Dehydration kinetics of nanoconfined water in beryl probed by high temperature single crystal synchrotron X-ray diffraction.

Understanding changes in material properties through external stimuli plays a key role in validating the expected performance of materials and engineering material properties in a controlled manner. Here, we introduce a fundamental protocol to deduce dehydration reactions kinetics of water confined in nanopore channels, with the cyclosilicate beryl as the scaffold of interest, using time-resolved synchrotron X-ray diffraction (SXRD), in the temperature interval of 298-1038 K. The temperature-dependent intensity ( I ) of the strongest reflection (112) was used as the crystallite variable. An estimation of an isobaric thermal crystallite coefficient, k , analogous with the isobaric thermal expansion coefficient, established the rate of relative crystallization as a function of temperature, ∂ I ∂ T . A plot of lnk and 1 T gives rise to two kinetic steps, indicating a slow dehydration stage up to ~ 700 K and a fast dehydration stage up to the investigated temperature 1038 K. The crystal structure of beryl determined up to 1038 K, in temperature increment as small as 10 K, indicates the presence of channel ions Na and Fe and a gradual decrease of water upon heating.

Enhancing the electrical properties of graphite nanoflake through gamma-ray irradiation.

Understanding changes in material properties through external stimuli is critical to validating the expected performance of materials as well as engineering material properties in a controlled manner. Here, we investigate a change in the c-axis electrical properties of graphite nanoflakes (GnFs) induced by gamma-ray irradiation, using conductive probe atomic force microscopy (CP-AFM). The fundamentals behind the change in their electrical properties are elucidated by analyzing the interlayer spacing, graphitization, and morphology. An increase in gamma-ray irradiation dose for GnFs leads to an exponential increase in the electrical conductance and a gradual decrease in the interlayer spacing, while accompanying indistinguishable changes in their morphology. Our experimental results suggest that the c-axis electrical conductance enhancement of GnFs with gamma-ray irradiation might be attributed to a reduction in interlayer spacing, though the created defects may also play a role. This study demonstrates that gamma-ray irradiation can be a promising route to tailor the electrical properties of GnFs.

Accurate crystal structure of ice VI from X-ray diffraction with Hirshfeld atom refinement.

Water is an essential chemical compound for living organisms, and twenty of its different crystal solid forms (ices) are known. Still, there are many fundamental problems with these structures such as establishing the correct positions and thermal motions of hydrogen atoms. The list of ice structures is not yet complete as DFT calculations have suggested the existence of additional and - to date - unknown phases. In many ice structures, neither neutron diffraction nor DFT calculations nor X-ray diffraction methods can easily solve the problem of hydrogen atom disorder or accurately determine their anisotropic displacement parameters (ADPs). Here, accurate crystal structures of H2O, D2O and mixed (50%H2O/50%D2O) ice VI obtained by Hirshfeld atom refinement (HAR) of high-pressure single-crystal synchrotron and laboratory X-ray diffraction data are presented. It was possible to obtain O-H/D bond lengths and ADPs for disordered hydrogen atoms which are in good agreement with the corresponding single-crystal neutron diffraction data. These results show that HAR combined with X-ray diffraction can compete with neutron diffraction in detailed studies of polymorphic forms of ice and crystals of other hydrogen-rich compounds. As neutron diffraction is relatively expensive, requires larger crystals which can be difficult to obtain and access to neutron facilities is restricted, cheaper and more accessible X-ray measurements combined with HAR can facilitate the verification of the existing ice polymorphs and the quest for new ones.

Tracing electron density changes in langbeinite under pressure.

Pressure is well known to dramatically alter physical properties and chemical behaviour of materials, much of which is due to the changes in chemical bonding that accompany compression. Though it is relatively easy to comprehend this correlation in the discontinuous compression regime, where phase transformations take place, understanding of the more subtle continuous compression effects is a far greater challenge, requiring insight into the finest details of electron density redistribution. In this study, a detailed examination of quantitative electron density redistribution in the mineral langbeinite was conducted at high pressure. Langbeinite is a potassium magnesium sulfate mineral with the chemical formula [K2Mg2(SO4)3], and crystallizes in the isometric tetartoidal (cubic) system. The mineral is an ore of potassium, occurs in marine evaporite deposits in association with carnallite, halite and sylvite, and gives its name to the langbeinites, a family of substances with the same cubic structure, a tetrahedral anion, and large and small cations. Single-crystal X-ray diffraction data for langbeinite have been collected at ambient pressure and at 1 GPa using a combination of in-house and synchrotron techniques. Experiments were complemented by theoretical calculations within the pressure range up to 40 GPa. On the basis of changes in structural and thermal parameters, all ions in the langbeinite structure can be grouped into 'soft' (potassium cations and oxygens) and 'hard' (sulfur and magnesium). This analysis emphasizes the importance of atomic basins as a convenient tool to analyse the redistribution of electron density under external stimuli such as pressure or temperature. Gradual reduction of completeness of experimental data accompanying compression did not significantly reduce the quality of structural, electronic and thermal parameters obtained in experimental quantitative charge density analysis.

