[show abstract][hide abstract] ABSTRACT: The shape of the Ba 4d–4f giant dipole resonance is studied for Ba atoms embedded inside
complex Si networks covering structures consisting of Si nanocages and nanotubes, i.e. the
clathrate Ba8Si46, the complex compound BaSi6, and the semiconducting BaSi2. Here,
non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of
the giant resonance in the vicinity of the Ba N_IV,V-edge. The distinct momentum transfer
dependence of the spectra is analyzed and discussed. The measurements are compared to
calculations of the giant resonance within time-dependent local density approximation in the
dipole limit. No modulation of the giant resonance’s shape for Ba atoms confined in different
local environments was observed, in contrast to the calculations. The absence of such shape
modulation for complex Ba/Si compounds is discussed providing important implications for
further studies of giant resonance phenomena utilizing both theory and experiment.
Journal of Physics B Atomic Molecular and Optical Physics 01/2014; 47(4):045102. · 2.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn21, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn21, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and Eact = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure.
Journal of the American Chemical Society 01/2014; · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the 1-D silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using x-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter Equation calculations. This is the first direct observation indicating that the
silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proofs that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions.
The Journal of Physical Chemistry C 12/2013; 118(2):1161. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: A combined experimental and theoretical study of hydrogen-rich ammonium borohydride (NH(4)BH(4)) subjected to pressures up to 10 GPa indicates two phase transitions, detected by synchrotron radiation powder X-ray diffraction, Raman spectroscopy and Car-Parrinello molecular dynamics calculations, at 1.5 and 3.4 GPa. The ambient pressure, face-centred cubic phase of NH(4)BH(4) transforms into a highly disordered intermediate structure which then evolves upon increasing pressure into an orthorhombic, distorted CsCl structure. The structure of the latter phase was solved using ab initio computational techniques and from a Rietveld full pattern refinement of the powder X-ray diffraction data.
Physical Chemistry Chemical Physics 04/2012; · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A technique to improve the ratio of diffraction signal to Compton background has been examined in the context of diamond anvil cell crystallography carried out using imaging plates. Specifically, imaging plates exhibit an abrupt discontinuity in absorption as a function of energy due to barium, the primary compositional element of the imaging plate phosphor. By carrying out diffraction at some energy just above the Ba K-edge, coherent scattering is efficiently absorbed by the detector while less-energetic Compton scattering is less efficiently absorbed. This yields an improvement in the signal-to-background ratio of approximately 33%. Data are presented detailing the absorption characteristics of imaging plates over a broad energy range of the continuous synchrotron radiation spectrum. Diffraction data are presented for Si powder contained in a diamond anvil cell. Diffraction images taken with incident radiation at three distinct energies near the Ba K-edge are analyzed to evaluate the effectiveness of the technique.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 03/2012; 668:9–13. · 1.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the "simple cubic" (SC) unit cell adopted by calcium over a broad pressure ranging from 30-90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the  plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.
[show abstract][hide abstract] ABSTRACT: Ba(8)Si(46) is the archetype of the Si clathrates family. X-ray diffractions have revealed an unusual homothetic isostructural transition at similar to 14-16 GPa. Raman experiments, however, suggested even more transitions at lower pressure. We present evidence showing that successive electronic topological transitions are responsible for the transformations. It is shown that the electronic structure of Ba(8)Si(46) is easily perturbed by the environment. Reverse Monte Carlo calculations and in-situ resistivity measurements revealed continual changes in the structure and electrical properties upon compression. This finding is corroborated by results of x-ray Raman scattering study in the vicinity of the Ba N(4,5) and Si L(2,3) absorption edges.
Physical Review B 11/2011; 84(18):184105. · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).
Journal of the American Chemical Society 03/2011; 133(15):6051-60. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the results of synchrotron X-ray and neutron diffraction studies of methane, argon, nitrogen, and xenon clathrate hydrates at high pressure and room temperature. The results reveal common features in the structural transition sequences. All phases transform initially to the hexagonal clathrate structure and all except xenon hydrate eventually form an orthorhombic dihydrate. Argon and nitrogen hydrates adopt the type-II clathrate structure at low pressures and have a tetragonal cage structure between the hexagonal and orthorhombic phases. At normal compression rates, the hexagonal form of xenon hydrate decomposes into ice and xenon at ~2.5 GPa. PACS Nos.: 61.50Ks, 61.10-i, 61.12ExNous rapportons ici les r�sultats d'une �tude par rayonnement X synchrotron et par diffusion de neutrons sur des hydrates clathrates de m�thane, d'argon, d'azote et de x�non � haute pression et � la temp�rature de la pi�ce. Les r�sultats r�v�lent des caract�ristiques communes dans les s�quences de transition de structure. Toutes les phases se transforment d'abord en structure clathrate hexagonale et toutes, sauf l'hydrate de x�non, �ventuellement forment des dihydrates orthorhombiques. Les hydrates d'argon et d'azote adoptent la structure clathrate de type II � basse pression et ont une structure de cage t�tragonale entre les phases hexagonale et orthorhombique. � des taux normaux de compression, l'hydrate hexagonal de x�non se d�compose en glace plus du x�non � ~2,5 GPa.[Traduit par la R�daction]
[show abstract][hide abstract] ABSTRACT: The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2(1)m and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.
