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The performances of a spherically bent Bragg analyzer and a cylindrically bent Laue analyzer in an X‐ray Raman/emission spectrometer are compared. The reflectivity and energy resolution are evaluated from the intensity of the elastic scattering and the width of the energy distribution on a SiO2 glass sample. Widely used, Bragg analyzers display excellent performance at the photon energy E ≤ 10 keV. However, at higher E, the reflectivity and the resolution gradually deteriorate as E increases, showing poor performance above 20 keV. On the other hand, the reflectivity of the Laue analyzer gradually increases at E > 10 keV, displaying excellent reflectivity and good resolution around 20 keV. The Laue analyzer is suitable for X‐ray absorption spectroscopy in high‐energy‐resolution fluorescence‐detection mode or X‐ray emission spectroscopy on 4d transition metal compounds. Furthermore, the X‐ray Raman features of the lithium K‐edge in LiF and the oxygen K‐edge feature in H2O, measured by nine Bragg analyzers (2 m radius) at E ≃ 9.9 keV and by five Laue analyzers (1.4 m radius) at E ≃ 19.5 keV, have been compared. Similar count rates and resolutions are observed. [ABSTRACT FROM AUTHOR]

A ternary catalyst comprising Iridium (Ir) single‐atoms (SA)s decorated on the Co‐oxide supported palladium (Pd) nanoparticles (denoted as CPI‐SA) is developed in this work. The CPI‐SA with 1 wt.% of Ir exhibits unprecedented high mass activity (MA) of 7173 and 770 mA mgIr−1, respectively, at 0.85 and 0.90 V versus RHE in alkaline ORR (0.1 m KOH), outperforming the commercial Johnson Matthey Pt catalyst (J.M.‐Pt/C; 20 wt.% Pt) by 107‐folds. More importantly, the high structural reliability of the Ir single‐atoms endows the CPI‐SA with outstanding durability, where it shows progressively increasing MA of 13 342 and 1372 mA mgIr−1, respectively, at 0.85 and 0.90 V versus RHE up to 69 000 cycles (3 months) in the accelerated degradation test (ADT). Evidence from the in situ partial fluorescence yield X‐ray absorption spectroscopy (PFY‐XAS) and the electrochemical analysis indicate that the Ir single‐atoms and adjacent Pd domains synergistically promote the O2 splitting and subsequent desorption of hydroxide ions (OH−), respectively. Whereas the Co‐atoms underneath serve as electron injectors to boost the ORR activity of the Ir single‐atoms. Besides, a progressive and sharp drop in the ORR performance is observed when Ir‐clusters and Ir nanoparticles are decorated on the Co‐oxide‐supported Pd nanoparticles. [ABSTRACT FROM AUTHOR]

Exploring advanced electrolysis techniques for attaining scene‐adaptive and on‐site green H2 production is an imperative matter of utmost practical significance but grand challenge remains. Herein, drawn inspiration from a spontaneous hydrazine‒H2O galvanic cell configured on a low‐valence Ru single atoms‐loaded Mo2C electrode (RuSA/v‐Mo2C), an alternative H2 energy solution utilizing self‐powered electrochemical hydrazine splitting (N2H4 → 2H2 + N2) instead of the stereotyped electricity‐consumed water splitting for green H2 production is proposed. This solution highlights a pH‐decoupled hydrazine‒H2O primary battery with notable open‐circuit voltage of 1.37 V and energy density up to 358 Wh gN2H4−1, which powerfully propels an alkaline hydrazine splitting cell, leading to bilateral H2 harvest with a remarkable rate of 18 mol h−1 m−2, i.e., 403.2 L h−1 m−2, setting a new record for the self‐sustaining electricity‐powered H2 production systems. The success of RuSA/v‐Mo2C for this solution is further decoded by tandem theoretical and in situ spectroscopic studies, cross‐verifying a Ru‒Mo dual‐site synergy in streamlining the overall energy barriers, thereby enhancing the kinetics of electrode reactions. This pioneering work, showcasing electrochemical H2 production free from both external energy and feedstock inputs, opens up a new horizon on way of the ultimate H2 energy solution. [ABSTRACT FROM AUTHOR]

