Alexander Kurnosov

BX90: A new diamond anvil cell design for X-ray diffraction and optical measurements

We present a new design of a universal diamond anvil cell, suitable for different kinds of experimental studies under high pressures. Main features of the cell are an ultimate 90-degrees symmetrical axial opening and high stability, making the presented cell design suitable for a whole range of techniques from optical absorption to single-crystal X-ray diffraction studies, also in combination with external resistive or double-side laser heating. Three examples of the cell applications are provided: a Brillouin scattering of neon, single-crystal X-ray diffraction of α-Cr(2)O(3), and resistivity measurements on the (Mg(0.60)Fe(0.40))(Si(0.63)Al(0.37))O(3) silicate perovskite.

Phase Diagram and High-Pressure Boundary of Hydrate Formation in the Carbon Dioxide−Water System

Experimental investigation of the phase diagram of the system carbon dioxide-water at pressures up to 2.7 GPa has been carried out in order to explain earlier controversial results on the decomposition curves of the hydrates formed in this system. According to X-ray diffraction data, solid and/or liquid phases of water and CO2 coexist in the system at room temperature within the pressure range from 0.8 to 2.6 GPa; no clathrate hydrates are observed. The results of neutron diffraction experiments involving the samples with different CO2/H2O molar ratios, and the data on the phase diagram of the system carbon dioxide-water show that CO2 hydrate of cubic structure I is the only clathrate phase present in this system under studied P-T conditions. We suppose that in the cubic structure I hydrate of CO2 multiple occupation of the large hydrate cavities with CO2 molecules takes place. At pressure of about 0.8 GPa this hydrate decomposes into components indicating the presence of the upper pressure boundary of the existence of clathrate hydrates in the system.

Evidence for a Fe3+-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data

The chemical composition of Earth’s lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements. However, the lack of laboratory data for Earth’s most abundant mineral, (Mg,Fe,Al)(Al,Fe,Si)O3 bridgmanite (also known as silicate perovskite), has hampered any conclusive result. Here we report single-crystal elasticity data on (Al,Fe)-bearing bridgmanite (Mg0.9Fe0.1Si0.9Al0.1)O3 measured using high-pressure Brillouin spectroscopy and X-ray diffraction. Our measurements show that the elastic behaviour of (Al,Fe)-bearing bridgmanite is markedly different from the behaviour of the MgSiO3 endmember. We use our data to model seismic wave velocities in the top portion of the lower mantle, assuming a pyrolitic mantle composition and accounting for depth-dependent changes in iron partitioning between bridgmanite and ferropericlase. We find excellent agreement between our mineral physics predictions and the seismic Preliminary Reference Earth Model down to at least 1,200 kilometres depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about two in shallow-lower-mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths greater than 1,200 kilometres, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle.

Peierls distortion, magnetism, and high hardness of manganese tetraboride

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB4, synthesized under high-pressure, high-temperature conditions. In contrast to superconducting FeB4 and metallic CrB4, which are both orthorhombic, MnB4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 μB /f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

