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When diamond anvil cell (DAC) sample chambers are outfitted with both thermal insulation and electrodes, two cutting-edge experimental methods are enabled: Joule heating with spectroradiometric temperature measurement and electrical resistance measurements of samples heated to thousands of kelvin. The accuracy of temperature and resistance measurements, however, often suffers from poor control of the shape and location of the sample, electrodes, and thermal insulation. Here, we present a recipe for the reproducible and precise fabrication of DAC sample, electrodes, and thermal insulation using a three-layer microassembly. The microassembly contains two potassium chloride thermal insulation layers, four electrical leads, a sample, and a buttressing layer made of polycrystalline alumina. The sample, innermost electrodes, and buttress layer are fabricated by focused-ion-beam milling. Three iron samples are presented as proof of concept. Each is successfully compressed and pulsed Joule heated while maintaining a four-point probe configuration. The highest pressure-temperature condition achieved is ∼ 150 GPa and 4000 K. [ABSTRACT FROM AUTHOR]

Decades of measurements of the thermophysical properties of hot metals show that pulsed Joule heating is an effective method to heat solid and liquid metals that are chemically reactive or difficult to contain. To extend such measurements to hundreds of GPa pressure, pulsed heating methods have recently been integrated with diamond anvil cells. The recent design used a low-side switch and active electrical sensing equipment that was prone to damage and measurement error. Here, we report the design and characterization of new electronics that use a high-side switch and robust, passive electrical sensing equipment. The new pulse amplifier can heat ∼ 5 to 50 μm diameter metal wires to thousands of kelvin at tens to hundreds of GPa using diamond anvil cells. Pulse durations and peak currents can each be varied over three orders of magnitude, from 5 µs to 10 ms and from 0.2 to 200 A. The pulse amplifier is integrated with a current probe. Two voltage probes attached to the body of a diamond anvil cell are used to measure voltage in a four-point probe geometry. The accuracy of four-point probe resistance measurements for a dummy sample with 0.1 Ω resistance is typically better than 5% at all times from 2 µs to 10 ms after the beginning of the pulse. [ABSTRACT FROM AUTHOR]

Accurate and precise measurements of spectroradiometric temperature are crucial for many high pressure experiments that use diamond anvil cells or shock waves. In experiments with sub-millisecond timescales, specialized detectors such as streak cameras or photomultiplier tubes are required to measure temperature. High accuracy and precision are difficult to attain, especially at temperatures below 3000 K. Here, we present a new spectroradiometry system based on multianode photomultiplier tube technology and passive readout circuitry that yields a 0.24 µs rise-time for each channel. Temperature is measured using five color spectroradiometry. During high pressure pulsed Joule heating experiments in a diamond anvil cell, we document measurement precision to be ±30 K at temperatures as low as 2000 K during single-shot heating experiments with 0.6 µs time-resolution. Ambient pressure melting tests using pulsed Joule heating indicate that the accuracy is ±80 K in the temperature range 1800–2700 K. [ABSTRACT FROM AUTHOR]

Hydrogen and helium are the most abundant elements in the universe, and they constitute the interiors of gas giant planets. Thus, their equations of states, phase, chemical state, and chemical reactivity at extreme conditions are of great interest. Applying Raman spectroscopy, visual observation, and synchrotron X-ray diffraction in diamond anvil cells, we performed experiments on H2–He 1:1 and D2–He 1:10 compressed gas mixtures up to 100 GPa at 300 K. By comparing with the available data on pure bulk materials, we find no sign of miscibility, chemical reactivity, and new compound formation. This result establishes a new baseline for future investigations of miscibility in the He–H2 system at extreme P-T conditions. [ABSTRACT FROM AUTHOR]

Brillouin spectroscopy at room temperature and pressures up to 40 GPa documents nearly identical elasticity and refractive index of amorphous CaSiO3 created by two different methods: temperature-quenching the melt at ambient pressure and pressure-amorphizing crystalline wollastonite at room temperature. We find reproducible hysteresis of 0 to 8% on pressure cycling that is small relative to the 30 to 60% changes in shear and longitudinal wave velocities over this pressure range. Together with observed changes in refractive index and previous results from Raman spectroscopy, these measurements reveal a continuous and reversible change in atomic packing induced by pressure. Unlike many other silicate glasses, amorphous CaSiO3 exhibits highly reproducible properties, behaving smoothly and reversibly under pressure cycling and possessing similar structure and elasticity regardless of synthesis paths for the starting material, which suggests that the amorphous solid may mimic the liquid over the pressure range investigated. [ABSTRACT FROM AUTHOR]