For the inelastic electron scattering of atoms and molecules, a consensus has been reached that the first Born approximation is easily approached by decreasing the momentum transfer at the same impact electron energy or increasing the impact electron energy at the same momentum transfer. Although this consensus is applicable for the elastic electron scattering of most atoms and molecules, it is violated for helium where the experimental differential cross sections deviate from the first Born approximation prediction gradually with the decrease of squared momentum transfer at the same impact electron energy. Since this anomalous phenomenon was observed more than 40 years ago, the intrinsic mechanism is not explicit. In the present work, using the high-resolution x-ray scattering, we isolate the scattering contribution from the nucleus and directly obtain the pure electronic structure of helium. Then, the anomalous asymptotic behavior of the elastic electron scattering of helium has been elucidated, i.e., in the small squared momentum transfer region, the scattering contribution from the target's electrons is counteracted by the one from the atomic nucleus, which results in the residual contribution beyond the first Born approximation being drastically enlarged. [ABSTRACT FROM AUTHOR]

In the ferrite magnets, the magnetoplumbite-type strontium hexaferrite (SrFe12O19) is an important base material with a relatively high Curie temperature. The La-Co substitution in the Sr hexaferrites ((Sr,La)(Fe,Co)12O19) significantly improves magnetic coercivity. We have investigated the electronic states of Sr1-xLaxFe12-yCoyO19 by 57Fe Mössbauer and Co Kβ x-ray emission spectroscopies using the single-crystalline samples. The Co Kβ x-ray emission spectrum shows that all Co2+ ions are in the high-spin state. The hyperfine parameters extracted from the observed 57Fe Mössbauer spectra clearly show the Co and La substitution dependences at two Fe sites of the five crystallographically nonequivalent Fe sites, revealing that most Co2+ ions occupy the 4f1 and 2a Fe sites in Sr1-xLaxFe12-yCoyO19. These Co site occupations are consistent with the higher magnetic coercivity without a decrease in remanent magnetization in the La-Co substituted Sr hexaferrites. [ABSTRACT FROM AUTHOR]

High-entropy-alloy (HEA) superconductors are a new class of disordered superconductors. However, commonality of superconducting characteristics of HEA materials is unclear. Here, we have investigated the crystal and electronic structure, and the robustness of superconducting states in a HEA-type metal telluride (MTe; M = Ag, In, Sn, Pb, Bi) under high pressure, and the results were compared with the pressure effects for a middle-entropy system (AgPbBiTe3) and a reference system of PbTe. When the crystal structure is CsCl-type, all phases show superconductivity under high pressure but exhibit different pressure dependences of the transition temperature (Tc). For PbTe, its Tc decreases with pressure. In contrast, the Tc of HEA-type AgInSnPbBiTe5 is almost independent of pressure, for pressures ranging from 13.0 to 35.1 GPa. Those results suggest that the robustness of superconductivity to external pressure is linked to the configurational entropy of mixing at the M site in MTe. Since the trend is quite similar to previous work on a HEA (Ti–Zr–Hf–Nb–Ta), where the robustness of superconductivity was observed up to ~ 200 GPa, we propose that the robustness of superconductivity under high pressure would be a universal feature in HEA-type superconductors. [ABSTRACT FROM AUTHOR]