The Sm:YAG primary fluorescence pressure scale

Primary pressure determinations involve the measurement of pressure without recourse to secondary standard materials. These measurements are essential for ensuring the accuracy of pressures measured in gasketed high-pressure devices. In this study, the wavelength of optical fluorescence bands and the density of single crystal Sm-doped yttrium aluminum garnet Y3Al5O12 (Sm:YAG) have been calibrated as a primary pressure scale up to 58 GPa. Absolute pressures were obtained by integrating the bulk modulus determined via Brillouin spectroscopy with respect to volumes measured simultaneously by X-ray diffraction. A third-order Birch-Murnaghan equation of state of Sm:YAG yields V0 = 1735.15(26) A3, KT0 = 185(1.5) GPa, and K` = 4.18(5). The accompanied pressure-induced shifts of the fluorescence lines Y1 and Y2 of Sm:YAG were calibrated to the primary pressure, thus creating a highly accurate fluorescence pressure scale. These shifts are described as P = (A/B) * {[1 + (Δλ/λ0)]B − 1} with A = 2089.91(23.04), B = −4.43(1.07) for Y1, and A = 2578.22(48.70), B = −15.38(1.62) for Y2 bands, where ∆λ = λ − λ0, λ and λ0 are wavelengths in nanometer at pressure and ambient conditions. The sensitivity in the pressure determination of the Sm:YAG fluorescence shift is 0.32 nm/GPa, which is identical to that of the ruby scale. Sm:YAG can be considered elastically isotropic up to 58 GPa, implying insensitivity of the determined pressure to the crystallographic orientation under nonhydrostatic or quasi-hydrostatic conditions. The Sm:YAG fluorescence shift is apparently also independent of crystallographic orientation, in contrast to that of ruby. Since the Y fluorescence band of Sm:YAG is insensitive to temperature changes, this material is highly suitable for the measurement of pressure at elevated temperatures.

Single-crystal X-ray diffraction at extreme conditions: a review

The latest developments in single-crystal X-ray diffraction at high pressure and high temperature are described. Advances in diamond anvil cell designs and X-ray sources allow collecting single-crystal diffraction data at pressures up and above 100 GPa and at temperatures above 1000°C. The technical details of single-crystal X-ray diffraction at high pressure such as the choice of pressure-transmitting media or the different methods for measuring pressures and temperatures have been reviewed. Examples of structural solution of complex structures and new materials, structural refinements of high pressure polymorphs as well as accurate compressibility data are described in order to outline the several advantages of using single crystals instead of powdered samples in high pressure diffraction experiments.

Revised calibration of the Sm:SrB4O7 pressure sensor using the Sm-doped yttrium-aluminum garnet primary pressure scale

The pressure-induced shift of Sm:SrB4O7 fluorescence was calibrated in a quasi-hydrostatic helium medium up to 60 GPa using the recent Sm-doped yttrium-aluminum garnet primary pressure scale as a reference. The resulting calibration can be written as P = −2836/14.3 [(1 + Δλ/685.51)−14.3 − 1]. Previous calibrations based on the internally inconsistent primary scales are revised, and, after appropriate correction, found to agree with the proposed one. The calibration extended to 120 GPa was also performed using corrected previous data and can be written as P = 4.20 Δλ (1 + 0.020 Δλ)/(1 + 0.036 Δλ).

In-situ infrared spectra of hydroxyl in wadsleyite and ringwoodite at high pressure and high temperature

Abstract The infrared spectra of hydroxyl in synthetic hydrous wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4) were measured at room temperature up to ∼18.8 GPa for wadsleyite and up to ∼21.5 GPa for ringwoodite. High-temperature spectra were measured in an externally heated diamond-anvil cell up to 650 °C at ∼14.2 GPa for wadsleyite and up to 900 °C at ∼18.4 GPa for ringwoodite. The synthetic samples reproduce nearly all the important OH bands previously observed at ambient conditions. Only subtle changes were observed in the infrared spectra of both minerals, both upon compression at room temperature and upon heating at high pressure. For wadsleyite, upon compression to ∼18.8 GPa, the frequencies of the bands at ∼3600 cm-1 remain almost unchanged, while the main bands at 3200-3400 cm-1 shift to lower frequencies. During heating at 14.2 GPa to 650 °C the bands at 3200-3400 cm-1 broaden and shift to slightly lower frequencies. For ringwoodite, upon compression to ∼21.5 GPa, the main bands at 3115 cm-1 progressively shift to lower frequencies. During heating at 18.4 GPa to 900 °C, no frequency shift was observed for the band at ∼3700 cm-1, but the band initially at ∼3115 cm-1 shifts very slightly to higher frequencies, which should yield almost the same band positions at 1300-1400 °C as those measured at ambient conditions. Our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions, except perhaps for subtle changes in hydrogen bonding. The low OH-stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large H/D fractionation during degassing of the deep mantle. This may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean.

