M.-L. Saboungi

Electron distribution in water

The x-ray structure factor of water measured under ambient conditions with synchrotron radiation is compared with those predicted on the basis of partial structure factors describing the nuclear positions obtained by neutron diffraction and of different assumptions for the electron distribution. The comparison indicates that a charge of approximately 0.5 e is transferred from each hydrogen atom to the oxygen on the same molecule, implying an effective dipole moment of 2.9 D, in good agreement with theoretical estimates.

Melting in Trivalent Metal Chlorides

We report a neutron diffraction study of the liquid structure of YCl3 and combine the structural data with macroscopic melting and transport data to contrast the behaviour of this molten salt with those of SrCl2, ZnCl2 and AlCl3 as prototypes of different melting mechanisms for ionic materials. A novel melting mechanism for trivalent metal chlorides, leading to a loose disordered network of edge-sharing octahedral units in the liquid phase, is thereby established. The various melting behaviours are related to bonding character with the help of Pettifors phenomenological chemical scale.

Quantum effects on the structure of water at constant temperature and constant atomic density

To explore quantum effects on the structure of liquid water, we have carried out high-energy x-ray diffraction measurements of quantum differences both under isothermal conditions at 24.5 °C and under isochoral conditions at 0.0997 atom/A3. The measured isothermal difference correlation function is approximately equivalent to an isochoric temperature differential (ITD) of 5.5 °C, reflecting the tendency of quantum effects to introduce more disorder into the liquid. The measured isochoral correlation function is about three times higher in amplitude and is approximately equivalent to an ITD of 19 °C. Since the isochoral measurements for H2O and D2O were made at temperatures 13.5 °C apart, the isothermal and isochoral functions are roughly consistent. The discrepancies are discussed with reference to data on the pressure dependence in the literature. They are comparable with differences in results obtained with different potential functions in path-integral molecular dynamics simulations. The present results lend further validity to the notion that quantum effects on the structure of liquid water are similar to those of thermal disorder, as long as the effects of differences in the equation-of-state of H2O and D2O are taken into account.To explore quantum effects on the structure of liquid water, we have carried out high-energy x-ray diffraction measurements of quantum differences both under isothermal conditions at 24.5 °C and under isochoral conditions at 0.0997 atom/A3. The measured isothermal difference correlation function is approximately equivalent to an isochoric temperature differential (ITD) of 5.5 °C, reflecting the tendency of quantum effects to introduce more disorder into the liquid. The measured isochoral correlation function is about three times higher in amplitude and is approximately equivalent to an ITD of 19 °C. Since the isochoral measurements for H2O and D2O were made at temperatures 13.5 °C apart, the isothermal and isochoral functions are roughly consistent. The discrepancies are discussed with reference to data on the pressure dependence in the literature. They are comparable with differences in results obtained with different potential functions in path-integral molecular dynamics simulations. The present result...

Combined X-ray and neutron diffraction from binary liquids and amorphous semiconductors

Abstract Neutron scattering and isotopic substitution has been established as a method of unambiguously determining the partial structure factors of binary liquid and amorphous materials. In this paper we present results of an extension of this method to include X-ray diffraction as a method of enhancing the structural information obtained. We have applied a combination of two neutron diffraction experiments using isotopic substitution and one X-ray diffraction experiment to obtain the partial structure factors and pair distribution functions of liquid TlSe. We have applied the anomalous X-ray diffraction technique to glassy PSe using the Se K-edge. The results show that the first sharp diffraction peak observed in this glass arises almost entirely from the P–P correlations. We have applied the combined X-ray anomalous scattering and neutron diffraction technique to obtain the partial structure factors of glassy GeO2. The results from two independent data sets have been compared and are in good agreement.

Local and Intermediate-Range Order in Cesium Germanate Glass

Complementary neutron diffraction and anomalous x-ray diffraction techniques are used to probe the nature and origin of local and intermediate-range order in cesium germanate glass. Both neutron and differential x-ray structure factors reveal peaks at low wave vector associated with intermediate-range order. The real-space correlation functions show that the local interatomic correlations in the glass are similar to those in the reference crystal Rb2Ge4O9 and imply a homogeneous distribution of Cs+ cations.