The synergistic collaboration between oxygen vacancies (O V) in the atomic Fe-oxide clusters and the adjacent Ni(OH) x resulted in the quantum leap of the CO 2 methanation performance of the NiFe-TiO 2 catalyst. [Display omitted] • Incorporation of atomic Fe oxide clusters induces oxygen vacancies both in atomic Fe-oxide clusters and Ni(OH)x. • Collaboration between oxygen vacancies and neighboring atoms facilitates CO 2 methanation performance of Ni(OH)x. • This material delivers the CH 4 productivity of ∼24,358 mmol g-1h−1 at 300 °C. • It shows no degradation in the stability test up to 330 h. The heterogeneous catalytic conversion of carbon dioxide (CO 2) to methane (CH 4) via CO 2 methanation offers a promising avenue for establishing the closed carbon loop. Nevertheless, the lack of effective catalysts limits its industrial applications. Considering this, we developed a novel heterogeneous catalyst comprising oxygen vacancies enriched atomic Fe-oxide clusters confined in the TiO 2 -supported Ni-hydroxide (denoted as NiFe-TiO 2) via wet chemical reduction method. This material delivers an unprecedently high CH 4 productivity of ∼24,358 mmol g-1h−1 in CO 2 methanation at 300 °C, surpassing the Ni-TiO 2 (12,481 mmol g-1h−1) by ∼ 95 %. On top of that, the high structural reliability of the Fe-oxide atomic clusters endows the NiFe-TiO 2 catalyst with outstanding durability, where it achieves an optimum CH 4 productivity of ∼ 36,399 mmol g-1h−1 after 116 cycles (155 h) with CH 4 selectivity of 90.5 % and retains the pristine performance up to 220 cycles (330 h) in the stability test. With evidence from in-situ X-ray absorption and ambient pressure X-ray photoelectron spectroscopy studies, the performance descriptors and reaction pathways were unveiled, where the oxygen vacancies in the atomic Fe-oxide clusters and the adjacent Ni-hydroxide domains synergistically boost the CO 2 activation and the H 2 dissociation, respectively. Such a potential synergy enables the simultaneous operation of all intermediate steps for enhanced CO 2 methanation kinetics on the NiFe-TiO 2 surface. Most importantly, these findings not only unravel the merits of oxygen vacancies in transition metals for CO 2 methanation but mark a step ahead for the rational design of heterogeneous catalysts in various catalytic applications. [ABSTRACT FROM AUTHOR]

X‐ray Compton scattering measurements using synchrotron radiation are carried out for liquid sodium (Na) from 473 K up to 1273 K. Utilizing the liquid state allows us to realize a continuous density reduction by controlling temperature and pressure. Valence Compton profiles (CPs) clearly exhibit thermodynamic state dependence as the temperature increases. Experimental CPs, CP differences, and electron kinetic energy are compared with those of the electron gas model. Combined with the analysis using the Fourier‐transformed CPs, the observed behaviors can be reasonably explained by the reduction in the electron density with volume expansion of the liquid. [ABSTRACT FROM AUTHOR]

Laser-excited N and R fluorescence lines of heavily doped ruby have been studied up to 26 GPa at low temperatures. While the intensity of the R lines at ambient pressure significantly decreases with decreasing temperature, the intensity of N lines originating from exchange-coupled Cr ion pairs is enhanced at low temperatures. The pressure induced wavelength shift of the N lines at 19 K is well fitted with an empirical formula similar to the equation for the R1 line, showing that the intense N line could be used as an alternative pressure scale at low temperatures. We also observe continuous increase in non-hydrostaticity with increasing pressure at low temperatures when silicone oil and 4:1 mixture of methanol and ethanol are used as pressure media. [ABSTRACT FROM AUTHOR]

The elastic squared form factor of gaseous carbon dioxide was measured for the first time by high resolution X-ray scattering to separate the elastic and inelastic scattering. Bond lengths of r CO  = 1.172 ± 0.008 Å and r OO  = 2.344 ± 0.016 Å of CO 2 were determined from the measured elastic squared form factors, which are in good agreement with the NIST benchmark data of r CO  = 1.1621 Å and r OO  = 2.3242 Å. Moreover, the obtained elastic squared form factor difference shows a clear binding effect in the region of small momentum transfers. The present work shows that high resolution X-ray scattering is a powerful tool to study the binding effect of molecules. [ABSTRACT FROM AUTHOR]

