Diamond Anvil Cell Experiments Research Articles

The Effect of Hydrostatic Pressure on Structure, Crystal‐Field Strength, and Emission Properties of Neat and Ni2+‐Activated KMgF3

AbstractTo understand excellent emission and sensitivity for hydrostatic pressure luminescent ions host material, the first principles calculations carried out within density functional theory (DFT) framework are performed to clarify the electronic structure of neat and doped with Ni2+ ions KMgF3 single crystals. The results of band structure calculations show that F2p states are the principal contributors to the KMgF3 valence band, mainly in its upper and central parts, while in the energy band gap of the KMgF3:Ni2+ phosphor, new electronic states associated with the Ni2+ 3d‐orbitals are formed. Furthermore, the zero phonon line (ZPL) spin‐forbidden transition emission energies, (3A2⇄1E) ZPL, (3A2⇄3T2) ZPL, strength of the octahedral crystal field, 10Dq (3A2→3T2)ZPL, are calculated for the KMgF3:Ni2+ phosphor. Any changes of the Em(3A2⇄1E)ZPL transition energy of the KMgF3:Ni2+ phosphor with pressure increasing from 0 to 20 GPa are not detected, while the crystal‐field strength increases linearly with increasing pressure. Present results bring a foresight tool for predicting physicochemical properties of undoped and doped wide‐gap fluorides; KMgF3:Ni2+, without any toxic/harmful or expensive rare‐earth can be effectively used as an optical manometer in 0–20 GPa, which covers the almost whole pressure range available at present in Diamond anvil cell experiments.

Molecular H2 in silicate melts

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H2 between H2O-saturated sodium aluminosilicate melts and H2O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H2 between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H2 in silicate melts saturated with H2O decreases as the temperature increases, with control exerted by the mole fraction of H2O in the melt. The dissolution of H2 in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:ln Km/f = 11.4 (±1.3) *1000 / T (K) − 18.7 (±1.1)where Km/f is the equilibrium constant for the reaction H2(g) = H2(melt). This equation was derived by integrating data from the current and prior experimental studies that include silicate melts with varying H2O saturation levels. It should be deemed applicable within the temperature range of 600–1450 °C and pressures ranging from 0.3 to 3 GPa. The implications are extended into developing an understanding of the H partitioning between H2-rich atmospheres blanketing magma oceans in the early history of planetary bodies. For example, transferring H from primordial atmospheric envelopes to the interior of rocky exoplanets may be less efficient than previously believed, which should be considered in models of volatile retention. Experimental data also suggest that minimal amounts of solar nebula H2 are likely to dissolve in the molten surface of primitive objects in the protoplanetary disk (∼10−5 to 10−9 mol faction H2 in melt), contradicting the highly reducing conditions observed in chondrule mineral compositions.

Exotic compounds of monovalent calcium synthesized at high pressure

It is well known that atoms of the same element in different valence states show very different chemical behaviors. Calcium is a typical divalent metal, sharing or losing both of its valence electrons when forming compounds. Attempts have been made to synthesize compounds of monovalent calcium ions for decades, but with very little success (e.g., in clusters). Pressure can result in substantial changes in the properties of atoms and chemical bonding, creating an extensive variety of unique materials with special valence states. In this study, using the ab initio evolutionary algorithm USPEX, we search for stable calcium–chlorine (Ca–Cl) system compounds at pressures up to 100 GPa. Besides the expected compound CaCl2, we predict three new compounds with monovalent Ca to be stable at high pressures, namely, CaCl, Ca5Cl6, and Ca3Cl4. According to our calculations, CaCl is stable at pressures above 18 GPa and is predicted to undergo a transition from nonmagnetic Fm-3m-CaCl to ferromagnetic Pm-3m-CaCl at 40 GPa. Ca5Cl6 and Ca3Cl4 are stable at pressures above 37 and 73 GPa, with space groups P-1 and R-3, respectively. Following these predictions, we successfully synthesized Pm-3m-CaCl in laser-heated diamond anvil cell experiments. The emergence of the unusual valence state at high pressures reveals exciting opportunities for creating entirely new materials in sufficiently large quantities for a variety of potential applications.

