coefficients, obtained from laser-heated diamond anvil cell experiments, which demonstrate that platinum partitioning into metal is lower at high pressures and temperatures. Consequently, the mantle was likely enriched in platinum immediately following core-mantle differentiation. Core formation models that incorporate these results and simultaneously account for collateral geochemical constraints, lead to excess platinum in the mantle. A subsequent process such as iron exsolution or sulfide segregation is therefore required to remove excess platinum and to explain the mantle’s modern HSE signature. A vestige of this platinum-enriched mantle can potentially account for 186Os-enriched ocean island basalt lavas.

Calibration of Manganin pressure gauge for diamond-anvil cells

Pressure calibration for most diamond-anvil cell (DAC) experiments is mainly based on the ruby scale, which is key to implementing this powerful tool for high-pressure study. However, the ruby scale can often hardly be used for programmably controlled DAC devices, especially the piezoelectric-driving cells, where a continuous pressure calibration is required. In this work, we present an effective pressure gauge for DACs made of Manganin metal based on the four-probe resistivity measurements. Pressure dependence of its resistivity is well established and shows excellent linear relations in the 0-30GPa pressure range with a slope of 23.4 (9) GPa for the first-cycle compression, in contrast to that of multiple-cycle compression and decompression having a nearly identical slope of 33.7 (4) GPa likely due to the strain effect. In addition, the such-established Manganin scale can be used for continuously monitoring the cell pressure of piezoelectric-driving DACs, and the reliability of this method is also verified by the fixed-point method with a Bi pressure standard. Realization of continuous pressure calibration for programmably controlled DACs would offer many opportunities for the study of dynamics, kinetics, and critical behaviors of pressure-induced phase transitions.

Bulk modulus of H2O across the ice VII–ice X transition measured by time-resolved x-ray diffraction in dynamic diamond anvil cell experiments

We have studied the ${\mathrm{H}}_{2}\mathrm{O}$ ice VII--ice X phase transition at room temperature by performing three quasicontinuous synchrotron time-resolved x-ray-diffraction experiments in a dynamic diamond anvil cell, reaching pressures of 180 GPa. The dense pressure coverage of our volume data allows us to directly derive the bulk modulus for ${\mathrm{H}}_{2}\mathrm{O}$ over the entire pressure range. Our data document three major changes in compression behavior in the ranges of 35--40, 50--55, and 90--110 GPa, likely corresponding to the formation of pretransition dynamically disordered ice VII and ice X, and static ice X, respectively. Our results confirm that ice X has a very high bulk modulus.

Development of a Multiphase Beryllium Equation of State and Physics-based Variations.

We construct a family of beryllium (Be) multiphase equation of state (EOS) models that consists of a baseline ("optimal") EOS and variations on the baseline to account for physics-based uncertainties. The Be baseline EOS is constructed to reproduce a set of self-consistent data and theory including known phase boundaries, the principal Hugoniot, isobars, and isotherms from diamond-anvil cell experiments. Three phases are considered, including the known hexagonal closed-packed (hcp) phase, the liquid, and the theoretically predicted high-pressure body-centered cubic (bcc) phase. Since both the high-temperature liquid and high-pressure bcc phases lack any experimental data, we carry out ab initio density functional theory (DFT) calculations to obtain new information about the EOS properties for these two regions. At extremely high temperature conditions (>87 eV), DFT-based quantum molecular dynamics simulations are performed for multiple liquid densities using the state-of-the-art Spectral Quadrature methodology in order to validate our selected models for the ion- and electron-thermal free energies of the liquid. We have also performed DFT simulations of hcp and bcc with different exchange-correlation functionals to examine their impact on bcc compressibility, which bound the hcp-bcc transition pressure to within 4 ± 0.5 Mbar. Our baseline EOS predicts the first density maximum along the Hugoniot to be 4.4-fold in compression, while the hcp-bcc-liquid triple-point pressure is predicted to be at 2.25 Mbar. In addition to the baseline EOS, we have generated eight variations to accommodate multiple sources of potential uncertainties such as (1) the choice of free-energy models, (2) differences in theoretical treatments, (3) experimental uncertainties, and (4) lack of information. These variations are designed to provide a reasonable representation of nonstatistical uncertainties for the Be EOS and may be used to assess its sensitivity to different inertial-confinement fusion capsule designs.

Macroscale Superlubricity and Polymorphism of Long-Chain n-Alcohols

Simple n-alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would result in enormous improvements in efficiency and durability.

