Yong Q. Cai

X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earths interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earths interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earths interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; [3]O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earths lower mantle.

Electronic structure of carbon dioxide under pressure and insights into the molecular-to-nonmolecular transition

Significance In this paper, both oxygen and carbon K-edge spectra of CO2 polymorphs at high pressure and room temperature were reported. The electronic structure of CO2 shows remarkable change from molecular to nonmolecular transition. Furthermore, a 532-eV feature was observed only at a limited pressure range from ∼37 to 53 GPa, suggesting the presence of a transient species in the nonmolecular phase. Knowledge of the high-pressure behavior of carbon dioxide (CO2), an important planetary material found in Venus, Earth, and Mars, is vital to the study of the evolution and dynamics of the planetary interiors as well as to the fundamental understanding of the C–O bonding and interaction between the molecules. Recent studies have revealed a number of crystalline polymorphs (CO2-I to -VII) and an amorphous phase under high pressure–temperature conditions. Nevertheless, the reported phase stability field and transition pressures at room temperature are poorly defined, especially for the amorphous phase. Here we shed light on the successive pressure-induced local structural changes and the molecular-to-nonmolecular transition of CO2 at room temperature by performing an in situ study of the local electronic structure using X-ray Raman scattering, aided by first-principle exciton calculations. We show that the transition from CO2-I to CO2-III was initiated at around 7.4 GPa, and completed at about 17 GPa. The present study also shows that at ∼37 GPa, molecular CO2 starts to polymerize to an extended structure with fourfold coordinated carbon and minor CO3 and CO-like species. The observed pressure is more than 10 GPa below previously reported. The disappearance of the minority species at 63(±3) GPa suggests that a previously unknown phase transition within the nonmolecular phase of CO2 has occurred.

Cost-effective upgrade of a focusing system for inelastic X-ray scattering experiments under high pressure

This paper describes a scheme utilizing a set of low-cost and compact Kirkpatrick–Baez mirrors for upgrading the optical system of the Taiwan Inelastic X-ray Scattering beamline at SPring-8 for high-pressure experiments using diamond-anvil cells. The scheme as implemented improves the focus to 13 µm × 16 µm with transmission of up to 72%.

Initial performances of first undulator-based hard x-ray beamlines of NSLS-II compared to simulations

Commissioning of the first X-ray beamlines of NSLS-II included detailed measurements of spectral and spatial distributions of the radiation at different locations of the beamlines, from front-ends to sample positions. Comparison of some of these measurement results with high-accuracy calculations of synchrotron (undulator) emission and wavefront propagation through X-ray transport optics, performed using SRW code, is presented.

Use of deep reactive ion etching in the fabrication of high-efficiency high-resolution crystal x-ray analyzers

Spherically bent silicon crystal x-ray analyzers have been employed in high-resolution inelastic x-ray scattering experiments to increase the counting efficiency due to the small cross-section of the inelastic scattering processes of interest. [1] In these bent analyzers, strain causes a distribution of lattice spacing, limiting the achievable energy resolution. Hence, the silicon wafers were diced using precision diamond saws into an array of ~1x1 mm2 blocks, and then acid etched to remove the saw damage, leaving blocks ~0.6x0.6 mm2 glued to a spherical concave substrate. With this method, meV energy resolution has been demonstrated with a bending radius of 6.5 m. [2] We seek to optimize the dicing process using the technique of deep reactive ion etching (DRIE) to develop highly efficient crystal analyzers. Ideally, each individual block subtends an angle that matches the acceptance (Darwin width) of the silicon reflection. This requires block sizes of about 500 μm2. DRIE offers the flexibility of selecting the block size, with finely controlled groove widths (i.e., minimal loss of material), and hence the possibility of controlling the energy width. We have made a prototype analyzer using DRIE with block size of 470 μm2, groove widths of 30 μm, and about 500 μm deep. The wafer was then bent and glued to a glass substrate with 2-meter radius. Tests showed encouraging results, with the DRIE analyzer performing at the 100 meV level. Details of the process and further refinements will be discussed.

High-pressure evolution ofFe2O3electronic structure revealed by x-ray absorption

We report the first high pressure measurement of the Fe K-edge in hematite (Fe{sub 2}O{sub 3}) by X-ray absorption spectroscopy in partial fluorescence yield geometry. The pressure-induced evolution of the electronic structure as Fe{sub 2}O{sub 3} transforms from a high-spin insulator to a low-spin metal is reflected in the x-ray absorption pre-edge. The crystal field splitting energy was found to increase monotonically with pressure up to 48 GPa, above which a series of phase transitions occur. Atomic multiplet, cluster diagonalization, and density-functional calculations were performed to simulate the pre-edge absorption spectra, showing good qualitative agreement with the measurements. The mechanism for the pressure-induced phase transitions of Fe{sub 2}O{sub 3} is discussed and it is shown that ligand hybridization significantly reduces the critical high-spin/low-spin gap pressure.

High-pressure evolution of Fe[subscript 2]O[subscript 3] electronic structure revealed by x-ray absorption

We report the first high pressure measurement of the Fe K-edge in hematite (Fe{sub 2}O{sub 3}) by X-ray absorption spectroscopy in partial fluorescence yield geometry. The pressure-induced evolution of the electronic structure as Fe{sub 2}O{sub 3} transforms from a high-spin insulator to a low-spin metal is reflected in the x-ray absorption pre-edge. The crystal field splitting energy was found to increase monotonically with pressure up to 48 GPa, above which a series of phase transitions occur. Atomic multiplet, cluster diagonalization, and density-functional calculations were performed to simulate the pre-edge absorption spectra, showing good qualitative agreement with the measurements. The mechanism for the pressure-induced phase transitions of Fe{sub 2}O{sub 3} is discussed and it is shown that ligand hybridization significantly reduces the critical high-spin/low-spin gap pressure.