The thermal conductivities of mantle and core materials have a major impact on planetary evolution, but their experimental determination requires precise knowledge of sample thickness at high pressure. Despite its importance, thickness in most diamond anvil cell (DAC) experiments is not measured but inferred from equations of state, assuming isotropic contraction upon compression or assuming isotropic expansion upon decompression. Here we provide evidence that in DAC experiments both assumptions are invalid for a range of mechanically diverse materials (KCl, NaCl, Ar, MgO, silica glass, Al2O3). Upon compression, these samples are ∼30–50% thinner than expected from isotropic contraction. Most surprisingly, all the studied samples continue to thin upon decompression to 10–20 GPa. Our results partially explain some discrepancies among the highly controversial thermal conductivity values of iron at Earth's core conditions. More generally, we suggest that in situ characterization of sample geometry is essential for conductivity measurements at high pressure. Plain Language Summary: The thermal and electrical conductivities of the materials making up Earth's core and lowermost mantle are crucial inputs for modeling Earth's interior and the geodynamo mechanism. Yet, large disagreements between published values of conductivity are common, including a factor‐of‐seven discrepancy in the thermal conductivity of iron at core‐mantle boundary conditions. One possible source of systematic uncertainty is the estimate of sample thickness during high‐pressure experiments. Here we show that common materials in compression experiments tend to thin by much more than previously assumed. Surprisingly, the thinning continues upon decompression. These thinning trends could lead to ∼30–50% systematic error, partially explaining the discrepancy in iron conductivity. In situ thickness measurements are thus crucial for accurate determination of conductivities of Earth's mantle and core. Key Points: Samples in diamond anvil cell experiments contract and expand in a strongly non‐isotropic fashion upon compression and decompressionExperimental reports on iron conductivity at high pressure contain errors due to the assumption of isotropic contraction or expansionAccurate in situ determination of sample geometry is necessary for thermal and electrical conductivity measurements at high pressure [ABSTRACT FROM AUTHOR]

If successfully developed, calorimetry at tens of GPa of pressure could help characterize phase transitions in materials such as high-pressure minerals, metals, and molecular solids. Here, we extend alternating-current calorimetry to 9 GPa and 300 K in a diamond anvil cell and use it to study phase transitions in H2O. In particular, water is loaded into the sample chambers of diamond-cells, along with thin metal heaters (1 µm-thick platinum or 20 nm-thick gold on a glass substrate) that drive high-frequency temperature oscillations (20 Hz to 600 kHz; 1 to 10 K). The heaters also act as thermometers via the third-harmonic technique, yielding calorimetric data on (1) heat conduction to the diamonds and (2) heat transport into substrate and sample. Using this method during temperature cycles from 300 to 200 K, we document melting, freezing, and proton ordering and disordering transitions of H2O at 0 to 9 GPa, and characterize changes in thermal conductivity and heat capacity across these transitions. The technique and analysis pave the way for calorimetry experiments on any non-metal at pressures up to ~100 GPa, provided a thin layer (several µm-thick) of thermal insulation supports a metallic thin-film (tens of nm thick) Joule-heater attached to low contact resistance leads inside the sample chamber of a diamond-cell. [ABSTRACT FROM AUTHOR]

Part I shows that quantitative measurements of heat capacity are theoretically possible inside diamond anvil cells via high-frequency Joule heating (100 kHz-10MHz), opening up the possibility of new methods to detect and characterize transformations at high-pressure such as the glass transitions, melting, magnetic orderings, and the onset of superconductivity. Here, we test the possibility outlined in Part I, using prototypes and detailed numerical models. First, a coupled electrical-thermal numerical model shows that specific heat of metals inside diamond cells can be measured directly using ~1MHz frequency, with <10% accuracy. Second, we test physical models of high-pressure experiments, i.e., diamond-cell mock-ups. Metal foils of 2-6 μm-thickness are clamped between glass insulation inside diamond anvil cells. Fitting data from 10 Hz to ~30 kHz, we infer the specific heat capacities of Fe, Pt, and Ni with ±20%-30% accuracy. The electrical test equipment generates -80 dBc spurious harmonics, which overwhelm the thermally induced harmonics at higher frequencies, disallowing the high precision expected from numerical models. An alternative Joule-heating calorimetry experiment, on the other hand, does allow absolute measurements with <10% accuracy, despite the -80 dBc spurious harmonics: the measurement of thermal effusivity, √ρck (ρ, c, and k being density, specific heat, and thermal conductivity), of the insulation surrounding a thin-film heater. Using a ~50 nm-thick Pt heater surrounded by glass and 10 Hz-300 kHz frequency, we measure thermal effusivity with ±6% accuracy inside the sample chamber of a diamond anvil cell. [ABSTRACT FROM AUTHOR]