High pressure polymorphism of LiBH4 and of NaBH4.

The pressure-induced structural changes in LiBH4 and in NaBH4 have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH4]− unit appears to be remarkably resistant under pressure. NaBH4 remains stable in the known Pnma γ-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH4 is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH4 phases should appear at about 290 and 428 GPa. A very slight solubility of H2 inside phases II, III and V of LiBH4 is observed. A NaBH4(H2)0.5 complex is predicted to be stable above 150 GPa.

Raman scattering from highly-stressed anvil diamond* *Project support by the National Natural Science Foundation of China (Grant No. 11774247).

The high-frequency edge of the first-order Raman mode of diamond reflects the stress state at the culet of anvil, and is often used for the pressure calibration in diamond anvil cell (DAC) experiments. Here we point out that the high-frequency edge of the diamond Raman phonon corresponds to the Brillouin zone (BZ) center Γ point as a function of pressure. The diamond Raman pressure gauge relies on the stability of crystal lattice of diamond under high stress. Upon the diamond anvil occurs failure under the uniaxial stress (197 GPa), the loss of intensity of the first-order Raman phonon and a stress-dependent broad Raman band centered at 600 cm−1 are observed, which is associated with a strain-induced local mode corresponding to the BZ edge phonon of the L 1 transverse acoustic phonon branch.

Solubility of Na2SO4 in silica-saturated solutions: Implications for REE mineralization

Abstract Sulfate is traditionally considered to have retrograde solubility in aqueous solutions. However, our recent hydrothermal diamond-anvil cell (HDAC) experiments have shown that the solubility of Na2SO4 changes from retrograde to prograde in the presence of silica, leading to the formation of sulfate-rich solutions at high temperatures, in line with observations on natural geofluids. In this study, we use synthetic inclusions of fused silica capillary capsules containing saturated Na2SO4 solutions and Na2SO4 crystals to quantitatively investigate the solubility of Na2SO4 at different temperatures in the Na2SO4-SiO2-H2O system. Sulfate concentrations were measured using Raman spectroscopy and calibrated using Cs2SO4 solutions with known concentrations. The solubility of crystalline Na2SO4 dropped slightly when heated from 50 to 225 °C and dramatically from 225 to 313 °C. At 313 °C, the Na2SO4 crystals began to melt, forming immiscible sulfate melt coexisting with the aqueous solution, with or without solid Na2SO4. With the formation of sulfate melt, the solubility of Na2SO4 was reversed to prograde (i.e., solubility increased considerably with increasing temperatures). The solubility of Na2SO4 in the measured solution was significantly higher than that predicted in the absence of SiO2 over the entire temperature range (except for temperatures around 313 °C). This indicates that the presence of SiO2 greatly changes the dissolution behavior of Na2SO4, which may be caused by the formation of a sulfate–silicate intermediates such as Si(OH)4SO42−. Considering that most crustal fluids are silica-saturated, the solubility curve of Na2SO4 obtained in this study can better reflect the characteristics of geofluids when compared to that of Na2SO4-H2O binary system. At temperatures of 313–425 °C, the solubility of Na2SO4 increases with temperature following the function Csulfate = –3173.7/T + 5.9301, where Csulfate and T represent the solubility of Na2SO4 in mol/kg H2O and temperature in Kelvin, respectively. As an application, this temperature-solubility relationship can be used to evaluate the sulfate contents in fluid inclusions that contain sulfate daughter minerals, based on the temperature of sulfate disappearance obtained from microthermometric analysis. The sulfate concentrations of the ore-forming fluids of the giant Maoniuping carbonatite-related rare earth element (REE) deposit (southwest China) were calculated to be 4.67–4.81 m (mol/kg H2O). These sulfate concentrations were then used as internal standards to calibrate the previously reported semi-quantitative results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of REE-forming stage fluid inclusions at this deposit. The calculated Ce concentrations in the REE-mineralizing fluid range from 0.42 to 0.49 wt%. The high fluid REE contents suggest that the sulfate-rich fluids are ideal solvents for REE transport. A mass-balance calculation was carried out to evaluate the minimal volume of carbonatite melt that was required for the formation of the giant Maoniuping REE deposit. The result indicates that the carbonatite dikes in the mining area are enough to provide the required fluids and metals, and thus a deep-seated magma chamber is not necessary for ore formation.