High-resolution partial-fluorescence-yield X-ray absorption and resonant X-ray emission spectra were used to characterize the temperature dependence of Sm 4f configurations and orbital/charge degree of freedom in SmB6. The variation of Sm 4f configurations responds well to the formed Kondo gap, below 140 K, and an in-gap state, below 40 K. The topological in-gap state is correlated with the fluctuating population of Sm 4f configurations that arises via carrier transfer between 3d94f6 and 3d94f5 states; both states are partially delocalized, and the mediating 5d orbital plays the role of a transfer path. Complementary results shown in this work thus manifest the importance of configuration fluctuations and orbital delocalization in the topological surface state of SmB6. [ABSTRACT FROM AUTHOR]

The ternary rare-earth metal fluoride CsEuF 3 adopts an ideal cubic perovskite structure [ABX 3 ] under ambient conditions. The B-cation site is occupied by the rare-earth Eu ion and EuF 6 octahedra are formed. In the present study, magnetic susceptibility and synchrotron X-ray absorption spectroscopy (XAS) analysis at the Eu- L 3 edge confirmed that Eu is in a divalent oxidation state under ambient conditions. Temperature-dependent magnetic susceptibility data revealed that the average Eu valence increased below 20 K due to a partial transition from the Eu2+ state to the Eu3+ state, thereby resulting in a mixed valence state with an average valence of +2.23. Direct evidence for valence fluctuation by the Eu ions in CsEuF 3 was obtained using the high pressure high energy resolution fluorescence detection-XAS technique, where continuous changes in valence were observed from 2.15+ at ambient pressure up to 2.5+ at 10.5 GPa. These findings indicate the possibility of discovering interesting physical properties associated with valence instabilities by rare-earth metals in similar systems. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study an analysis strategy towards using the resonant inelastic X-ray scattering (RIXS) technique more effectively compared with X-ray absorption spectroscopy (XAS) is presented. In particular, the question of when RIXS brings extra information compared with XAS is addressed. To answer this question the RIXS plane is analysed using two models: (i) an exciton model and (ii) a continuum model. The continuum model describes the dipole pre-edge excitations while the exciton model describes the quadrupole excitations. Applying our approach to the experimental 1 s2 p RIXS planes of VO2 and TiO2, it is shown that only in the case of quadrupole excitations being present is additional information gained by RIXS compared with XAS. Combining this knowledge with methods to calculate the dipole contribution in XAS measurements gives scientists the opportunity to plan more effective experiments. [ABSTRACT FROM AUTHOR]

Diopside, CaMgSi 2 O 6 , is an important analogue for depolymerized silicate melts. We have used the complimentary spectroscopies, X-ray Raman Scattering (XRS), X-ray absorption near edge structure (XANES) and Raman, to investigate diopside glass in situ to 20 GPa. We observe a stark change in behavior of CaMgSi 2 O 6 near 4 GPa that corresponds to a major change in the rate of inter-tetrahedral angle (∠Si–O–Si) closure. Below 4 GPa, the ∠Si–O–Si closes rapidly at 2–3°/GPa whereas above 4 GPa it decreases by ∼1°/GPa. A distinct shift to higher wavenumbers of the 870 and 905 cm −1 Raman bands are observed at 1.3 GPa suggesting that Q 0 species have been completely converted to Q 1 or higher Q n species. XRS measurements at the O K -edge suggest that [5] Si is formed by 3 GPa. This formation is accompanied by a rapid decrease in the ∠Si–O–Si and a decrease in Q 0 species. A linear increase in the geometric mean of the high frequency envelope, the χ b value, from 999 to 1018 cm −1 suggests that the conversion of NBO to BO is continuous up to 14 GPa. Above 14 GPa, the Raman spectra show an obvious negative shift in both, the high frequency peak maximum, and the χ b position. Simultaneously, the low frequency envelope looses its asymmetry at 14 GPa. This may be explained by either a loss of a vibrational mode in the range 1000–1200 cm −1 and/or the formation of [6] Si. The structural evolution of CaMgSi 2 O 6 correlates well with a major change in the compressibility and diffusivity around 5 GPa. [ABSTRACT FROM AUTHOR]