Numerical simulations of heat transport in diamond anvil cells reveal a possibility for absolute measurements of specific heat via high-frequency modulation calorimetry. Such experiments could reveal and help characterize temperature-driven phase transitions at high-pressure, such as melting, the glass transition, magnetic and electric orderings, or superconducting transitions. Specifically, we show that calorimetric information of a sample cannot be directly extracted from measurements at frequencies slower than the timescale of conduction to the diamond anvils (10 s-100 s of kHz) since the experiment is far from adiabatic. At higher frequencies, laser-heating experiments allow relative calorimetric measurements, where changes in specific heat of the sample are discriminated from changes in other material properties by scanning the heating frequency from ~1 MHz to 1 GHz. But laser-heating generates large temperature gradients in metal samples, preventing absolute heat capacities to be inferred. High-frequency Joule heating, on the other hand, allows accurate, absolute specific heat measurements if it can be performed at high-enough frequency: assuming a thin layer of KBr insulation, the specific heat of a 5 μm-thick metal sample heated at 100 kHz, 1MHz, or 10MHz frequency would be measured with 30%, 8%, or 2% accuracy, respectively. [ABSTRACT FROM AUTHOR]

Many high-pressure high-temperature studies using laser-heated diamond cells have documented plateaus in the increase of temperature with increasing laser power or with time. By modeling heat transfer in typical laser-heated diamond anvil cell experiments, we demonstrate that latent heat due to melting or other phase transformation is unlikely to be the source of observed plateaus in any previously published studies, regardless of whether pulsed or continuous lasers were used. Rather, large increases (∼10-fold) in thermal conductivity can explain some of the plateaus, and modest increases in reflectivity (tens of percent) can explain any or all of them. Modeling also shows that the sub-microsecond timescale of heating employed in recent pulsed heating experiments is fast enough compared to heat transport into and through typical insulations, but too slow compared to heat transport into metallic laser absorbers themselves to allow the detection of a large plateau due to latent heat of fusion. Four new designs are suggested for future experiments that could use the simple observation of a latent heat-induced plateau to provide reliable high-pressure melting data. [ABSTRACT FROM AUTHOR]

Abstract: Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen‐rich simple compounds having new clathrate‐like structures and remarkable electronic properties including room‐temperature superconductivity. X‐ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH10 having cages of thirty‐two hydrogen atoms surrounding each La atom. Upon decompression, the fcc‐based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H−H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized. [ABSTRACT FROM AUTHOR]

(a) Schematic cross-section of experimental setup, including electron backscatter image of sample recovered from high pressure and sliced in half. The pyrolite sample is coated, insulated and compressed to 80 GPa in a diamond anvil cell. Lasers 1 and 2 are continuous and pulsed lasers, respectively, which heat the pyrolite sample to the temperature of interest. (b) Measured temperature evolution from left side (red) and right side (blue) at 80 GPa during the heating experiment represented in (a), along with modeled temperature evolutions (black). • Lattice thermal conductivity of pyrolite is between 2.8 and 5.3 W/m/K at 80 GPa, 2000 to 2500 K. • Lattice thermal conductivity of pyrolite is between 3.6 and 9.9 W/m/K at 124 GPa, 2000 to 3000 K. • Measurements combined diamond anvil cells, continuous and pulsed laser heating, and spectroradiometry. • Data analysis used finite element modeling. Knowledge of thermal conductivity of mantle minerals is crucial for understanding heat transport from the Earth's core to mantle. At the pressure-temperature conditions of the Earth's core-mantle boundary, calculations of lattice thermal conductivity based on atomistic models have determined values ranging from 1 to 14 W/m/K for bridgmanite and bridgmanite-rich mineral assemblages. Previous studies have been performed at room temperature up to the pressures of the core-mantle boundary, but correcting these to geotherm temperatures may introduce large errors. Here we present the first measurements of lattice thermal conductivity of mantle minerals up to pressures and temperatures near the base of the mantle, 120 GPa and 2500 K. We use a combination of continuous and pulsed laser heating in a diamond anvil cell to measure the lattice thermal conductivity of pyrolite, the assemblage of minerals expected to make up the lower mantle. We find a value of 3.9 + 1.4 − 1.1 W/m/K at 80 GPa and 2000 to 2500 K and 5.9 + 4.0 − 2.3 W/m/K at 124 GPa and 2000 to 3000 K. These values rule out the highest calculations of thermal conductivity of the Earth's mid-lower mantle (i.e. k < 6 W/m/K at 80 GPa), and are consistent with both the high and low calculations of thermal conductivity near the base of the lower mantle. [ABSTRACT FROM AUTHOR]