Fe4(OAc)10[EMIM]2: Novel Iron-Based Acetate EMIM Ionic Compound.

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280-325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC-CH3, OH-CO, H3C-CO-CH3, and H3C-O-CO-CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.

Room temperature facile synthesis of olivine-Co2SiO4 nanoparticles utilizing a mechanochemical method.

Co2SiO4 is a ceramic pigment and promising battery material of significant technological interest, as well as a model end-member of one of the most important mineral families in the Earth's crust and upper mantle. All previously developed methods for synthesis of Co2SiO4 require high-temperature processing, which promotes grain growth, while the nanocrystalline form is required for some important technological applications. Here, we report a successful method for synthesizing nanocrystalline Co2SiO4 via a simple and inexpensive high-energy ball milling mechanochemical process. Products of the synthesis were characterized by a combination of XRD and TEM, and their crystal structures and elemental compositions are reported.

Synthesis, Characterization, and Crystal Structures of Two New Manganese Aceto EMIM Ionic Compounds with Chains of Mn2+ Ions Coordinated Exclusively by Acetate.

We synthesized and determined crystal structures of two manganese(II) aceto EMIM coordination compounds with simplified empirical formulas Mn4(OAc)10[EMIM]2 and Mn4(OAc)10[EMIM]2·2H2O. Both compounds feature extended chains of Mn2+ octahedrally coordinated exclusively by acetate anions, which has been observed for the first time. The EMIM moieties and water molecules participate in hydrogen bonding with acetate anions but do not directly interact with the metal cation. Both compounds have melting temperatures around 120 °C and can be considered as (non-room-temperature) ionic liquids. The structural arrangement represented by the two title compounds is robust in terms of accommodating other types of cations and allows for tuning of physical properties of the ionic liquid by means of cation substitution. Thermal analysis results obtained using TGA-DSC and VT IR suggest melting phase transitions around 120 °C, followed by structural rearrangement in the molten state taking place around 140-160 °C. Compounds I and II have a higher thermal stability range compared to [EMIM][OAc] ionic liquid, with an onset decomposition temperature above 260 °C.

Remote Raman spectroscopy of natural rocks.

We report the remote Raman spectra of natural igneous, metamorphic, and sedimentary rock samples at a standoff distance of 5 m. High-quality remote Raman spectra of unprepared rocks are necessary for accurate and realistic analysis of future Raman measurements on planetary surfaces such as Mars. Our results display the ability of a portable compact remote Raman system (CRRS) to effectively detect and isolate various light- and dark-colored mineral phases in natural rocks. The CRRS easily detected plagioclase and potassium feldspar end members, quartz, and calcite in rocks with high fluorescence backgrounds. Intermediate feldspars and quartz, when found in rocks with complex mineralogies, exhibited band shifts and broadening in the ${504{-}510}\,\,{{\rm cm}^{ - 1}}$504-510cm-1 and ${600{-}1200}\,\,{{\rm cm}^{ - 1}}$600-1200cm-1 regions. A good approximation of intermediate plagioclase feldspars was possible by using overall Raman spectral shape and assigning other minor Raman peaks in addition to the $ 504{-}510\,\,{{\rm cm}^{ - 1}}$504-510cm-1 peaks. Detection of olivine and pyroxene in mafic rocks allowed for compositional characterization.

Fast identification of mineral inclusions in diamond at GSECARS using synchrotron X-ray microtomography, radiography and diffraction.

Mineral inclusions in natural diamond are widely studied for the insight that they provide into the geochemistry and dynamics of the Earth's interior. A major challenge in achieving thorough yet high rates of analysis of mineral inclusions in diamond derives from the micrometre-scale of most inclusions, often requiring synchrotron radiation sources for diffraction. Centering microinclusions for diffraction with a highly focused synchrotron beam cannot be achieved optically because of the very high index of refraction of diamond. A fast, high-throughput method for identification of micromineral inclusions in diamond has been developed at the GeoSoilEnviro Center for Advanced Radiation Sources (GSECARS), Advanced Photon Source, Argonne National Laboratory, USA. Diamonds and their inclusions are imaged using synchrotron 3D computed X-ray microtomography on beamline 13-BM-D of GSECARS. The location of every inclusion is then pinpointed onto the coordinate system of the six-circle goniometer of the single-crystal diffractometer on beamline 13-BM-C. Because the bending magnet branch 13-BM is divided and delivered into 13-BM-C and 13-BM-D stations simultaneously, numerous diamonds can be examined during coordinated runs. The fast, high-throughput capability of the methodology is demonstrated by collecting 3D diffraction data on 53 diamond inclusions from Juína, Brazil, within a total of about 72 h of beam time.