Based on numerical results from density-functional perturbation theory calculations, we show that the magnetic spin transition in Fe2+-rich MgSiO3 perovskite can be identified as changes in the powder X-ray diffraction (XRD) pattern and the vibrational spectra. In particular theory predicts how the symmetry breaking and the volume reduction associated with the spin transition affects both structural and vibrational properties. The XRD measurements of (Mg0.5Fe0.5)SiO3 perovskite indeed demonstrated that the new diffraction peaks and the peak broadening formed during the spin transition can be explained by the associated symmetry breaking. We also show computationally that certain vibrational peaks exhibit a shift at the transition; the Grüneisen parameters of certain modes are affected by the transition, thus bearing on the thermodynamical properties. Raman and/or infrared measurements before and after the spin transition could identify these changes [ABSTRACT FROM AUTHOR]

The correlation between the 4f-conduction electron (c-f) hybridization and the transition temperature T0 into the unconventional antiferromagnetically (AFM) ordered state in Ce(Ru1-xFex)2Al10 has been investigated. The impact of pressure on the crystal structure and the electronic structure has been measured using x-ray diffraction and resonant x-ray emission spectroscopy at the Ce L3 absorption edge in a diamond anvil cell. Our results show that the lattice spacings imply some correlation with the disappearance of T0. Analyses of the measured valence states, however, show only a weak correlation with the corresponding critical disappearance of the AFM order within experimental errors. Our results may be explained by a scenario based on the Kondo-Heisenberg model beyond the conventional Doniach phase diagram. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report on direct measurements of the Yb valence in the heavy Fermion compound YbCuAl as a function of temperature and pressure using resonant x-ray emission spectroscopy. The increase of the Yb2+ component at T < 100 K and ambient pressure, well described by the single impurity Anderson model, is found to compensate for the thermal contraction of the unit cell volume. Under pressure, the Yb valence increases continuously up to 25 GPa, albeit a marked leveling off close to the critical pressure, at P ≥ 13 GPa. This finding is reminiscent of a recent report on YbCu2Si2 and further confirms the interplay between electronic and magnetic fluctuations near the magnetic instability point of rare-earth intermediate-valence materials. [ABSTRACT FROM AUTHOR]

Abstract: The spin transition and substitution of Fe3+ in Mg0.85Fe3+ 0.15Al0.15Si0.85O3 post-Mg-perovskite (PPv) synthesized at 165–170GPa, 2100K were examined by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) during decompression to 37GPa without annealing. XRD showed that samples were single phase PPv down to 49GPa, although the samples may have partly become amorphous. The result of XES measurement indicates that Fe3+ is fully low spin (LS) for 165–100GPa and Fe3+ largely changes from LS to high spin (HS) between 80 and ∼40GPa. These results combined with previous reports indicate that Al and LS Fe3+ may occupy the A-(dodecahedral) and B-(octahedral) sites, respectively, as the favorable cation sites for 165–100GPa in Al-bearing PPv. Based on the present results on Fe3+ and recent theoretical reports on Fe2+, the spin states of iron in lower mantle PPv are discussed. [Copyright &y& Elsevier]

Temperature and pressure dependences of the electronic structure of the heavy-fermion system CePd2Si2 have been investigated using partial fluorescence yield x-ray absorption spectroscopy and resonant x-ray emission spectroscopy at the Ce L3 edge. The temperature dependence has also been measured for CeRh2Si2 for comparison. In both compounds Ce is in a weakly mixed valence state at ambient pressure, mostly ∫1 with a small contribution from the ∫0 component. No temperature dependence of the Ce valence is observed at temperatures as low as 8 K. In CePd2Si2 at 19 K, however, the Ce valence shows a continuous increase with pressure, indicating pressure-induced derealization of the 4∫ states. Theoretical calculations based on the single impurity Anderson model reproduce the experimental results well. Pressure dependence of the difference between the ground state valence and the measured valence including the final state effect is also discussed. [ABSTRACT FROM AUTHOR]

