| 1. |
- Dogiel, Vladimir A., et al.
(författare)
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Origin of Thermal and Non-Thermal Hard X-Ray Emissionfrom the Galactic Center
- 2009
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Ingår i: Publications of the Astronomical Society of Japan. - : Oxford University Press. - 0004-6264 .- 2053-051X. ; 61:5, s. 1099-1105
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Tidskriftsartikel (refereegranskat)abstract
- We analyse new results of Chandra and Suzaku Observatories which found a flux of hard X-ray emission from the compact region around Sgr A∗ (r ∼ 100 pc). We suppose that this emission is generated by accretion processes onto the central supermassive blackhole when an unbound part of captured stars obtains an additional momentum. As a result a flux of subrelativistic protons is generated near the galactic center which heats the background plasma up to temperatures about 6–10 keV and produces by inverse bremsstrahlung a flux of non-thermal X-ray emission in the energy range above 10 keV.
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| 2. |
- Dmitriev, Vladimir, et al.
(författare)
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Pressure-temperature phase diagram of LiBH4: Synchrotron x-ray diffraction experiments and theoretical analysis
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 77:17
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Tidskriftsartikel (refereegranskat)abstract
- An in situ combined high-temperature high-pressure synchrotron radiation diffraction study has been carried out on LiBH4. The phase diagram of LiBH4 is mapped to 10 GPa and 500 K, and four phases are identified. The corresponding structural distortions are analyzed in terms of symmetry-breaking atomic position shifts and anion ordering. Group-theoretical and crystal-chemical considerations reveal a nontrivial layered structure of LiBH4. The layers and their deformations define the structural stability of the observed phases.
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| 3. |
- Filinchuk, Yaroslav, et al.
(författare)
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Cation Size and Anion Anisotropy in Structural Chemistry of Metal Borohydrides. The Peculiar Pressure Evolution of RbBH4
- 2010
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 49:11, s. 5285-5292
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Tidskriftsartikel (refereegranskat)abstract
- The pressure evolution of RbBH4 has been characterized by synchrotron powder X-ray diffraction and Raman spectroscopy up to 23 GPa. Diffraction experiments at ambient temperature reveal three phase transitions, at 3.0, 10.4, and 18 GPa (at 2.6, 7.8, and 20 GPa from Raman data), at which the space group symmetry changes in the order Fm-3m(Z=4) → P4/nmm(2) → C222(2) → I-42m(4). Crystal structures and equations of state are reported for all four phases. The three high-pressure structure types are new in the crystal chemistry of borohydrides. RbBH4 polymorphs reveal high coordination numbers (CNs) for cation and anion sites, increasing with pressure from 6 to 8, via an intermediate 4 + 4 coordination. Different arrangements of the tetrahedral BH4 group in the Rb environment define the crystal symmetries of the RbBH4 polymorphs. The structural evolution in the MBH4 series is determined by the cation’s size, as it differs drastically for M = Li (CNs = 4, 6), Na (CN = 6), and Rb. The only structure common to the whole MBH4 family is the cubic one. Its bulk modulus linearly decreases as the ionic radius of M increases, indicating that the compressibility of the material is mainly determined by the repulsive BH4···BH4 interactions.
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| 4. |
- Lo, Chun Wan Timothy, et al.
(författare)
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Synthesis, Structure, and Thermoelectric Properties of α-Zn3Sb2 and Comparison to β-Zn13Sb10
- 2017
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 29:12, s. 5249-5258
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Tidskriftsartikel (refereegranskat)abstract
- Zn-Sb compounds (e.g., ZnSb, β-Zn13Sb10) are known to have intriguing thermoelectric properties, but studies of the Zn3Sb2 composition are largely absent. In this work, α-Zn3Sb2 was synthesized and studied via temperature-dependent synchrotron powder diffraction. The α-Zn3Sb2 phase undergoes a phase transformation to the β form at 425 °C, which is stable until melting at 590 °C. Rapid quenching was successful in stabilizing the α phase at room temperature, although all attempts to quench β-Zn3Sb2 were unsuccessful. The structure of α-Zn3Sb2 was solved using single crystal diffraction techniques and verified through Rietveld refinement of the powder data. α-Zn3Sb2 adopts a large hexagonal cell (R 3-space group, a = 15.212(2), c = 74.83(2) Å) containing a well-defined framework of isolated Sb3- anions but highly disordered Zn2+ cations. Dense ingots of both the α-Zn3Sb2 and β-Zn13Sb10 phases were formed and used to characterize and compare the low temperature thermoelectric properties. Resistivity and Seebeck coefficient measurements on α-Zn3Sb2 are consistent with a small-gap, degenerately doped, p-type semiconductor. The temperature-dependent lattice thermal conductivity of α-Zn3Sb2 is unusual, resembling that of an amorphous material. Consistent with the extreme degree of Zn disorder observed in the structural analysis, phonon scattering in α-Zn3Sb2 appears to be completely dominated by point-defect scattering over all temperatures below 350 K. This contrasts with the typical balance between point-defect scattering and Umklapp scattering seen in β-Zn13Sb10. Using the Debye-Callaway interpretation of the lattice thermal conductivity, we use the differences between α-Zn3Sb2 and β-Zn13Sb10 to illustrate the potential significance of cation/anion disorder in the Zn-Sb system.
