Thermal Equation of State of Cubic Silicon Carbide at High Pressures

We have performed in situ X-ray diffraction measurements of cubic silicon carbide (SiC) with a zinc-blende crystal structure (B3) at high pressures and temperatures using multi-anvil apparatus. The ambient volume inferred from the compression curves is smaller than that of the starting material. Using the 3$^{rd}$-order Birch-Murnaghan equation of state and the Mie-Grüneisen-Debye model, we have determined the thermoelastic parameters of the B3-SiC to be K$_0$=228±3 GPa, K$_0$’,=4.4±0.4, q=0.27±0.37, where K$_0$, K$_0$’ and q are the isothermal bulk modulus, its pressure derivative and logarithmic volume dependence of the Grüneisen parameter, respectively. Using the 3rd-order Birch-Murnaghan EOS with the thermal expansion coefficient, the thermoelastic parameters have been found as K$_0$=221±3 GPa, K$_0$’,=5.2±0.4, α$_0$=0.90±0.02 ⋅ 10$^{−5}$ ⋅ K$^{−1}$, where α$_0$ is the thermal expansion coefficient at room pressure and temperature. We have determined that paired B3-SiC – MgO calibrants can be used to estimate pressure and temperature simultaneously in ultrahigh-pressure experiments up to 60 GPa

Crystal chemistry and compressibility of Fe0.5Mg0.5Al0.5Si0.5O3 and FeMg0.5Si0.5O3 silicate perovskites at pressures up to 95 GPa

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe3+-rich phases, FeMg0.5Si0.5O3 and Fe0.5Mg0.5Al0.5Si0.5O3, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg0.5Si0.5O3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe0.5Mg0.5Al0.5Si0.5O3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe3+ in a (Fe0.7,Mg0.3)O6 octahedral site of the FeMg0.5Si0.5O3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe0.5Mg0.5Al0.5Si0.5O3 phase as all Fe3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al0.5,Si0.5)O6

Stabilization Of The CN₃⁵− Anion In Recoverable High-pressure Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates

A series of isostructural Ln(3)O(2)(CN3) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln(3)O(2)(CN3) solids are composed of the hitherto unknown CN35- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln(3)O(2)(CN3) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln(3)O(2)(CN3) compounds are recoverable to ambient conditions. The stabilization of the CN35- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.Funding Agencies|National Science Foundation; DOE Office of Science by Argonne National Laboratory; UKRI Future Leaders Fellowship; Deutsche Forschungsgemeinschaft (DFG) [DE-AC02-06CH11357, MR/V025724/1, 2009 00971]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Projekt DEAL; [DU 954-11/1]; [DU 393-9/2]; [DU 393-13/1]; [DU 945/15-1]; [EAR- 1634415]</p

A New Approach Determining a Phase Transition Boundary Strictly Following a Definition of Phase Equilibrium: An Example of the Post-Spinel Transition in Mg₂SiO₄ System

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition’s normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary

Small effect of water incorporation on dislocation mobility in olivine: Negligible creep enhancement and water-induced fabric transition in the asthenosphere

To constrain the effect of water on upper mantle dynamics, we measured the annihilation rate coefficients (k) of [100](010) and [001](100) dislocations in olivine, referred to as a-dislocations and c-dislocations, respectively, as a function of water content. Natural olivine single crystals were doped with 5–800 wt. ppm water and sheared in the [100] or [001] directions along the (010) and (100) planes to produce a- and c-dislocations, respectively, and then annealed under quasi-hydrostatic conditions at a constant pressure and temperature of 5 GPa and 1473 K. The obtained annihilation rate coefficients are fitted to a power-law equation, yielding astonishingly small water-content exponents of $0.0 \pm 0.1$ and $0.2 \pm 0.2$ for the a- and c-dislocations, respectively. The overall effect of water on dislocation mobility is therefore small because these two slip systems are considered to be the least and most sensitive to water, respectively. These results imply that water incorporation does not effectively increase the dislocation-creep rate and that a water-induced fabric transition is unlikely. The effects of water on asthenospheric dynamics may thus be limited, and the lateral seismic anisotropy changes observed in the asthenosphere may solely reflect changes of mantle flow geometry

Determination of phase relations of the olivine–ahrensite transition in the Mg2SiO4–Fe2SiO4 system at 1740 K using modern multi-anvil techniques

The phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)2SiO4 system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures.BMBFH2020 European Research Council http://dx.doi.org/10.13039/100010663Deutsches Elektronen-Synchrotron (DESY) (4201