Journal of the American Chemical Society 10/2010; 132(45):16212-24. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Synchrotron powder X-ray diffraction, ab initio molecular dynamics calculations and solid state (1)H and (2)H NMR are used to refine the structure of crystalline NH(4)BH(4) including H atoms. Rapid reorientations of both ions mean that on average half-hydrogens occupy the corners of a cube around B or N.
Chemical Communications 10/2010; 46(48):9164-6. · 6.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Variable pressure and temperature conductivity measurements on the bisthiaselenazolyl radical dimer [1a](2) have established the presence of a weakly metallic state over the pressure range 5-9 GPa. To explore the origin of this metallization we have examined the crystal and molecular structure of [1a](2) as a function of pressure. At ambient pressure the dimer consists of two radicals linked by a hypervalent 4-center 6-electron S...Se-Se...S sigma-bond into an essentially coplanar arrangement. The dimers are packed in cross-braced slipped pi-stack arrays running along the x-direction of the monoclinic (space group P2(1)/c) unit cell. Pressurization to 4 GPa induces little change in the molecular structure of [1a](2) or in the slipped pi-stack crystal architecture. Near 5 GPa, however, stress on the dimer leads to buckling of the two halves of the molecule and a contraction in the metrics of the S...Se-Se...S unit. These structural changes can be understood in terms of an electronic configurational switch from a 4-center 6-electron sigma-bonded dimer to a more conventional pi-bonded arrangement. At the same time the slipped pi-stack arrays undergo a concertina-like compression, and the crystal structure experiences highly anisotropic changes in cell dimensions. DFT calculations on the molecular electronic structure of the dimer indicate a marked decrease in the HOMO-LUMO gap as the dimer buckles. Related solid-state calculations indicate a rapid closure of the valence/conduction band gap in the same pressure region and the formation of a quasi-metallic state. Metallization of [1a](2) thus arises as much from intramolecular changes, which give rise to a collapse of the HOMO-LUMO gap and near coalescence of the valence and conduction bands, as from increased intermolecular interactions, which cause widening and overlap of the band edges.
Journal of the American Chemical Society 03/2010; 132(13):4876-86. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Application of physical pressure to a ferromagnetic bisdiselenazolyl radical leads to a decrease in pi-stack slippage. Initially, this leads to an increase in the ferromagnetic ordering temperature T(C), which reaches a maximum of 21 K near 1 GPa. At higher pressures, as the pi-stacks become more nearly superimposed, the value of T(C) diminishes.
Journal of the American Chemical Society 11/2009; 131(44):16012-3. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hexagonal CaLi2 has been predicted to undergo a bifurcated lattice distortion into polymorphs that may be candidates for superconductivity at high pressure. In this study, density functional and experimental X-ray diffraction techniques are employed to investigate the high-pressure stability of hexagonal CaLi2. Calculated enthalpies of hexagonal CaLi2 and elemental Ca and Li indicate that CaLi2 becomes unstable against segregation at pressures above 7 GPa. Our predictions are corroborated by powder X-ray diffraction measurements which show that the application of non-hydrostatic pressure leads to a partial decomposition of CaLi2 to elemental species of Ca and Li. A quantitative phase analysis of hexagonal CaLi2 and cubic Ca clearly shows an increase of the latter for pressures above 14 GPa. Finally, it is suggested that the recently reported superconductivity of CaLi2 at high pressure may contain decomposed Ca and Li.
[show abstract][hide abstract] ABSTRACT: The distinct X-ray transmission profile obtained by scanning a sample in a diamond anvil cell across a collimated X-ray beam is used to monitor sample displacement brought about by rotation. This measured displacement can in turn be used to calculate, and subsequently correct, the sample position with respect to a centre of rotation. This centring method differs from others also based on transmission in that it does not require a 180 degrees sample rotation, nor does it require prior positioning of the rotation axis in the path of the X-ray beam. After a full description of the method, an example is presented together with an extended record of use to evaluate the method in a practical setting. The practice and benefits of rocking polycrystalline samples during X-ray exposure have also been quantitatively examined. Changing the orientation of the sample grains with respect to the incident beam yields the expected result of a more homogeneous intensity distribution along Debye rings. Interestingly, by limiting the amount of time large grains assume a particular orientation, rocking brings about the added effect of significantly reducing detector saturation. Sample rocking yields more reliable relative intensities, a more appropriate line shape and narrower line width. Data are presented for a calibration standard at ambient pressure as well as a research sample at high pressure.