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| 5. |
- Mondal, Swastik, et al.
(författare)
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Electron-Deficient and Polycenter Bonds in the High-Pressure gamma-B-28 Phase of Boron
- 2011
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Ingår i: PHYSICAL REVIEW LETTERS. - : American Physical Society. - 0031-9007. ; 106:21, s. 215502-
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Tidskriftsartikel (refereegranskat)abstract
- The peculiar bonding situation in gamma boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B-12 clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B-2 dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
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| 6. |
- Simonov, Arkadiy, et al.
(författare)
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Hidden diversity of vacancy networks in Prussian blue analogues
- 2020
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Ingår i: Nature. - : NATURE PUBLISHING GROUP. - 0028-0836 .- 1476-4687. ; 578:7794, s. 256-260
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Tidskriftsartikel (refereegranskat)abstract
- Prussian blue analogues (PBAs) are a diverse family of microporous inorganic solids, known for their gas storage ability(1), metal-ion immobilization(2), proton conduction(3), and stimuli-dependent magnetic(4,5), electronic(6) and optical(7) properties. This family of materials includes the double-metal cyanide catalysts(8,9) and the hexacyanoferrate/ hexacyanomanganate battery materials(10,11). Central to the various physical properties of PBAs is their ability to reversibly transport mass, a process enabled by structural vacancies. Conventionally presumed to be random(12,13), vacancy arrangements are crucial because they control micropore-network characteristics, and hence the diffusivity and adsorption profiles(14,15). The long-standing obstacle to characterizing the vacancy networks of PBAs is the inaccessibility of single crystals(16). Here we report the growth of single crystals of various PBAs and the measurement and interpretation of their X-ray diffuse scattering patterns. We identify a diversity of non-random vacancy arrangements that is hidden from conventional crystallographic powder analysis. Moreover, we explain this unexpected phase complexity in terms of a simple microscopic model that is based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancynetwork polymorphs with very different micropore characteristics. Our results establish a foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy and transport efficiency.
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| 7. |
- Steele, Julian A., et al.
(författare)
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How to GIWAXS: Grazing Incidence Wide Angle X-Ray Scattering Applied to Metal Halide Perovskite Thin Films
- 2023
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Ingår i: Advanced Energy Materials. - : WILEY-V C H VERLAG GMBH. - 1614-6832 .- 1614-6840. ; 13:27
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Tidskriftsartikel (refereegranskat)abstract
- The frequency of reports utilizing synchrotron-based grazing incident wide angle X-ray scattering (GIWAXS) to study metal halide perovskite thin films has exploded recently, as this technique has proven invaluable for understanding several structure-property relationships that fundamentally limit optoelectronic performance. The GIWAXS geometry and temporal resolution are also inherently compatible with in situ and operando setups (including ISOS protocols), and a relatively large halide perovskite research community has deployed GIWAXS to unravel important kinetic and dynamic features in these materials. Considering its rising popularity, the aim here is to accelerate the required learning curve for new experimentalists by clearly detailing the underlying analytical concepts which can be leveraged to maximize GIWAXS studies of polycrystalline thin films and devices. Motivated by the vast range of measurement conditions offered, together with the wide variety of compositions and structural motifs available (i.e., from single-crystal and polycrystalline systems, to quantum dots and layered superlatices), a comprehensive framework for conducting effective GIWAXS experiments is outlined for different purposes. It is anticipated that providing a clear perspective for this topic will help elevate the quality of future GIWAXS studies-which have become routine-and provide the impetus required to develop novel GIWAXS approaches to resolve unsettled scientific questions.
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| 8. |
- Tumanov, Nikolay A., et al.