We have performed hard x-ray photoemission spectroscopy (HAXPES) for Yb-based Kondo lattice compounds; an antiferromagnetic heavy-fermion system YbNi3Al9 and a valence fluctuation system YbNi3Ga9. The Yb 3d5/2 spectra of YbNi3Ga9 showed both Yb2+ and Yb3+-derived structures indicating strong valence fluctuation, and the intensity of Yb2+ (Yb3+) structures gradually increased (decreased) on cooling. The Yb 3d5/2 spectra of YbNi3Al9 mostly consisted of Yb3+-derived structures and showed little temperature dependence. The Yb valences of YbNi3Ga9 and YbNi3 Al9 at 22 Κ were evaluated to be 2.43 and 2.97, respectively. Based on the results of the Ni 2p and valence-band HAXPES spectra together with soft x-ray valence-band spectra, we described that the difference of physical properties of YbNi3X9 (X = Al, Ga) is derived from the differences of the 4f-hole level relative to the Fermi level (EF) and Ni 3d density of states at EF. The HAXPES results on the Yb valences were consistent with those obtained by x-ray absorption spectroscopy using the partial fluorescence yield mode and resonant x-ray emission spectroscopy at the Yb L3 edge. [ABSTRACT FROM AUTHOR]

Abstract: There are still large discrepancies among the previous reports on the spin transition of iron in Mg–perovskite (Pv). To alleviate this problem, we examined the spin state of Fe3+ in Mg0.85Fe3+ 0.15Al0.15Si0.85O3 Pv up to 200GPa by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD). The gradual decrease of the high spin (HS) ratio of Fe3+ by low temperature annealing of the samples above ~60GPa in XES and the change of the trend of unit cell volumes with pressure by annealing at 50–60GPa in XRD indicate that Fe3+ occupies the A-site and is HS below ~50GPa but above 50–60GPa it gradually replaces Al at the B-site through cation exchange reaction by annealing and becomes low spin (LS), while Fe3+ remaining at the A-site is HS up to 200GPa. This means that the spin state of Fe3+ depends on Fe3+ occupancies between the A- and B-sites and these Fe3+ occupancies are strongly controlled by the synthesis condition and annealing temperature of the samples through the cation exchange reaction. The present results combined with the previous reports indicate that in Al-bearing Mg–Pv in the lower mantle Fe2+ occupies the A-site and remains HS for the whole lower mantle, while Fe3+ occupies the A-site and is HS below ~50GPa but above 50–60GPa it replaces Al at the B-site and becomes LS, on the assumption that spin transition pressure of Fe2+ at the A-site is higher than that of Fe3+ at the same site. [Copyright &y& Elsevier]

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe4Sb12 and Yb0.88Fe4Sb12, using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x = 0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe4Sb12 is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe4Sb12 and 17 GPa for Yb0.88Fe4Sb12. [ABSTRACT FROM AUTHOR]

We report high-spin to low-spin crossover of iron in NiAs (B8)-type Fe0.96O (FeO hereafter) at high pressure by a combination of in-situ x-ray emission spectroscopy (XES) and x-ray diffraction (XRD) measurements. The XES spectra show a loss of spin in BS-type FeO approximately above 119 GPa at 300 K, consistent with the ~2.5% volume reduction around 120 GPa. The high-temperature XRD study also demonstrates a similar volume decrease at 1560-1780 K under the same pressure condition. While the crystal structure of FeO remains to be B8 type across such volume reduction, the atomic arrangements of Fe and O change from inverse to normal NiAs form with considerable decrease in c/a axial ratio. With recent electrical resistance measurements, these suggest that iron spin crossover, inverse-normal structural change, and insulator-metal transition occur concurrently in B8 FeO around 120 GPa. [ABSTRACT FROM AUTHOR]