[show abstract][hide abstract] ABSTRACT: Powder x-ray diffraction experiments and first-principles calculations have been carried out to investigate the possibility of a structural phase transition, characterized by a change from ionic to covalent bonding, in strontium hydride at pressures greater than 50 GPa. The powder x-ray diffraction results confirm a previously reported transition from the cotunnite structure to the Ni2In structure at approximately 8 GPa. The Ni2In phase remained stable up to the maximum experimental pressure of 113 GPa. The first-principles calculations, however, predict that under hydrostatic conditions a transition from the Ni2In structure to the AlB2 structure will occur at 115 GPa. A comparison of the pressure-dependent volume yielded by the respective experimental and theoretical studies suggests that in many cases the bulk modulus obtained from experiments carried out under non-hydrostatic conditions may be overestimated. Raman spectroscopy experiments corroborated the previously proposed Ni2In structure, as the spectra obtained at pressures greater than 8 GPa exhibited two Raman-active modes, consistent with those expected from the Ni2In structure.
Solid State Communications 01/2009; 149:830-834. · 1.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: The structure of BaH2 at high pressure up to 58 GPa has been investigated with synchrotron powder x-ray diffraction. A structural phase transition from the low pressure Ni2In structure to a simple hexagonal structure, which started at 40 GPa and was completed at approximately 45 GPa, was confirmed. A loss of Raman signal in this pressure range suggests that an insulator to metal transition has occurred. This observation is corroborated with first-principles calculations. The investigation of the electron–phonon coupling shows that the metallic phase is a very weak superconductor with an estimated superconducting critical temperature Tc in the range of mK.
Solid State Communications 01/2009; 149(43):1944-1946. · 1.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: Resistance measurements of a ``111'' type superconductor LixFeAs (x=0.8,1.0,1.1) were performed at high pressure. The superconductivity transition temperature (Tc) is found to decrease almost linearly with increasing pressure and the pressure derivative, dTc/dP , becomes smaller with increasing Li content. Electron spin resonance experiment at ambient pressure on a stoichometric sample shows gradual decrease in the spin moment and the loss of coherence of spin coupling as the temperature is lowered. A very weak Fe local moment is observed when the temperature is lowered to Tc . In situ synchrotron radiation powder x-ray diffraction at high pressure shows changes in the resistivity are not related to a structural transition.
[show abstract][hide abstract] ABSTRACT: In this report, we present a detailed powder x-ray diffraction study of the structural properties and charge density topology of structure I Xe clathrate hydrate under high pressure and room temperature. The pressure dependence of the structural parameters was determined by applying a Rietveld analysis to the experimental data. The combined Rietveld/maximum entropy method was used to derive the most probable charge density distribution at each pressure. Our results show that the charge density distribution of the encaged Xe atoms differs depending on the type of host cage at all pressures. Spherical electron density distributions were observed for the Xe atoms in the small cages while the atoms in the large cages showed longitudinal elongated electronic distributions. Along with the observed cage deformations, the change in electronic density distribution represents a clear indication that the guest-host interaction differs significantly between the small and large cages at high pressures. A similar behavior has been previously reported in low-temperature studies of methane clathrate hydrate.
The Journal of chemical physics 01/2009; 129(24):244507. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: Powder x-ray diffraction and Raman spectroscopy experiments, together with first-principles calculations, have been carried out to corroborate and clarify the pressure-dependent structural evolution of barium fluoride. X-ray diffraction experiments were performed both with and without a pressure-transmitting medium. The latter serve to reproduce past experimental results, while the former show a marked difference. Specifically, the experiments employing helium as a pressure-transmitting medium yield observed relative intensities and volume compression consistent with the proposed Ni(2)In structure of barium fluoride at pressures greater than 14 GPa. The Raman spectroscopy measurements corroborate the proposed Ni(2)In structure, as the spectra exhibit the two modes expected for the high-pressure phase. The experiments also demonstrate that barium fluoride remains an insulator up to at least 77 GPa, and the results of first-principles calculations suggest that a subsequent insulator-metal transition may not be expected well into the megabar regime.
Physical Review B 01/2009; 79. · 3.77 Impact Factor