Crystal structure determination at extreme pressures is currently possible at synchrotron beamlines optimized for such a purpose. We report the description of the experimental setup available at ...European Synchrotron Radiation Facility ID09 beamline (Grenoble, France) and, with two examples, we illustrate the state-of-the-art experiments currently performed at third-generation synchrotrons. The first example concerns the determination of the equation of state and the structural behavior of low-spin Fe-bearing siderite in the megabar pressure range. Siderite, in fact, undergoes a first-order isosymmetric transition at 45 GPa, and, above this pressure, it features Fe
2+
in electronic low-spin configuration. The local configuration of Fe coordination polyhedra, determined by structural refinements, significantly deviates from a regular octahedron. Nevertheless, no further structural transition is detected up to the maximum pressure reached in our experiments, 135 GPa. The analysis of the Fe-O bond length extrapolated to ambient pressure, which indicates that the difference in ionic radii between the high- and the low-spin state of Fe
2+
is 0.172 Å, in excellent agreement with the tabulated data by Shannon and Prewitt Effective ionic radii in oxides and fluorides. Acta Crystallogr. 1969;B25:925-946. The second example concerns the determination and refinement of the oP8 structure adopted by sodium in the pressure interval 118-125 GPa, using an experimental dataset collected at 118 GPa. The orthorhombic a=4.7687(15) Å, b=3.0150(6) Å, c=5.2423(7) Å, V=75.4(3) Å
3
oP8 structure is topologically related to the MnP structure, with two non-equivalent atoms in the unit cell. Despite the weak scattering factor of Na atoms, the quality of the data also allows meaningful displacement parameters refinements (R
1
=4.6%, 14 parameters, 190 diffractions, and 105 unique) demonstrating that the current accuracy of diffraction data at extreme pressures can be comparable with ambient condition measurements.
In this study the first in situ high‐pressure single‐crystal X‐ray diffraction experiments at Xpress, the Indo‐Italian beamline of the Elettra synchrotron, Trieste (Italy), are reported. A ...description of the beamline experimental setup and of the procedures for single‐crystal centring, data collection and processing, using diamond anvil cells, are provided. High‐pressure experiments on a synthetic crystal of clinoenstatite (MgSiO3), CaCO3 polymorphs and a natural sample of leucophoenicite Mn7Si3O12(OH)2 validated the suitability of the beamline experimental setup to: (i) locate and characterize pressure‐induced phase transitions; (ii) solve ab initio the crystal structure of high‐pressure polymorphs; (iii) perform fine structural analyses at the atomic scale as a function of pressure; (iv) disclose complex symmetry and structural features undetected using conventional X‐ray sources.
The experimental setup of the Xpress beamline at the Elettra synchrotron (Trieste, Italy) is described, along with the procedures for single‐crystal centring and X‐ray diffraction data collection and processing. The reported results demonstrate the possibility to perform in situ high‐pressure single‐crystal X‐ray diffraction experiments at the Xpress beamline.
Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements ...demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.
Abstract
High-pressure high-temperature syntheses that involve volatile-bearing aqueous fluids are typically accomplished by enclosing the samples in gas-tight welded shut noble-metal capsules, from ...which the bulk volatile content must be extracted to be analyzed with mass spectroscopy, hence making the analysis non-replicable. Here we describe a novel non-destructive method that ensures the identification and the quantitative estimate of the volatiles directly in the sealed capsule, focusing on fluid H
2
O–CO
2
mixtures equilibrated with graphite at conditions of geological interest (1 GPa, 800 °C). We used a high-energy (77 keV) synchrotron X-ray radiation combined with a cryostat to produce X-ray diffraction patterns and X-ray diffraction microtomographic cross-sections of the volatile-bearing samples down to –180 °C, thus encompassing the conditions at which crystalline phases-solid CO
2
and clathrate (CO
2
hydrate)-form. The uncertainty of the method is < 15 mol%, which reflects the difference between the volatile proportion estimated by both Rietveld refinement of the diffraction data and by image analysis of the microtomograms, and the reference value measured by quadrupole mass spectrometry. Therefore, our method can be reliably applied to the analysis of frozen H
2
O–CO
2
mixtures and, moreover, has the potential to be extended to experimental fluids of geological interest containing other volatiles, such as CH
4
, SO
2
and H
2
S.