Discovery of terrestrial allabogdanite (Fe,Ni)2P, and the effect of Ni and Mo substitution on the barringerite-allabogdanite high-pressure transition

Abstract Minerals formed at high pressures are sensitive indicators of extreme pressure-temperature conditions that occur in nature. The discovery of the high-pressure polymorph of (Fe,Ni)2P, allabogdanite in the surficial pyrometamorphic rocks of the Hatrurim Formation (the Mottled Zone) surrounding the Dead Sea basin in Israel is the first terrestrial occurrence of a mineral previously only found in iron meteorites. Stepwise annealing experiments demonstrate that allabogdanite is metastable at ambient pressure and that it irreversibly transforms into its low-pressure polymorph, barringerite, upon heating to 850 ± 50 °C. High-pressure, high-temperature diamond-anvil cell (DAC) experiments confirm the results of annealing experiments. The DAC data indicate that Hatrurim allabogdanite is metastable below 7.4 GPa, and the low-to high-pressure phase transition (barringerite → allabogdanite) occurs at 25 ± 3 GPa and 1400 ± 100 °C. The observed transition pressure of Hatrurim allabogdanite is significantly higher than that of pure synthetic Fe2P (8 GPa) due to partial substitution of Fe for Ni (4 wt%) and Mo (2.5 wt%). Because the influence of substituting impurities on the conditions of phase transitions can be unexpectedly strong, our findings confirm that caution should be exercised when extrapolating data from experiments on synthetic compounds to natural systems. Based on the discovery of terrestrial allabogdanite (Fe,Ni)2P coupled with experiments probing the phase transitions in this natural composition, we contend that terrestrial allabogdanite formed via transformation from barringerite and posit potential scenarios of its formation.

Transport properties of Fe-Ni-Si alloys at Earth's core conditions: Insight into the viability of thermal and compositional convection

Thermal and compositional convection in Earth's core are thought to be the main power sources driving geodynamo. The viability and strength of thermally and compositionally-driven convection over Earth's history depend on the adiabatic heat flow across the core-mantle boundary (CMB) which is governed by the thermal conductivity of a constituent Fe-Ni-light element alloy at the pressure-temperature (P-T) conditions relevant to the core. Silicon is often proposed to be an abundant light element alloyed with Fe along with ∼5 wt% Ni, but the thermal transport properties of Fe-Ni-Si alloys at high P-T remain largely uncertain. Here we measured the electrical resistivities of Fe-10wt%Ni and Fe-1.8wt%Si alloys up to ∼142 GPa and ∼3400 K using four-probe van der Pauw method in laser-heated diamond anvil cell experiments. Our results show that the resistivities of hcp-Fe-1.8Si and Fe-10Ni display quasi-linear temperature dependence from ∼1500 to 3400 K at each given high pressure. Addition of ∼2 wt% Si in hcp-Fe significantly increases its resistivity by ∼25% at ∼138 GPa and 4000 K, but Fe-10wt%Ni has similar resistivity to pure hcp-Fe at near CMB P-T conditions. Using our measured values of electrical resistivities, we model thermal conductivities via the Wiedemann-Franz law, giving a nominal thermal conductivity of ∼50 Wm−1K−1 for liquid Fe-5Ni-8Si alloy at the topmost outer core, implying an adiabatic (conductive) core heat flow of ∼8.0 TW. The outer core has a much lower thermal conductivity than the inner core due to light-element differentiation across the solidifying inner-core boundary. Our studies imply that the adiabatic core heat flow is low enough to enable thermal convection to drive the geodynamo over most and possibly all of Earth's history, while the strength of compositional convection increases with the inner-core growth and accounts for ∼83% of the buoyancy flux to the present-day geodynamo.

