Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_e$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_e$ and the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_e$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also displays a significant number of residual dimers and trimers.Comment: 14 pages, 5 figures, 1 tabl

Phonon dispersion and lifetimes in MgB2

We measure phonon dispersion and linewidth in a single crystal of MgB_2 along the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We use Density Functional Theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E_2g phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.Comment: 4 pages, 3 figure

Structural phase transitions in aluminium above 320 GPa

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Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation

International audiencePressure induced phase transitions in GeTe, a prototype phase change material have been studied to date with diffraction which is not sensitive to anharmonicity induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high pressure Raman scattering and ab initio pseudopotential density functional calculations to unambiguously establish the high pressure phase diagram and identify three phases up to 57 GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase and a high pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases

Functional Monochalcogenides: Raman Evidence Linking Properties, Structure, and Metavalent Bonding

International audiencePressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of “incipient” metals

Unveiling the electrochemical mechanisms of the Li 2 Fe(SO 4 ) 2 polymorphs by neutron diffraction and density functional theory calculations

International audienceThe quest for new sustainable iron-based positive electrode materials for lithium-ion batteries recently led to the discovery of a new family of compounds with the general formula Li2M(SO4)2 with M = transition metal, which presents monoclinic and orthorhombic polymorphs. In terms of electrochemical performances, although both Li2Fe(SO4)2 polymorphs present a similar potential of ∼3.8 V vs. Li+/Li0, the associated electrochemical processes drastically differ in terms of polarization and reaction redox mechanisms. We herein provide an explanation to account for such a behavior. While monoclinic Li2Fe(SO4)2 directly transforms into Li1.0Fe(SO4)2 upon oxidation, the orthorhombic counterpart forms a distinct intermediate Li1.5Fe(SO4)2 phase leading to a two-step delithiation process involving an unequal depopulation of the two Li sites pertaining to the structure as deduced by neutron powder diffraction experiments and confirmed by both density functional theory and Bond Valence Energy Landscape calculations. Moreover, to access band gap information, both polymorphs are studied by UV/Vis spectroscopy. Lastly, the possibility of transforming the monoclinic phase to the orthorhombic phase under pressure is explored

Disorder-order phase transition at high pressure in ammonium fluoride

Solid NH4F displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, X-ray diffraction and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made

Structural and electronic phase changes in GaGeTe with pressure

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Publisher's Note: Magnetostructural coupling and magnetodielectric effects in the A -site cation-ordered spinel LiFeCr₄O₈ [Phys. Rev. B 96 , 214439 (2017)]

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