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2.

In situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn2Bi2 undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. Puckered Mn-Mn honeycomb layer converts to quasi-one-dimensional (1D) zigzag chains above the phase transition pressure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic 2 orders of magnitude drop of resistivity. Although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as surprisingly, are two resistivity anomalies with opposite pressure dependences, while one of them could be a magnetic transition and the other originates from Fermi surface instability. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn2Bi2 indicates that ferrimagnetism is thermodynamically favored.

Enhanced Néel temperature in EuSnP under pressure.

We present the combined results of single crystal X-ray diffraction, physical properties characterization, and theoretical assessment of EuSnP under high pressure. Single crystals of EuSnP prepared using Sn self-flux crystallize in the tetragonal NbCrN-type crystal structure (S.G. P4/nmm) at ambient pressure. Previous studies have shown that for Eu ions, seven unpaired electrons impart a 2+ oxidation state. Assuming the oxidation states of Eu to be +2 and P to be -3, each Sn will donate one electron, with one p valence electron left for forming a weak Sn-Sn bond. According to the high-pressure single crystal X-ray diffraction measurements, no structural phase transition was observed up to ∼6.2 GPa. Temperature-dependent resistivity measurements up to 2.15 GPa on single crystals indicate that the phase-transition temperature occurring at the Néel temperature (TN) is significantly enhanced under high pressure. The robust crystallography and enhanced antiferromagnetic transition temperatures can be rationalized by the electronic structure calculations and chemical bonding analysis. The increasing Eu-P bonding interaction is consistent with the lattice parameter changing and enhanced TN. Moreover, the molecular orbital diagram shows that the weak Sn-Sn bond can be squeezed under pressure, acting as a compression buffer to stabilize the structure.

Cr2.37Ga3Se8: A Quasi-Two-Dimensional Magnetic Semiconductor.

We present a novel magnetic semiconductor, Cr2.37Ga3Se8, synthesized by partially replacing magnetic Cr3+ in antiferromagnetic Cr5+δSe8 with nonmagnetic Ga3+. The crystal structure of Cr2.37Ga3Se8 was determined by both powder and single-crystal X-ray diffraction. The title compound crystallizes in a monoclinic structure with space group C2/ m (No. 12). In Cr2.37Ga3Se8, the Cr atoms are surrounded by 6 Se atoms and form filled octahedral clusters, while Ga atoms are centered in the Se4 tetrahedral clusters. The two kinds of clusters pack alternatingly along the c-axis, which results in a quasi-two-dimensional layered structure. The magnetization ( M) measurement shows the development of short-range ferromagnetic coupling below the Curie-Weiss (CW) temperature θCW ∼ 92 K, evidenced by the nonlinear field dependence of M. However, the magnetic susceptibility exhibits a peak at low fields at ∼18 K, indicating the existence of an antiferromagnetic interaction as well. Electronic structure calculations using the WIEN2k program in the local spin density approximation indicate that the magnetism arises exclusively from local moments of the Cr atoms. The electrical resistivity measurement of the Cr2.37Ga3Se8 sample confirms that this material is a semiconductor with the band gap ∼0.26 eV. Meanwhile, the experimental band gap (∼0.26 eV) is close to the theoretical prediction using the WIEN2k program (∼0.35 eV).

High Pressure Single Crystal Diffraction at PX2.