Pressure stabilizes ferrous iron in bridgmanite under hydrous deep lower mantle conditions

Earth’s lower mantle is a potential water reservoir. The physical and chemical properties of the region are in part controlled by the Fe3+/ΣFe ratio and total iron content in bridgmanite. However, the water effect on the chemistry of bridgmanite remains unclear. We carry out laser-heated diamond anvil cell experiments under hydrous conditions and observe dominant Fe2+ in bridgmanite (Mg, Fe)SiO3 above 105 GPa under the normal geotherm conditions corresponding to depth > 2300 km, whereas Fe3+-rich bridgmanite is obtained at lower pressures. We further observe FeO in coexistence with hydrous NiAs-type SiO2 under similar conditions, indicating that the stability of ferrous iron is a combined result of H2O effect and high pressure. The stability of ferrous iron in bridgmanite under hydrous conditions would provide an explanation for the nature of the low-shear-velocity anomalies in the deep lower mantle. In addition, entrainment from a hydrous dense layer may influence mantle plume dynamics and contribute to variations in the redox conditions of the mantle.

High-pressure Phase Transition of Olivine-type Mg2GeO4 to a Metastable Forsterite-III type Structure and their Equation of States

Abstract Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg2GeO4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a 3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1 (3) Å3, K0 = 124.6 (14) GPa and K0′ = 3.86 (fixed) for forsterite- and V0 = 263.5 (15) Å3, K0 = 175 (7) GPa and K0′ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs and interpreting the results of shock compression experiments.

Hyper-enrichment of gold via quartz fracturing and growth of polymetallic melt droplets

Abstract Gold precipitation in hydrothermal systems is traditionally attributed to supersaturation of gold due to decreasing gold complex stability triggered by changes in physicochemical conditions of the ore fluid. However, ultrahigh-grade gold veins in orogenic (shear zone related) gold deposits can contain kilograms per tonne of gold or more, in marked contrast to the typically very low gold concentrations (tens of parts per billion) in fluid. The gold mineral assemblage is commonly restricted to native gold and/or Au-(Ag)-tellurides and occurs in micro-fractures of sheared quartz veins. Textural and compositional characterization of such assemblages, coupled with hydrothermal diamond anvil cell experiments and heating-freezing experiments, provides evidence for an alternative ultrahigh-grade gold enrichment mechanism via growth of polymetallic melt droplets induced by quartz fracturing. We propose that polymetallic melt droplets of Au-Ag-Te-Bi–rich composition form through adsorption-reduction of metal complexes on fractured quartz surfaces, where surface silanol groups and hydrogen serve as reductants. The melt droplets subsequently grow by catalyzing reduction of metal complexes and absorbing metals from fluids percolating in the fractured quartz network. The mobile and reactive polymetallic melt droplets can repeatedly react with the fluid on protracted quartz fracturing and efficiently continue to scavenge gold from multiple pulses of gold-undersaturated ore fluids.

Uncovering the Effects of Non-Hydrostaticity on Pressure-Induced Phase Transformation in Xenotime-Structured TbPO4

The pressure-induced phase transformations of rare earth orthophosphates (REPO4s) have become increasingly relevant in ceramic matrix composite (CMC) research; however, understanding of the shear-dependence of these transformations remains limited. This study employs diamond anvil cell experiments with three pressure media (neon, KCl, sample itself/no medium) to systematically assess the effect of shear on the phase transformations of TbPO4. Results show a lowering of the TbPO4 transformation onset pressure (Ponset) as well as an extension of the xenotime–monazite phase coexistence range under non-hydrostatic conditions. The TbPO4 Ponset under no medium (4.4(3) GPa) is the lowest REPO4 Ponset reported to date and represents a ~50% drop from the hydrostatic Ponset. Enthalpic differences likely account for lower Ponset values in TbPO4 compared to DyPO4. Experiments also show scheelite may be the post-monazite phase of TbPO4; this phase is consistent with observed and predicted REPO4 transformation pathways.

Stability of hydrides in sub-Neptune exoplanets with thick hydrogen-rich atmospheres.

Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant core. From an assumption that there is no chemical reaction between hydrogen and silicates/metals at the atmosphere-interior boundary, the cores of sub-Neptunes have been modeled with molten silicates and metals (magma) in previous studies. In large sub-Neptunes, pressure at the atmosphere-magma boundary can reach tens of gigapascals where hydrogen is a dense liquid. A recent experiment showed that hydrogen can induce the reduction of Fe[Formula: see text] in (Mg,Fe)O to Fe[Formula: see text] metal at the pressure-temperature conditions relevant to the atmosphere-interior boundary. However, it is unclear whether Mg, one of the abundant heavy elements in the planetary interiors, remains oxidized or can be reduced by H. Our experiments in the laser-heated diamond-anvil cell found that heating of MgO + Fe to 3,500 to 4,900 K (close to or above their melting temperatures) in an H medium leads to the formation of Mg[Formula: see text]FeH[Formula: see text] and H[Formula: see text]O at 8 to 13 GPa. At 26 to 29 GPa, the behavior of the system changes, and Mg-H in an H fluid and H[Formula: see text]O were detected with separate FeH[Formula: see text]. The observations indicate the dissociation of the Mg-O bond by H and subsequent production of hydride and water. Therefore, the atmosphere-magma interaction can lead to a fundamentally different mineralogy for sub-Neptune exoplanets compared with rocky planets. The change in the chemical reaction at the higher pressures can also affect the size demographics (i.e., "radius cliff") and the atmosphere chemistry of sub-Neptune exoplanets.

Quantifying uncertainty in analysis of shockless dynamic compression experiments on platinum. I. Inverse Lagrangian analysis

Absolute measurements of solid-material compressibility by magnetically driven shockless dynamic compression experiments to multi-megabar pressures have the potential to greatly improve the accuracy and precision of pressure calibration standards for use in diamond anvil cell experiments. To this end, we apply characteristics-based inverse Lagrangian analysis (ILA) to 11 sets of ramp-compression data on pure platinum (Pt) metal and then reduce the resulting weighted-mean stress–strain curve to the principal isentrope and room-temperature isotherm using simple models for yield stress and Grüneisen parameter. We introduce several improvements to methods for ILA and quasi-isentrope reduction, the latter including calculation of corrections in wave speed instead of stress and pressure to render results largely independent of initial yield stress while enforcing thermodynamic consistency near zero pressure. More importantly, we quantify in detail the propagation of experimental uncertainty through ILA and model uncertainty through quasi-isentrope reduction, considering all potential sources of error except the electrode and window material models used in ILA. Compared to previous approaches, we find larger uncertainty in longitudinal stress. Monte Carlo analysis demonstrates that uncertainty in the yield-stress model constitutes by far the largest contribution to uncertainty in quasi-isentrope reduction corrections. We present a new room-temperature isotherm for Pt up to 444 GPa, with 1-sigma uncertainty at that pressure of just under ±1.2%; the latter is about a factor of three smaller than uncertainty previously reported for multi-megabar ramp-compression experiments on Pt. The result is well represented by a Vinet-form compression curve with (isothermal) bulk modulus K0=270.3±3.8 GPa, pressure derivative K0′=5.66±0.10, and correlation coefficient RK0,K0′=−0.843.

Combined First-Principles and Experimental Investigation into the Reactivity of Codeposited Chromium-Carbon under Pressure.

High-pressure synthesis in the diamond anvil cell suffers from the lack of a general approach for the control of precursor stoichiometry and homogeneity. Here, we present results from a new method we have developed that uses magnetron cosputtering to prepare stoichiometrically precise and atomically mixed amorphous films of Cr:C. Laser-heated diamond anvil cell experiments carried out on a flake of this sample at pressures between 13.5 and 24.3 GPa lead to the observation of Cr3C (Pnma) over the entire pressure range-in good agreement with our in-house theoretical predictions-but also reveal two other metastable phases that were not expected: a novel monoclinic chromium carbide phase and the NaCl-type CrC (Fm3̅m) phase. The unexpected stability of CrC is investigated by using first-principles methods, revealing a large stabilizing effect tied to substoichiometry at the carbon site. These results offer an important case study into the current limitations of crystal structure prediction methods with regard to phase complexity and bolster the growing need for advanced theoretical approaches that can more completely survey experimentally unexplored phase space.