A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core-mantle boundary region. Recent theoretical calculations combined with estimates of iron partitioning between (Mg,Fe)SiO3 perovskite and melt at shallower mantle conditions suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ∼76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg0.95Fe0.05)SiO3 glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO3 liquid becomes more dense than coexisting solid at ∼1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ∼1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core-mantle boundary region. [ABSTRACT FROM AUTHOR]

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa1.15Si0.85 and nonsuperconducting ternary germanide YbGaxGe2-x (x = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa1.15Si0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7–300 K, while the valence shows a drastic increase under pressure from the Yb2+ state partially including itinerant electrons to the localized Yb3+ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa1.15Si0.85 and YbGaxGe2-x, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa1.15Si0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity. [ABSTRACT FROM AUTHOR]

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's 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 Earth's 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 Earth's 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 Earth's lower mantle. [ABSTRACT FROM AUTHOR]

Microstructure and morphology of particles play key roles in optimizing the properties of shape-selected ZnO particles, which are essential factors for flexible and reliable applications. In particular, chemical understanding and physical measurement with scientific theory must be further integrated for the realization of finely tuned ZnO nano/microstructures with desired sizes and shapes. Herein, we deliver a detailed description of the mechanism that mimics the formation of finely-tuned, spherical ZnO nanoparticles (NPs) at the computational level. We tackled issues that significantly affect the favorable structural motifs of the spherical ZnO NPs grown hydrothermally from ethanolic solution leading to their advancing chemical and physical properties. The excellent photocatalytic activity of the spherical ZnO was addressed by an apparent-rate constant of 9.7(2)x10-2 min-1 efficiently degrading the Rhodamine B solution by ∼99% in 50 min. The apparent-rate constant for tubular ZnO particles is almost six times lower than that of spherical ZnO NPs. Comparative results revealed that the diversity of size and shape of ZnO particles distinguishes the wurtzite-to-rocksalt transformation reversibility phenomena by dictating the microstructure-dependent deformation behavior and ultimately leading to different transition-induced elastic strain responses to hydrostatic pressure up to 30 GPa. [Display omitted] • Mechanical response up to 30 GPa of ZnO NPs and parameters influencing the phase transformation are discussed in detail. • Spherical ZnO NPs feature apparent-rate constant of 9.7(2) × 10-2 min-1 and degrade ∼ 99% of RhB dye in 50 min • The apparent-rate constant for tubular ZnO particles is demonstrated six times lower than that of spherical ZnO NPs. • The crystal growth of ZnO NPs featuring enhanced photocatalytic activity is elucidated through theory calculations. [ABSTRACT FROM AUTHOR]

The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO 3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO 4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L 2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO 4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO 3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L 2,3 edge of LaSrCoO 4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO 3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO 4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively. • Co3+ ion in LaSrCoO 4 with an elongated distortion has a mixed spin state rather than a disputable intermediate spin state. • It is demonstrated that 2:3 ratio of HS:LS for LaSrCoO 4 against 1:3 for LaCoO 3 at 300 K and ambient pressure. • The scenario of intermediate spin state of Co3+ ion in LaSrCoO 4 is safely excluded based on the theoretical calculations. [ABSTRACT FROM AUTHOR]