In this report we describe detailed procedures for carrying out single crystal X-ray diffraction experiments with a diamond anvil cell (DAC) at the GSECARS 13-BM-C beamline at the Advanced Photon Source. The DAC program at 13-BM-C is part of the Partnership for Extreme Xtallography (PX^2) project. BX-90 type DACs with conical-type diamond anvils and backing plates are recommended for these experiments. The sample chamber should be loaded with noble gas to maintain a hydrostatic pressure environment. The sample is aligned to the rotation center of the diffraction goniometer. The MARCCD area detector is calibrated with a powder diffraction pattern from LaB6. The sample diffraction peaks are analyzed with the ATREX software program, and are then indexed with the RSV software program. RSV is used to refine the UB matrix of the single crystal, and with this information and the peak prediction function, more diffraction peaks can be located. Representative single crystal diffraction data from an omphacite (Ca0.51Na0.48)(Mg0.44Al0.44Fe2+0.14Fe3+0.02)Si2O6 sample were collected. Analysis of the data gave a monoclinic lattice with P2/n space group at 0.35 GPa, and the lattice parameters were found to be: a = 9.496 ±0.006 Å, b = 8.761 ±0.004 Å, c = 5.248 ±0.001 Å, ß = 105.06 ±0.03º, α = γ = 90º.

Hydrogen-Stuffed, Quartz-like Water Ice.

Through use of in situ Raman spectroscopy and single-crystal/powder X-ray diffraction, we resolve the "C0" phase structure discovered recently in the H2 + H2O system. This phase forms at ∼400 MPa and 280 K with the nominal composition (H2O)2H2 and three formula units per unit cell. The hexagonal structure is chiral, consisting of interpenetrating spiral chains of hydrogen-bonded water molecules and rotationally disordered H2 molecules, and shows topological similarities with the mineral quartz. Like other clathrate hydrates and forms of ice, the protons of H2O molecules within C0 are disordered. The large zeolite-like channels accommodate significant amounts of hydrogen (5.3% by weight) in a unique hydrogen-bonded lattice, which might be applicable to the thermodynamic conditions found on icy planetary bodies.

Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites.

The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.

High-pressure phase transition makes B4.3C boron carbide a wide-gap semiconductor.

Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and inter-relation of atomic distances, optical properties and Raman-active phonons up to ~70 GPa. The anomalous pressure evolution of the gap width to higher energies is striking. This is obtained from observations of transparency, which most rapidly increases around 55 GPa. Full visible optical transparency is approached at pressures of >60 GPa indicating that the band gap reaches ~3.5 eV; at high pressure, boron carbide is a wide-gap semiconductor. The reason is that the high concentration of structural defects controlling the electronic properties of boron carbide at ambient conditions initially decreases and finally vanishes at high pressures. The structural parameters and Raman-active phonons indicate a pressure-dependent phase transition in single-crystal (nat)B4.3C boron carbide near 40 GPa, likely related to structural changes in the C-B-C chains, while the basic icosahedral structure appears to be less affected.

Structure and stability of solid Xe(H2)n.

Mixtures of xenon and molecular hydrogen form a series of hexagonal, van der Waals compounds at high pressures and at 300 K. Synchrotron, x-ray, single crystal diffraction studies reveal that below 7.5 GPa, Xe(H2)8 crystallizes in a P3̄m1 structure that displays pressure-induced occupancy changes of two pairs of xenon atoms located on the 2c and 2d sites (while the third pair on yet another 2c site remains fully occupied). The occupancy becomes 1 at the P3̄m1 to R3 transition and all the xenon atoms occupy the 3d sites in the high-pressure structure. These pressure-induced changes in occupancy coincide with volume changes that maintain the average Xe:H2 stoichiometry fixed at 1:8. The synchrotron x-ray diffraction and Raman measurements show that this unique hydrogen-bearing compound that can be synthesized at 4.2 GPa and 300 K, quenched at low temperatures to atmospheric pressure, and retained up to 90 K on subsequent warming.

The first X-ray diffraction measurements on Mars.

The Mars Science Laboratory landed in Gale crater on Mars in August 2012, and the Curiosity rover then began field studies on its drive toward Mount Sharp, a central peak made of ancient sediments. CheMin is one of ten instruments on or inside the rover, all designed to provide detailed information on the rocks, soils and atmosphere in this region. CheMin is a miniaturized X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument that uses transmission geometry with an energy-discriminating CCD detector. CheMin uses onboard standards for XRD and XRF calibration, and beryl:quartz mixtures constitute the primary XRD standards. Four samples have been analysed by CheMin, namely a soil sample, two samples drilled from mudstones and a sample drilled from a sandstone. Rietveld and full-pattern analysis of the XRD data reveal a complex mineralogy, with contributions from parent igneous rocks, amorphous components and several minerals relating to aqueous alteration. In particular, the mudstone samples all contain one or more phyllosilicates consistent with alteration in liquid water. In addition to quantitative mineralogy, Rietveld refinements also provide unit-cell parameters for the major phases, which can be used to infer the chemical compositions of individual minerals and, by difference, the composition of the amorphous component.

Synthesis and microdiffraction at extreme pressures and temperatures.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 (1) will be shown.