An in-situ experimental HP/HT study on bromine release from a natural basalt

We present the first in-situ partitioning data for bromine between a natural basaltic melt and a coexisting fluid. For this study hydrothermal diamond anvil cell experiments at pressures up to 1.7 GPa were conducted. We combined laser heating to melt the basalt glass with external heating to lower the temperature gradient in the cell and to initiate circulation for the aqueous fluid. Bromine concentrations were measured in-situ with X-ray fluorescence in the basaltic melts, glasses, and in the fluid. From the results we calculated partition coefficients of DBrfluid/melt = 1.19 to 3.92 in the range of 0.4 to 1 GPa for aqueous fluids. Experiments with neon as the surrounding fluid (DBrfluid/melt = 0.38 ± 0.01 at 1.1 GPa) suggest that Br-release from a basalt into volatiles that have no bonding affinity with Br is weak. This should be the case for dry intra-plate volcanic eruptions. From the experimentally gained partition coefficients and from global Br concentration values in melt inclusions of arc magmas, we calculated an annual global Br flux of 23.5–72.9 × 109 g/y.

Pressure-dependent deformation in brittle diamond

Distinguished from ductile metals, in brittle covalent materials like diamond, hydrostatic pressure is a prerequisite for triggering the plastic deformation. Consequently, the theoretical framework rooted in equilibrium lattice proves inadequate in describing the behavior of brittle covalent materials, making it essential to include extrinsic hydrostatic pressure effects. Here, we take diamond as a model and introduce hydrostatic pressure in the calculation of generalized stacking fault energies (GSFE). A deformation mechanism transition from perfect dislocation slipping on {200} plane to the partial dislocation slipping on {111} plane when the hydrostatic pressure exceeds 100 GPa is revealed by the pressure-coupled GSFE and further authenticated by nanocompression and diamond anvil cell experiments. Such pressure-dependent deformation mechanism is attributed to the different pressure sensitivities of lattice volume expansion during different slip systems operating. Our works provide insights into the activation of slip systems in diamond at the atomic scale and a route to study the plastic deformation behavior of brittle covalent materials.

Synthesis and characterization of XeAr2 under high pressure.

The binary Xe-Ar system has been studied in a series of high pressure diamond anvil cell experiments up to 60GPa at 300K. In-situ x-ray powder diffraction and Raman spectroscopy indicate the formation of a van der Waals compound, XeAr2, at above 3.5GPa. Powder x-ray diffraction analysis demonstrates that XeAr2 adopts a Laves MgZn2-type structure with space group P63/mmc and cell parameters a = 6.595 Å and c = 10.716 Å at 4GPa. Density functional theory calculations support the structure determination, with agreement between experimental and calculated Raman spectra. Our DFT calculations suggest that XeAr2 would remain stable without a structural transformation or decomposition into elemental Xe and Ar up to at least 80GPa.

MHz free electron laser x-ray diffraction and modeling of pulsed laser heated diamond anvil cell

A new diamond anvil cell experimental approach has been implemented at the European x-ray Free Electron Laser, combining pulsed laser heating with MHz x-ray diffraction. Here, we use this setup to determine liquidus temperatures under extreme conditions, based on the determination of time-resolved crystallization. The focus is on a Fe-Si-O ternary system, relevant for planetary cores. This time-resolved diagnostic is complemented by a finite-element model, reproducing temporal temperature profiles measured experimentally using streaked optical pyrometry. This model calculates the temperature and strain fields by including (i) pressure and temperature dependencies of material properties, and (ii) the heat-induced thermal stress, including feedback effect on material parameter variations. Making our model more realistic, these improvements are critical as they give 7000 K temperature differences compared to previous models. Laser intensities are determined by seeking minimal deviation between measured and modeled temperatures. Combining models and streak optical pyrometry data extends temperature determination below detection limit. The presented approach can be used to infer the liquidus temperature by the appearance of SiO2 diffraction spots. In addition, temperatures obtained by the model agree with crystallization temperatures reported for Fe–Si alloys. Our model reproduces the planetary relevant experimental conditions, providing temperature, pressure, and volume conditions. Those predictions are then used to determine liquidus temperatures at experimental timescales where chemical migration is limited. This synergy of novel time-resolved experiments and finite-element modeling pushes further the interpretation capabilities in diamond anvil cell experiments.