Rare-earth (RE) fullerides are an intriguing family of materials in which electronic instabilities on the RE sublattice couple to the electronic and lattice degrees-of-freedom of the strongly-correlated fulleride sublattice. In particular, insulating Sm 2.75 C 60 adopts an orthorhombic superstructure, arising from long-range ordering of Sm partial vacancies, and can be driven to a lattice-collapsed metallic state due to a valence transition towards Sm3+ upon pressurization. Here we use synchrotron X-ray absorption spectroscopy (XAS) at the Sm L 3 edge at ambient conditions to authenticate the mixed valence character of the material by identifying two features due to distinct Sm2+ and Sm3+ components – their relative intensity allows a direct measure of the average Sm valence in Sm 2.75 C 60 at +2.07(3). We then attempt to mimic the physical pressure effect on the electronic properties by co-intercalation of the smaller-size valence-precise Ca2+ ion to form the series of ternary solid solutions, (Sm 1- x Ca x) 2.75 C 60 (0 ≤ x ≤ ⅔). XAS measurements in their high-resolution partial fluorescence yield (PFY) variance find that chemical pressure leads to an increase of the Sm3+ contribution to the average valence by >10% in the most contracted member, (Sm ⅓ Ca ⅔) 2.75 C 60 of the present study with the average valence reaching a value of +2.33(2). Assuming full charge transfer between the metal ions and C 60 , the charge on the C 60 units remains invariant throughout at approximately −5.78. This opens the possibility that the system can be tuned further towards the higher average valences needed for the high-pressure insulator-to-metal transition in Sm 2.75 C 60 to be shifted to ambient pressure with a single-valence Sm3+ state accessible at a Ca content of x ~0.9. [ABSTRACT FROM AUTHOR]

Non-Fermi-liquid (NFL), a significant deviation from Fermi-liquid theory, usually emerges near an order-disorder phase transition at absolute zero. Recently, a diverging susceptibility toward zero temperature was observed in a quasicrystal (QC). Since an electronic long-range ordering is normally absent in QCs, this anomalous behaviour should be a new type of NFL. Here we study high-resolution partial-fluorescence-yield x-ray absorption spectroscopy on Yb-based intermediate-valence icosahedral QCs and cubic approximant crystals (ACs), some of which are new materials, to unveil the mechanism of the NFL. We find that for both forms of QCs and ACs, there is a critical lattice parameter where Yb-valence and magnetism concomitantly exhibit singularities, suggesting a critical-valence-fluctuation-induced NFL. The present result provides an intriguing structure–property relationship of matter; size of a Tsai-type cluster (that is a common local structure to both forms) tunes the NFL whereas translational symmetry (that is present in ACs but absent in QCs) determines the nature of the NFL against the external/chemical pressure. [ABSTRACT FROM AUTHOR]

The neutron diffraction pattern of D2O ice was recently measured at pressures up to 52 GPa by Guthrie et al., who proposed an octahedral interstitial model for ice at pressures above 13 GPa to account for the deviation of the observed crystal structure from that of ice VII. In this article, the octahedral interstitial model was re-examined in terms of the interstitial occupancy and X-ray Raman spectroscopy (XRS) spectra. The interstitial occupancy calculated using first-principles molecular dynamics simulations was negligibly small compared to that of the interstitial model. The oxygen K-edge spectra calculated for the interstitial model exhibited two additional low-energy peaks originating from water molecules and hydroxides that are interacting with interstitial protons, respectively, whereas these low-energy peaks were not observed in the experimentally measured spectra. These results suggest that the interstitial model cannot explain the XRS spectra of ice VII at pressures above 13 GPa and that more precise structure measurements and analyses are necessary to reveal the nature of the pressure-induced transition. [ABSTRACT FROM AUTHOR]

Electronic states of iron in Earth's mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mössbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25) O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe2+ (a lower-energy 4T1g state, a 4T2g state, and a higher-energy 4T1g state) and a single ²Eg state for low-spin Fe2+, attributed to the (t2g)0(eg)³ excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe2+ site. The K pre-edge RXES spectra at the incident energy of 7112 eV, which are similar to the Fe L-edge spectra, are also used successfully to quantitatively obtain consistent results on the spin transition of Fe2+ in ferropericlase under high pressures. Owing to the superior sensitivity of the RXES technique, the observed electronic states and their energy separations provide direct information on the local electronic structures and crystal-field splitting energies of the 3d electronic shells of Fe2+ in ferropericlase at relevant pressures of the Earth's lower mantle. [ABSTRACT FROM AUTHOR]