Ordering of hydrogen bonds in high-pressure low-temperature ices

The hydrogen bonds linking the water molecules are known to give rise to a rich variety of stable and metastable phases of H2O under specific temperature and pressure conditions (Fig. 1) [1]. Study of the electronic structure of the hydrogen bonds in various phases of H2O provides valuable information on the change of hydrogen, covalent and ionic bonding of the H2O framework that is essential for understanding the icy planetary interiors as well as the physical and chemical properties of organic and biological systems at high pressure. Such information can in principle be obtained from analysis of the near-edge fine structure from X-ray absorption spectroscopy (XAS). For low-Z materials whose core-level electrons are in the soft Xray region, however, conventional XAS is difficult to perform. An alternative technique, based on the inelastic scattering of hard X-rays (~ 10 keV) by corelevel excitations, known as X-ray Raman scattering (XRS), provides the same information as XAS when the momentum transfer of XRS is small enough that the dipole approximation is valid. As our work demonstrated, the inherent bulk sensitivity and good penetration depth of XRS make it especially valuable for studies under extreme conditions such as high pressure. We have studied the near K-edge structure of oxygen in liquid water and ices III, II, and IX at 0.25 GPa and several low temperatures down to 4 K on the Taiwan Inelastic X-ray Scattering Beamline BL12XU [2]. The spectra obtained are summarized in Fig. 2, which reveal detailed spectral changes across the various phases that contain important information on the change of the hydrogen bonds of the H2O framework. The most prominent changes are observed in the pre-edge region (535-537 eV). First-principles density functional calculations (DFT) for liquid water [3] have identified that the pre-edge feature is caused by oxygen 2p and 2s orbital hybridization in water molecules with an uncoordinated (broken or distorted) donor hydrogen bond. The observed increase of the pre-edge intensity in liquid water upon high-pressure compression at 300 K therefore suggests the increase of the number of uncoordinated hydrogen bonds with pressure. With decreasing temperature at 0.25 GPa, the H2O framework undergoes structural changes firstly from liquid to ice III with the ordering of the oxygen network, then from ice III to ices II and IX. Ice IX is the low-temperature proton-ordered phase of ice III with an identical tetragonal crystal structure, whereas ice II has a completely proton-ordered rhombohedral lattice. From Fig. 2, the ordering of the oxygen network causes only a small decrease of the pre-edge intensity, whereas the ordering of the proton network, or equivalently the ordering of the hydrogen bonds, dramatically reduces the pre-edge intensity, which can be interpreted as a result of the diminishing number of uncoordinated hydrogen bonds in the proton-ordered lattice of ices II and IX. The remaining pre-edge intensities observed in ices II and IX are, however, unexpected as all (most) of the water molecules in the proton-ordered lattice of ice II (IX) are fully coordinated with symmetric hydrogen bonds, which should lead to a diminishing pre-edge intensity. Our DFT calculations of the nearedge XAS spectrum for ice IX indicate that the remaining intensity may be due to the influence of the local electronic structure by the Madelung potential of the crystal lattice, estimated by point charges placed at the hydrogen and oxygen periodic lattice positions. The calculated XAS spectrum including the point charges reproduces qualitatively the major features of the experimental spectrum (Fig. 3). We therefore conclude that the Madelung potential of the protonordered lattice causes the remaining pre-edge intensity. This is in contrast to liquid water where the near-edge structure is determined pre-dominantly by the first coordination shell of the water molecules [4]. These results can be reconciled, however, by Vapor

Pressure-induced electron transfer in cobalt-iron Prussian blue complex studied by RIXS

Resonant Inelastic X-ray Scattering (RIXS) recently has become one of the most advanced techniques that probe electronic excitations in solids, combining both advantages of a high resolution and bulk sensitivity. In our measurement, we attempted to study the charge transfer in K0.2Co1.4[Fe(CN)6]•XH2O as a function of pressure by RIXS. The photoinduced magnetization at low temperature in Co-Fe Prussian blue analogues was explained by the presence of diamagnetic Co(III)-Fe(II) low spin pairs, this step can be pushed by low temperature or high pressure. Then the photoinduced electron transfer from Fe(II) to Co(III) can be happened. We had performed a preliminary Resonant inelastic x-ray scattering (RIXS) studies on the K0.2Co1.4[Fe(CN)6]•XH2O at 0.33GPa under Diamond Anvil Cell(DAC) to study and confirmed the charge transfer behavior successfully during this time. From the comparison of the title compound at 0.33GPa pressure and ambient pressure, we can see the Co(III) ratio increase very clearly, that mean the charge transfer Fe(III)-Co(II)→Fe(II)-Co(III) happened. This confirms the outstanding resolving power of RIXS and fruitful quantitative determinate the ligand field strength and also the Co(II)/[Co(II)+Co(III)] ratio can be determinated from this kind of measurement. In here, we will present the measurement results on Iron K-edge and Cobalt K-edge partial fluorescence yield mode (PFY) by RIXS experiment to get the ligand field strength and charge transfer information related with pressure.