The insulator‐to‐metal transition in dense fluid hydrogen is an essential phenomenon in the study of gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using direct fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, an onset of metal‐like reflectance is observed in the visible spectral range at P >150 GPa and T ≥ 3000 K. The reflectance increases rapidly with decreasing photon energy indicating free‐electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectance spectra also suggest much longer electronic collision time (≥1 fs) than previously inferred, implying that metallic hydrogen at the conditions studied is not in the regime of saturated conductivity (Mott–Ioffe–Regel limit). The results confirm the existence of a semiconducting intermediate fluid hydrogen state en route to metallization. [ABSTRACT FROM AUTHOR]

Zaghoo et al. [Phys. Rev. B 93, 155128 (2016)] report on the observation of a first-order phase transition to metallic hydrogen at pressures of 100-170 GPa and high temperatures of 1100-1800 K. Here, based on the analysis of their optical spectroscopy data and finite element calculations, we show that the presented data do not support the existence of such a transition at their claimed Tc and are likely related to a continuous band-gap reduction. [ABSTRACT FROM AUTHOR]

FeSi remains crystalline up to at least 2350 (±200) K at 23 GPa and 2770 (±200) K at 47 GPa in a laser-heated diamond-anvil cell, showing that addition of silicon does not cause a large amount of melting point depression; the melting temperature of pure iron ranges from 2300 (±100) K to 2700 (±150) K between 20 and 50 GPa. The transition between ε (B20) and B2 (CsCl-structured) crystalline phases occurs at 30 (±2) GPa at all temperatures from 1200 to 2400 K. The resulting 5% density increase may cause an increase in the miscibility of silicon in iron at P > 30 GPa, with potential implications for the cores of small rocky planets such as Mars and Mercury. [ABSTRACT FROM AUTHOR]

We performed optical absorption and photoluminescence experiments under high pressure up to 10 GPa on two good quality InGaN epilayers with ∼40% indium. The pressure coefficient of about 30 meV/GPa for the absorption edge is close to the bandgap pressure coefficients of InN and GaN, indicating similar pressure dependence of the fundamental band gap in the whole composition range. In contrast, the pressure coefficient of the photoluminescence peak energy shows much weaker pressure dependence which we attribute to an increasing role of highly localized defects when the conduction band approaches the Fermi level stabilization energy at higher indium contents. [ABSTRACT FROM AUTHOR]

Recent predictions and experimental observations of high Tc superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including four-probe electrical transport measurements that display significant drops in resistivity on cooling up to 260 K and 180-200 GPa, and resistivity transitions at both lower and higher temperatures in other experiments. Additional current-voltage measurements, critical current estimates, and low-temperature x-ray diffraction are also obtained. We suggest that the transitions represent signatures of superconductivity to near room temperature in phases of lanthanum superhydride, in good agreement with density functional structure search and BCS theory calculations. [ABSTRACT FROM AUTHOR]

The insulator-to-metal transition in dense fluid hydrogen is an essential phenomenon in the study of gas giant planetary interiors and the physical and chemical behavior of highly compressed condensed matter. Using direct fast laser spectroscopy techniques to probe hydrogen and deuterium precompressed in a diamond anvil cell and laser heated on microsecond timescales, an onset of metal-like reflectance is observed in the visible spectral range at P >150 GPa and T ≥ 3000 K. The reflectance increases rapidly with decreasing photon energy indicating free-electron metallic behavior with a plasma edge in the visible spectral range at high temperatures. The reflectance spectra also suggest much longer electronic collision time (≥1 fs) than previously inferred, implying that metallic hydrogen at the conditions studied is not in the regime of saturated conductivity (Mott-Ioffe-Regel limit). The results confirm the existence of a semiconducting intermediate fluid hydrogen state en route to metallization., Competing Interests: The authors declare no conflict of interest., (© 2019 The Authors. Published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.)