Face‐Centered Cubic Platinum Hydride and Phase Diagram of PtH

Recent research on superconductivity of high‐pressure hydrides generated many phase stability calculations with a lack of their experimental verification; a typical example is Pt–H system. The stability of eight PtH structures was predicted, while the experiments revealed the existence of only hexagonal close‐packed (hcp) and trigonal PtH. Face‐centered cubic (fcc) PtH was predicted to be nearly isoentalpic to the hcp PtH and stable near 100 GPa, but never observed experimentally. Here we report the first synthesis of the fcc PtH using laser‐heated diamond anvil cell. It was found to occupy a high‐temperature area of the phase diagram in a wide pressure range of 20–100 GPa, being metastable at room temperature. Our results look promising for uncovering weak approximations in current high‐pressure hydrides stability ab initio calculations

Determination of phase relations of the olivine-ahrensite transition in the Mg2SiO4−Fe2SiO4Mg_{2}SiO_{4}-Fe_{2}SiO_{4} system at 1740 K using modern multi-anvil techniques

The phase relations of iron-rich olivine and its high-pressure polymorphs are important for planetary science and meteoritics because these minerals are the main constituents of terrestrial mantles and meteorites. The olivine–ahrensite binary loop was previously determined by thermochemical calculations in combination with high-pressure experiments; however, the transition pressures contained significant uncertainties. Here we determined the binary loop of the olivine–ahrensite transition in the (Mg,Fe)$_2$SiO$_4$ system at 1740 K in the pressure range of 7.5–11.2 GPa using a multi-anvil apparatus with the pressure determined using in situ X-ray diffraction, compositional analysis of quenched run products, and thermochemical calculation. Based on the determined binary loop, a user-friendly software was developed to calculate pressure from the coexisting olivine and ahrensite compositions. The software is used to estimate the shock conditions of several L6-type chondrites. The obtained olivine–ahrensite phase relations can also be applied for precise in-house multi-anvil pressure calibration at high temperatures

Ferric Iron Substitution Mechanism in Bridgmanite under SiO2_2 -Saturated Conditions at 27 GPa

The chemistry of bridgmanite is a crucial parameter for understanding the structure and dynamics of the Earth’s lower mantle. Incorporating Al, Fe$^{2+}$, and Fe$^{3+}$ into MgSiO$_3$ bridgmanite can significantly affect its physical properties. We investigated the substitution mechanisms of Fe$^{3+}$ in bridgmanite under SiO$_2$- and Fe$_2$O$_3$-saturated conditions at 27 GPa and 1700–2000 K using a multianvil apparatus. The fraction of Fe$^{3+}$ in bridgmanite increases from 0.085 to 0.202 atoms pfu with increasing temperature. We observed that only the charge-coupled mechanism takes place in bridgmanite under the studied conditions by forming the FeFeO$_3$ component, which increases from 4.0 ± 0.6 mol % at 1700 K to 9.9 ± 0.7 mol % at 2000 K. The absence of vacancies in bridgmanite under SiO$_2$-saturated conditions implies that bridgmanite in mid-ocean ridge basalt layers of subducted slabs in the lower mantle should have higher viscosity and lower electrical conductivity than that in the surrounding mantle. These differences in bridgmanite properties should affect lower-mantle dynamics, for example, enhance slab penetration more deeply into the lower mantle due to viscosity difference between mid-ocean ridge basalt and surrounding bridgmanite

Does It “Rain” Diamonds on Neptune and Uranus?

The methane phase diagram is extremely important to understand the physical properties of the ice giants─Uranus and Neptune. Several experimental studies using diamond anvil cells (DACs) combined with laser heating have reported the formation of diamond from methane at pressures of 10–80 GPa and temperatures above 2000 K, corresponding to the interiors of these planets. These results, however, are probably affected by the presence of metallic heat absorbers, widely used in all previously reported experiments and interacting with hydrogen at high pressure. In the present work, the effect of metallic heat absorbers on the decomposition of methane into diamond was studied at 20–95 GPa and 1300–3700 K using laser-heated DACs with platinum (as hydride-forming) and gold (as non-hydride-forming) metals. In the case of a platinum heat absorber, diamond formation was observed from 50 to 95 GPa near 2000 K simultaneously with platinum hydride formation. In contrast, in the case of a gold heat absorber, diamond formation was not observed below 95 GPa and 3700 K. Thus, the hypothesis of diamond precipitation in the Uranus and Neptune interiors should be reconsidered, taking into account the effect of metallic heater reactivity on the experimentally observed reactions