Negligible fractionation between fluorine and chlorine during magma ocean crystallization and implications for the origin of Earth’s volatiles

Halogens are powerful tracers for the accretion of Earth’s volatiles. Their sensitivity to Earth’s differentiation, such as core formation, magma ocean crystallization, and planetary evaporation allows us to track such major events and the origin of Earth’s building blocks. Here we examine the role of magma ocean crystallization in fractionating F/Cl ratio of the BSE by experimentally determining the partitioning behavior of F and Cl between silicate melt and crystallized minerals. Previous experimental data, due to technical challenges, are only available at pressures <25 GPa. To simulate relevant conditions for deep magma oceans, we perform laser-heated diamond anvil cell experiments to determine partition coefficients of F and Cl between bridgmanite and silicate melt at temperatures of ∼4000–4500 K and pressures of 33–55 GPa. Our data show that F and Cl are both strongly favored in silicate melt, with partition coefficients of F and Cl in the range of 0.12–0.16 and 0.007–0.017, respectively. Given plausible magma ocean scenarios, our results indicate that F and Cl are not significantly fractionated during crystallization of a deep magma ocean. Neglecting other potential processes that may fractionate F and Cl, the F/Cl ratio in the BSE should reflect that of Earth’s building blocks. It indicates Earth’s volatiles may be largely derived from inner solar system, in alignment with other isotopic systems such as nitrogen and hydrogen.

In-situ redox conditions in hydrothermal diamond-anvil cell experiments using various metal gaskets

Hydrothermal diamond-anvil cells (HDACs) have been widely used in various research fields since they were introduced in 1993. Unlike temperature (T) and pressure (P), the redox states of the samples in HDAC experiments were poorly controlled or measured; they were usually estimated by observing the reduction/disappearance of redox-sensitive mineral(s) (e.g., Mo metal) and corresponding formation and growth of such mineral(s) (e.g., MoO2) without in-situ redox measurement. In this study, we demonstrated the generation of H2 during experiments using various metal gaskets, including inert (Re) and chemically more reactive metals (Ni, Co, Mo, and W), in the pure H2O system under various P-T conditions. A technique for quantitative Raman spectroscopic analysis of H2 in HDAC is designed. Based on this technique, the oxygen fugacity (fO2) values of Mo-MoO2 buffer at the experimental P-T conditions were then calculated from these measured H2 pressures. We demonstrated that steady-state redox control can be maintained at a fixed T between 400 and 500 °C in HDAC experiments using a Mo gasket for durations of up to two hours and for those using a Re gasket with an additional Mo rod in the sample between 400 and 800 °C for durations of up to 90 min, regardless of the considerable changes in sample pressure. This demonstration was established by the balance between the rates of H2 generation and H2 loss from the sample chamber. The loss of H2 enhanced the consumption of H2O and therefore reduced the sample pressures during the experiments.We believe that hydrogen contents in every HDAC experiment, which attempt to control the redox state, need in situ measurement, then estimate the real redox state and compare it to thermodynamic equilibrium prediction. To enhance in situ redox control and measurements, modifications of HDAC are required for improving the quantitative Raman spectroscopic analysis technique in both sensitivity and accuracy.

Melting Curve of Cobalt using Molecular Dynamics Simulation

Molecular dynamics simulation is used to estimate the melting point of cobalt using the embedded atomic model (EAM) potential by heat until melting, void, hysteresis and interface methods. For instance, the estimated melting temperature are 2102 K, 1944.15 K, 1731 K and 1725±25 K using these methods, respectively. Then, the melting points at different pressures are calculated. A graph depicting the variation of melting point with pressure is drawn and compared with the available simulation and experimental results. The melting point at a low-pressure range is similar to the previous diamond anvil cell experiments. Besides, using the Simon equation, we calculated the melting slope at 0 GPa pressure of 36 K/GPa for one phase and 40 K/GPa for two-phase methods.

The intrinsic effects of using rhenium gaskets in hydrothermal diamond anvil cell experiments on background fluorescence, contamination, and redox control