(författare)
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High-Pressure Study of Mn(BH4)(2) Reveals a Stable Polymorph with High Hydrogen Density
- 2016
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 28:1, s. 274-283
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure behavior of alpha-Mn(BH4)(2) was studied up to 29.4 GPa in diamond anvil cells using powder Xray diffraction combined with DFT calculations and Raman spectroscopy, and two new polymorphs were discovered. The first polymorph, delta-Mn(BH4)(2), forms near 1 GPa and is isostructural to the magnesium analogue delta-Mg(BH4)(2). This polymorph is stable upon decompression to ambient conditions and can also be obtained by compression of alpha-Mn(BH4)(2) in a large-volume steel press as well as by high-energy ball milling. It shows a high volumetric density of hydrogen of 125 g H-2/L at ambient conditions. delta-Mn(BH4)(2) was refined in the space group I4(1)/acd with the cell parameters a = 7.85245(6), c = 12.1456(2) angstrom, and V = 748.91(1) angstrom(3) at ambient conditions; it can also be described in a stable P-4n2 superstructure. Its thermal stability was studied by in situ X-ray powder diffraction and thermal analysis coupled with mass-spectroscopy. delta-Mn(BH4)(2) transforms back to alpha-Mn(BH4)(2) upon heating in the temperature range of 67-109 degrees C in Ar (1 bar) or H-2 (100 bar) atmosphere, and a decomposition is initiated at 130 degrees C with the release of hydrogen and some diborane. Mn(BH4)(2) undergoes a second phase transition to delta'-Mn(BH4)(2) in the pressure range of 8.6-11.8 GPa. delta'-phase is not isostructural to the second high-pressure phase of Mg(BH4)(2), and its structure was determined in the root 2a X c supercell compared to the delta-phase and refined in the space group Fddd with a = 9.205(17), b = 9.321(10), c = 12.638(15) angstrom, and V = 1084(3) angstrom(3) at 11.8 GPa. Equations of state were determined for alpha- and delta-Mn(BH4)(2).
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| 9. |
- Venkateshwarlu, Sarangi, et al.
(författare)
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Large electromechanical strain and unconventional domain switching near phase convergence in a Pb-free ferroelectric
- 2020
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- In many ferroelectrics, large electromechanical strains are observed near regions of composition- or temperature- driven phase coexistence. Phenomenologically, this is attributed to easy re-orientation of the polarization vector and/or phase transition, although their effects are highly convoluted and difficult to distinguish experimentally. Here, we used synchrotron X-ray scattering and digital image correlation to differentiate between the microscopic mechanisms leading to large electrostrains in an exemplary Pb-free piezoceramic Sn-doped barium calcium zirconate titanate. Large electrostrains of ~0.2% measured at room-temperature are attributed to an unconventional effect, wherein polarization switching is aided by a reversible phase transition near the tetragonal-orthorhombic phase boundary. Additionally, electrostrains of ~0.1% or more could be maintained from room temperature to 140 °C due to a succession of different microscopic mechanisms. In situ X-ray diffraction elucidates that while 90° domain reorientation is pertinent below the Curie temperature (TC), isotropic distortion of polar clusters is the dominant mechanism above TC.
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| 10. |
- Vorobiev, Alexei, et al.
(författare)
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Graphene oxide hydration and solvation : an in situ neutron reflectivity study
- 2014
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Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 6:20, s. 12151-12156
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Tidskriftsartikel (refereegranskat)abstract
- Graphene oxide membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Using isotope contrast, neutron reflectivity was applied to evaluate the amounts of solvents intercalated into a membrane from pure and binary vapors and to evaluate the selectivity of the membrane permeation. Particularly, the effect of D2O, ethanol and D2O–ethanol vapours on graphene oxide (GO) thin films (25 nm) was studied. The interlayer spacing of GO and the amount of intercalated solvents were evaluated simultaneously as a function of vapour exposure duration. The significant difference in neutron scattering length density between D2O and ethanol allows distinguishing insertion of each component of the binary mixture into the GO structure. The amount of intercalated solvent at saturation corresponds to 1.4 molecules per formula unit for pure D2O (1.4 monolayers) and 0.45 molecules per formula unit (one monolayer) for pure ethanol. This amount is in addition to H2O absorbed at ambient humidity. Exposure of the GO film to ethanol–D2O vapours results in intercalation of GO with both solvents even for high ethanol concentration. A mixed D2O–ethanol layer inserted into the GO structure is water enriched compared to the composition of vapours due to slower ethanol diffusion into GO interlayers
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