High-temperature structural behaviors of anhydrous wadsleyite and forsterite

Abstract The thermal expansion of anhydrous Mg2SiO4 wadsleyite and forsterite was comprehensively studied over the temperature ranges 297-1163 and 297-1313 K, respectively, employing X-ray powder diffraction. Experiments were carried out with two separately synthesized samples of wadsleyite (numbered z626 and z627), for which room temperature unit-cell volumes differed by 0.05%, although the determined thermal expansions were identical within error. The high-temperature thermal expansions of wadsleyite and forsterite were parameterized on the basis of the first-order Grüneisen approximation using a Debye function for the internal energy. Values for hypothetical volume at T = 0 K, Debye temperature and Grüneisen parameter are 536.86(14) Å3, 980(55) K, 1.28(2) and 537.00(13) Å3, 887(50) K, 1.26(1) for z626 and z627, respectively, with the bulk modulus fixed to a literature determination of 161 GPa. For forsterite, the respective values are 288.80(2) Å3, 771(9) K, and 1.269(2) with a constrained bulk modulus of 125 GPa. These quantities are in good agreement with literature values obtained independently from sound velocity and heat capacity measurements, giving strong support to the applicability of Grüneisen theory in describing the thermal expansion of wadsleyite and forsterite. In addition, high-temperature structural variations were determined for wadsleyite from Rietveld analysis of the X-ray diffraction data. The pronounced anisotropy in thermal expansion of wadsleyite with a more expandable c-axis, similar to the compressional anisotropy, arises from specific features of the crystal structure consisting of the pseudolayers of MgO6 octahedra parallel to the a-b plane with cross-linking Si2O7 dimers along the c-axis. Although anisotropic compression and expansion originate from the same structural features, the details of structural changes with pressure differ from those caused by temperature. The longest Mg-O bonds, which are roughly parallel to the c-axis in all three octahedral sites of wadsleyite, dominate the compression, but these bonds do not exhibit the largest expansivities.

Compressibility of Gas Hydrates

Experimental data on the pressure dependence of unit cell parameters for the gas hydrates of ethane (cubic structure I, pressure range 0-2 GPa), xenon (cubic structure I, pressure range 0-1.5 GPa) and the double hydrate of tetrahydrofuran+xenon (cubic structure II, pressure range 0-3 GPa) are presented. Approximation of the data using the cubic Birch-Murnaghan equation, P=1.5B(0)[(V(0)/V)(7/3)-(V(0)/V)(5/3)], gave the following results: for ethane hydrate V(0)=1781 Å(3) , B(0)=11.2 GPa; for xenon hydrate V(0)=1726 Å(3) , B(0)=9.3 GPa; for the double hydrate of tetrahydrofuran+xenon V(0)=5323 Å(3) , B(0)=8.8 GPa. In the last case, the approximation was performed within the pressure range 0-1.5 GPa; it is impossible to describe the results within a broader pressure range using the cubic Birch-Murnaghan equation. At the maximum pressure of the existence of the double hydrate of tetrahydrofuran+xenon (3.1 GPa), the unit cell volume was 86% of the unit cell volume at zero pressure. Analysis of the experimental data obtained by us and data available from the literature showed that 1) the bulk modulus of gas hydrates with classical polyhedral structures, in most cases, are close to each other and 2) the bulk modulus is mainly determined by the elasticity of the hydrogen-bonded water framework. Variable filling of the cavities with guest molecules also has a substantial effect on the bulk modulus. On the basis of the obtained results, we concluded that the bulk modulus of gas hydrates with classical polyhedral structures and existing at pressures up to 1.5 GPa was equal to (9±2) GPa. In cases when data on the equations of state for the hydrates were unavailable, the indicated values may be recommended as the most probable ones.