Incompatibility of argon during magma ocean crystallization

We report results from multi-anvil (MA) and laser-heated diamond anvil cell (LH-DAC) experiments that synthesize high-pressure phases, including bridgmanite, ferropericlase, stishovite, and ultramafic liquid, in the presence of an argon-rich fluid. The goal of the experiments is to constrain the equilibrium distribution of argon in magma ocean environments. Argon concentrations in LH-DAC experiments were quantified by electron microprobe analysis, while argon concentrations in MA experiments were quantified by laser-ablation mass spectrometry and electron microprobe analysis. Our LH-DAC experiments demonstrate that argon solubility in ultramafic liquid is near or above 1.5 wt.% at conditions between 13–101 GPa and 2300–6300 K. Argon concentrations in bridgmanite and ferropericlase synthesized in LH-DAC experiments range from below detection to 0.58 wt.%. Argon concentrations in bridgmanite and ferropericlase synthesized in MA experiments range from below detection to 2.16 wt.% for electron microprobe measurements and laser-ablation measurements. We interpret this wide range of argon concentrations in minerals to reflect the variable presence of argon-rich fluid inclusions in analytical volumes. Our analyses therefore provide upper limit constraints for argon solubility in high-pressure minerals (<0.015 wt.%) across all mantle pressures and temperatures. The combination of relatively high argon solubility in ultramafic liquid (∼1.5 wt.%) and low argon solubility in minerals implies argon incompatibility (Dbridgmanite−meltAr < 0.01, Dferropericlase−meltAr < 0.01) during magma ocean crystallization and that the initial distribution of argon, and likely other neutral species, may be controlled by liquids trapped in a crystallizing magma ocean. We thus predict a basal magma ocean would be enriched in noble gases relative to other regions of the mantle. Moreover, we predict that the noble gas parent-daughter ratio of magma ocean cumulates pile will increase with crystallization, assuming refractory and incompatible behavior for parent elements.

In situ micro-FTIR spectroscopic investigations of synthetic ammonium phengite under pressure and temperature

Abstract. Phengite is known to be an important mineral in the transport of alkalis and water to upper mantle depths. Since ammonium (NH4+) can substitute for K+ in K-bearing minerals, phengite is thus a potential host to transport nitrogen into the mantle. However, the temperature and pressure conditions at which devolatilisation of NH4-bearing phengite occurs are not well constrained. In this study, NH4-phengite (NH4)(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2 was synthesised in piston-cylinder experiments at 700 ∘C and 4.0 GPa. Its devolatilisation behaviour was studied by means of in situ micro-FTIR (Fourier transform infrared) spectroscopy under low and high temperatures from −180 up to 600 ∘C at ambient pressure using a Linkam cooling–heating stage and pressures up to 42 GPa at ambient temperature in diamond anvil cell (DAC) experiments. In addition to these short-term in situ experiments, we performed quenched experiments where the samples were annealed for 24 h at certain temperatures and analysed at room conditions by micro-FTIR spectroscopy. Our results can be summarised as follows: (1) an order–disorder process of the NH4+ molecule takes place with temperature variation at ambient pressure; (2) NH4+ is still retained in the phengite structure up to 600 ∘C, and the expansion of the NH4+ molecule with heating is reversible for short-term experiments; (3) kinetic effects partly control the destabilisation of NH4+ in phengite; (4) ammonium loss occurs at temperatures near dehydration; (5) NH4+ in phengite is apparently distorted above 8.6 GPa at ambient temperature; and (6) the local symmetry of the NH4+ molecule is lowered/descended/reduced by increasing pressure (P) or decreasing temperature (T), and the type and mechanism of this lowered symmetry is different in both cases. The current study confirms the wide stability range of phengite and its volatiles and thus has important implications for the recycling of nitrogen and hydrogen into the deep Earth. Moreover, it is considered as a first step in the crystallographic determination of the orientation of the NH4+ molecule in the phengite structure.

Prediction of chlorine and fluorine crystal structures at high pressure using symmetry driven structure search with geometric constraints.

The high-pressure properties of fluorine and chlorine are not yet well understood because both are highly reactive and volatile elements, which have made conducting diamond anvil cell and x-ray diffraction experiments a challenge. Here, we use ab initio methods to search for stable crystal structures of both elements at megabar pressures. We demonstrate how symmetry and geometric constraints can be combined to efficiently generate crystal structures that are composed of diatomic molecules. Our algorithm extends the symmetry driven structure search method [R. Domingos et al., Phys. Rev. B 98, 174107 (2018)] by adding constraints for the bond length and the number of atoms in a molecule while still maintaining generality. As a method of validation, we have tested our approach for dense hydrogen and reproduced the known molecular structures of Cmca-12 and Cmca-4. We apply our algorithm to study chlorine and fluorine in the pressure range of 10 GPa-4000 GPa while considering crystal structures with up to 40 atoms per unit cell. We predict chlorine to follow the same series of phase transformations as elemental iodine from Cmca to Immm to Fm3¯m, but at substantially higher pressures. We predict fluorine to transition from a C2/c to Cmca structure at 70 GPa, to a novel orthorhombic and metallic structure with P42/mmc symmetry at 2500 GPa, and finally to its cubic analog form with Pm3¯n symmetry at 3000 GPa.