Because of the great stiffness and high melting point of rhenium (Re) metal, Re gaskets are widely used in hydrothermal diamond anvil cell (HDAC) experiments that involve aqueous fluids and even hydrous melts under high pressure–temperature (P–T) conditions. This metal was usually considered chemically inert in hydrothermal experiments, but some concerns have been raised about its reactivity with the fluids. Therefore, this communication addresses the intrinsic effects of using Re gaskets in HDAC experiments, including background fluorescence in Raman spectroscopy, contamination, and redox control. A series of hydrothermal experiments involving rhenium were performed in pure water, H2O-NaCl, and SiO2-COH systems using the HDAC up to 28.9 kbar and 850 °C and fused silica capillary capsule (FSCC) up to 87 bar and 300 °C, and Raman spectroscopy was employed for in situ analysis. The results indicate that (1) strong background fluorescence, often occurring between 200 and 700 °C, is caused by the dissolution of Re gaskets and the formation of aqueous species of Re with a low oxidation state, such as a Re2+cation and eventually, its hydroxide complexes; (2) in addition to the fluorescent Re species and unwanted carbon species produced from the diamond-water reaction catalyzed by Re, a considerable amount of ReO4−, which does not cause fluorescence and shows a dominant Raman peak at ∼970 cm−1, is detected in the fluids either in situ at high temperatures above 700 °C or under cooling conditions, and it can reduce the activity of H2O and the pH of the fluids and interfere with the assignments of Raman peaks of other species in the samples; and (3) the production of oxidizing aqueous species at high temperatures, including ReO4− and CO2, is attributed to the substantial loss of H2 and the corresponding increase in oxygen fugacity. Because of these intrinsic effects of using Re gaskets in HDAC experiments, implications for hydrothermal experiments are discussed with several previously reported examples such as the discrepancy in the solubility of rutile in water under elevated P–T conditions. Based on the results of the current and previous studies, four suggestions have also been made to minimize the unwanted effects of using Re gaskets, including the generation of a reduced experimental condition, a pre-heating treatment of Re gaskets, a gasket pre-indentation approach, and a gold-lined gasket method. Armed with the knowledge of these intrinsic effects as well as the advantages of using different HDAC metal gaskets, the researchers will be able to create appropriate experimental designs.

Formation of hydrocarbons favored by high pressure at subduction zone conditions

Subduction zones enable carbon transport to the deep Earth. Inorganic and organic carbon in fluids released from the subducted slab are important in water-rock interactions. However, the potential role of reduced carbon species in fluids during subduction is still unclear. In particular, the pressure, temperature, and oxidation state conducive to the formation of fluid hydrocarbons are poorly understood. Here we focused on the role of pressure influencing the formation of hydrocarbons using diamond anvil cell experiments starting with 0.5 M Ca-acetate solutions over a broad range of pressures, from 1.6 to 4.6 GPa at temperatures of 300 and 350 °C. Droplets of immiscible hydrocarbon fluid coexisting with aqueous carbonate or bicarbonate and crystals of calcite or aragonite were formed at peak pressure and temperature conditions in the experiments. Greater quantities of hydrocarbons formed at higher pressure as revealed by measuring the in situ volume percent of the droplets. Measurement of the in situ Raman spectra of the co-existing aqueous solutions and analysis of the peak area ratios of acetate to HCO3− showed faster decrease at higher pressure, which indicated that the reactions forming hydrocarbons were accelerated by pressure. Compared to previous Na-acetate experiments at 300 °C and 3.0 GPa, this study found hydrocarbons that were more methane-rich and had more propane and less isobutane. In experiments at 350 °C, isobutane transformed to one or more aromatic hydrocarbons in an immiscible fluid. The air-dried aromatic hydrocarbons were measured using a UV laser, and show peaks of bitumen. Overall, our study supports the hypothesis that high pressures and high temperatures facilitate the possible occurrence of immiscible hydrocarbon fluids in the deep carbon cycle.

Anti-Stokes/Stokes temperature calibration and its application in laser-heating diamond anvil cells

Anti-Stokes/Stokes Raman peak intensity ratio was used to infer sample temperatures, but the influence factors of system correction factors were not clear. Non-contact in-situ anti-Stokes/Stokes temperature calibration was carried out for up to 1500 K based on six different samples under two excitation light sources (±50 K within 1000 K, ±100 K above 1000 K), and the system correction factor γ was systematically investigated. The results show that the correction factor γ of anti-Stokes/Stokes thermometry is affected by the wavelength of the excitation light source, Raman mode peak position, temperature measurement region and other factors. The anti-Stokes/Stokes thermometry was applied to the laser-heating diamond anvil cell (LHDAC) experiment to investigate the anharmonic effect of hBN under high temperature and high pressure. It is concluded that the strong anharmonic effect caused by phonon scattering at low pressure gradually changes into the predominance of localized molecular lattice thermal expansion at high pressure.