Abstract
Synchrotron X-ray tomography enables the examination of the internal structure of materials at submicron spatial resolution and subsecond temporal resolution. Unavoidable experimental ...constraints can impose dose and time limits on the measurements, introducing noise in the reconstructed images. Convolutional neural networks (CNNs) have emerged as a powerful tool to remove noise from reconstructed images. However, their training typically requires collecting a dataset of paired noisy and high-quality measurements, which is a major obstacle to their use in practice. To circumvent this problem, methods for CNN-based denoising have recently been proposed that require no separate training data beyond the already available noisy reconstructions. Among these, the Noise2Inverse method is specifically designed for tomography and related inverse problems. To date, applications of Noise2Inverse have only taken into account 2D spatial information. In this paper, we expand the application of Noise2Inverse in space, time, and spectrum-like domains. This development enhances applications to static and dynamic micro-tomography as well as X-ray diffraction tomography. Results on real-world datasets establish that Noise2Inverse is capable of accurate denoising and enables a substantial reduction in acquisition time while maintaining image quality.
The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth's lower mantle. However, their range of stability, ...crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth's geotherm at pressures to ∼50 GPa FeCO
partially dissociates to form various iron oxides. At higher pressures FeCO
forms two new structures-tetrairon(III) orthocarbonate Fe
C
O
, and diiron(II) diiron(III) tetracarbonate Fe
Fe
C
O
, both phases containing CO
tetrahedra. Fe
C
O
is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth's lower mantle.
Carbon-bearing solids, fluids, and melts in the Earth's deep interior may play an important role in the long-term carbon cycle. Here we apply synchrotron X-ray single crystal micro-diffraction ...techniques to identify and characterize the high-pressure polymorphs of dolomite. Dolomite-II, observed above 17 GPa, is triclinic, and its structure is topologically related to CaCO₃-II. It transforms above 35 GPa to dolomite-III, also triclinic, which features carbon in 3 + 1 coordination at the highest pressures investigated (60 GPa). The structure is therefore representative of an intermediate between the low-pressure carbonates and the predicted ultra-high pressure carbonates, with carbon in tetrahedral coordination. Dolomite-III does not decompose up to the melting point (2,600 K at 43 GPa) and its thermodynamic stability demonstrates that this complex phase can transport carbon to depths of at least up to 1,700 km. Dolomite-III, therefore, is a likely occurring phase in areas containing recycled crustal slabs, which are more oxidized and Ca-enriched than the primitive lower mantle. Indeed, these phases may play an important role as carbon carriers in the whole mantle carbon cycling. As such, they are expected to participate in the fundamental petrological processes which, through carbon-bearing fluids and carbonate melts, will return carbon back to the Earth's surface.
Three single-crystals of magnesium silicate perovskite with differing chemical compositions have been studied by means of synchrotron X-ray diffraction in diamond anvil cells with He as pressure ...transmitting medium from room pressure up to 75GPa. In addition to the end-member MgSiO3 composition, a perovskite containing 4mol% of the Fe2+SiO3 component (Mg,Fe)SiO3 and one containing 37mol% of an Fe3+AlO3 component (Mg,Fe)(Al,Si)SiO3 were investigated. The high-quality of the collected data allows a detailed examination of the effect of different chemical substitutions on the compression mechanism of perovskite and on its equation of state (EoS). The bulk modulus and first pressure derivative determined for MgSiO3 perovskite obtained by fitting a 3rd-order Birch–Murnaghan are found to be quite insensitive to the maximum pressure to which the data are fitted. The EoS parameters obtained by fitting data from room pressure to 10, 40 or 75GPa are almost identical. This is not the case, however, for either (Mg,Fe)SiO3 or (Mg,Fe)(Al,Si)SiO3 perovskites, for which volumes calculated from an EoS obtained from fitting data up to 40GPa deviate from the experimental data above 40GPa. In the case of (Mg,Fe)SiO3 perovskite this deviation appears to be related to a change in octahedral tilting during compression, as revealed by analysis of the lattice strain variation with pressure. The tilting change is a likely consequence of a high-spin to intermediate spin transition of Fe2+ but the effect on the density and bulk modulus is almost negligible and unlikely to cause seismically observable changes in the mantle. In the case of (Mg,Fe)(Al,Si)SiO3 perovskite, the deviation is clearly due to a change in the compressibility of the c-axis and no evidence for effects due to a change in Fe spin state is observed. Substitution of Fe2+SiO3 has a significant negative effect on the bulk sound velocity while increasing density, whereas the effect of Fe3+AlO3 substitution on both the bulk sound velocity and density are in the same direction but more modest. The latter substitution may, therefore, be more compatible with some aspects of seismic anomalies observed at the base of the lower mantle.
► This is the first study of silicate perovskite single-crystals up to 75GPa. ► The peculiar axial compressibility of Fe,Al-pv explain different K0 values reported. ► High-quality data reveal the small effect of Fe2+ spin transition on compressibility. ► Fe2+ substitution has a large pressure dependent effect on bulk sound velocity. ► Fe,Al substitution may explain observed seismic anomalies in the lower mantle.
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