Lars Ehm

The structure of ferrihydrite, a nanocrystalline material.

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P63mc; a = ∼5.95 angstroms, c = ∼9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis δ-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

Pressure-Induced Disordered Substitution Alloy in Sb2Te3

A new type of disordered substitution alloy of Sb and Te at above 15.1 GPa was discovered by performing in situ high-pressure angle-dispersive X-ray diffraction experiments on antimony telluride (Sb(2)Te(3)), a topological insulator and thermoelectric material, at room temperature. In this disordered substitution alloy, Sb(2)Te(3) crystallizes into a monoclinic structure with the space group C2/m, which is different from the corresponding high-pressure phase of the similar isostructural compound Bi(2)Te(3). Above 19.8 GPa, Sb(2)Te(3) adopts a body-centered-cubic structure with the disordered atomic array in the crystal lattice. The in situ high-pressure experiments down to about 13 K show that Sb(2)Te(3) undergoes the same phase-transition sequence with increasing pressure at low temperature, with almost the same phase-transition pressures.

High-pressure phase transitions, amorphization, and crystallization behaviors in Bi2Se3

The phase transition, amorphization, and crystallization behaviors of the topological insulator bismuth selenide (Bi2Se3) were discovered by performing in situ high-pressure angle-dispersive x-ray diffraction experiments during an increasing, decreasing, and recycling pressure process. In the compression process, Bi2Se3 transforms from the original rhombohedral structure (phase I(A)) to a monoclinic structure (phase II) at about 10.4 GPa, and further to a body-centered tetragonal structure (phase III) at about 24.5 GPa. When releasing pressure to ambient conditions after the complete transformation from phase II to III, Bi2Se3 becomes an amorphous solid (AM). In the relaxation process from this amorphous state, Bi2Se3 starts crystallizing into an orthorhombic structure (phase I(B)) about five hours after releasing the pressure to ambient. A review of the pressure-induced phase transition behaviors of A2B3-type materials composed from the V and VI group elements is presented.

Studies of Local and Intermediate Range Structure in Crystalline and Amorphous Materials at High Pressure Using High-Energy X-rays

The method of high-energy total elastic X-ray scattering to determine the atomic structure of nanocrystalline, highly disordered, and amorphous materials is presented. The current state of the technique, its potential, and limitations are discussed with two successful studies on the pressure induced phase transition in mackinawite (FeS) and the high-pressure behavior of liquid gallium.

Maskelynite formation via solid-state transformation: Evidence of infrared and X-ray anisotropy

We present the results of a combined study of shocked labradorite from the Lonar crater, India, using optical microscopy, micro-Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, high-energy X-ray total scattering experiments, and micro-Fourier transform infrared (micro-FTIR) spectroscopy. We show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase. However, there are slight but significant differences—preservation of original preimpact igneous zoning, anisotropy at infrared wavelengths, X-ray anisotropy, and preservation of some intermediate range order—which are all consistent with a solid-state transformation from plagioclase to maskelynite.

β‐diopside, a new ultrahigh‐pressure polymorph of CaMgSi2O6with six‐coordinated silicon

Minerals containing silicon in four-fold coordination (IVSi4+) are common in crustal rocks, while those involving six-coordinated silicon (VISi4+) dominate the Earths lower mantle and determine its properties. Here we show a new type of phase transition determined by single-crystal high pressure X-ray diffraction experiments in a diamond anvil cell (DAC) using natural diopside (CaMgSi2O6), the archetypic member of clinopyroxene family, and one of the most abundant minerals of the Earths upper mantle. Above 50 GPa at ambient temperature diopside transforms to a previously unknown post-clinopyroxene phase,β-diopside, with half of the tetrahedralIVSi4+ layers converted to octahedral VISi4+coordination. This phase is most probably a metastable state that is kinetically accessible at room temperature and the transformation is fully reversible on decompression. This new type of phase transition provides important clues to the exact mechanisms of breakdown of clinopyroxene in the Earths mantle and may be expected to take place in other pyroxenes at pressures higher than previously explored.

High pressure and Multiferroics materials. A happy marriage

The effect of high pressure on the synthesis, properties and atomic structure of multiferroic materials with perovskite or perovskite-related structures is reviewed.

Structural changes in nanocrystalline mackinawite (FeS) at high pressure

The high-pressure behavior of nanocrystalline mackinawite (FeS) with particle sizes of 6, 7 and 8 nm has been investigated by high-energy X-ray total scattering and pair distribution function analysis. An irreversible first-order structural phase transition from tetragonal mackinawite to orthorhombic FeS-II was observed at about 3 GPa. The transition is induced by the closure of the van der Waals gap in the layered mackinawite structure. A grain size effect on the transition pressure and the compressibility was observed.

Polyhedral units and network connectivity in GeO2 glass at high pressure: An X-ray total scattering investigation

We report a pressure-induced dense tetrahedral intermediate state via Ge–O–Ge rotation formed at 3–5 GPa and the polyhedral relations in GeO2 glass up to 17.5 GPa using in situ X-ray total scattering and X-ray absorption (XAFS) techniques. It was found that the nearest-neighbor Ge-Ge correlations show a decrease reaching a minimum between 4 and 6 GPa, and exhibit negative compression behavior at 7–17.5 GPa. The Ge–Ge distance determined by XAFS shows a substantial reduction, i.e., normal compression behavior, at 7–17.5 GPa. The comparison with the theoretical g(r) function for rutile-type GeO2 (16.1 GPa) indicates that the negative compression of intermediate range order reflects the direct formation of GeO6 octahedral units. Results of coordination number analysis show that GeO2 glass undergoes a transition from tetrahedral GeO4, to GeO5 units (possibly triangular bipyramidal), and finally to octahedral GeO6 units. The present investigation provides the structural details of the polyhedral units and thei...

High-energy X-ray focusing and applications to pair distribution function investigation of Pt and Au nanoparticles at high pressures.

We report development of micro-focusing optics for high-energy x-rays by combining a sagittally bent Laue crystal monchromator with Kirkpatrick-Baez (K–B) X-ray focusing mirrors. The optical system is able to provide a clean, high-flux X-ray beam suitable for pair distribution function (PDF) measurements at high pressure using a diamond anvil cell (DAC). A focused beam of moderate size (10–15 μm) has been achieved at energies of 66 and 81 keV. PDF data for nanocrystalline platinum (n-Pt) were collected at 12.5 GPa with a single 5 s X-ray exposure, showing that the in-situ compression, decompression, and relaxation behavior of samples in the DAC can be investigated with this technique. PDFs of n-Pt and nano Au (n-Au) under quasi-hydrostatic loading to as high as 71 GPa indicate the existence of substantial reduction of grain or domain size for Pt and Au nanoparticles at pressures below 10 GPa. The coupling of sagittally bent Laue crystals with K–B mirrors provides a useful means to focus high-energy synchrotron X-rays from a bending magnet or wiggler source.