Longitudinal excitations in Mg–Al–O refractory oxide melts studied by inelastic x-ray scattering

The dynamic structure factor S(Q,omega) of the refractory oxide melts MgAl2O4 and MgAl4O7 is studied by inelastic x-ray scattering with aerodynamic levitation and laser heating. This technique allows the authors to measure simultaneously the elastic response and transport properties of melts under extreme temperatures. Over the wave vector Q range of 1-8 nm-1 the data can be fitted with a generalized hydrodynamic model that incorporates a slow component described by a single relaxation time and an effectively instantaneous fast component. Their study provides estimates of high-frequency sound velocities and viscosities of the Mg-Al-O melts. In contrast to liquid metals, the dispersion of the high-frequency sound mode is found to be linear, and the generalized viscosity to be Q independent. Both experiment and simulation show a weak viscosity maximum around the MgAl4O7 composition.

The molten state of trivalent metal halides and oxides : recent progress.

Abstract The structures of trivalent metal halides and oxides have been reviewed with emphasis on the stability of a six-fold coordination of metal ions, e.g. in YCl 3 and YO 3 , and its competition with a four-fold coordination, e.g. in AlCl 3 and AbO 3 . Upon melting in some cases the crystalline coordination is preserved, leading to the formation of either an ionic network or a molecular structure, while in other cases a transition to a different kind of structure occurs, such as that from an ionic to a molecular-type structure. Specific examples will be selected from recent experiments in which structural information at the partial level and Raman spectra have been obtained, and the X-ray average structure has been complemented by electrical transport data. Directions for future work with a potential impact on our understanding of the complex structure-property relations in the crystalline and liquid state have been proposed.

Application of new synchrotron powder diffraction techniques to anomalous scattering from glasses

We have applied two synchrotron powder diffraction techniques to the measurement of high quality anomalous scattering diffraction data for amorphous materials. One of these methods, which uses a curved perfect crystal analyzer to simultaneously diffract multiple powder lines into a position sensitive detector has been shown to possess high resolution, low background, and very high counting rates. This data measurement technique provides excellent energy resolution while minimizing systematic errors resulting from detector nonlinearity. Anomalous scattering data for a Cesium Germanate glass collected using this technique will be presented. The second powder diffraction technique uses a flat analyzer crystal to deflect multiple diffraction lines out of the equatorial plane. Calculations show that this method possesses sufficient energy resolution for anomalous scattering experiments when a perfect crystal analyzer is used and is experimentally much simpler. Future studies will make use of a rapid sample cha...

Raman scattering in liquid K-Te.

Raman-scattering measurements from crystalline and liquid K{sub 0.12}Te{sub 0.88} and K{sub 0.50}Te{sub 0.50} semiconducting alloys are reported. The spectra of liquid K{sub 0.12}Te{sub 0.88} are characterized by a single, weakly polarized band, markedly different from those obtained from pure liquid Te and bearing a greater similarity to the spectra reported for amorphous Te. The spectra of K{sub 0.50}Te{sub 0.50} in the crystalline and liquid phases show remarkable similarities, confirming the presence of the Te{sub 2}{sup 2-} Zintl ion in both phases. Substantial softening of the symmetric stretching mode occurs relative to the equivalent mode in vapor and matrix-isolated Te{sub 2}. This is most likely due to the charge transfer interaction with the K{sup +} matrix. A comparison of structural data available for liquid and amorphous Te with those for liquid K{sub 0.12}Te{sub 0.88} and K{sub 0.50}Te{sub 0.50} indicates a transition to true 2-fold chainlike covalent bonding of Te in the liquid on addition of potassium. The importance of 2-fold bonding for the semiconducting behavior of Te-rich alloy systems in the liquid and solid states is discussed.

Survival of polyanions in expanded liquid Zintl alloys

Abstract Neutron diffraction experiments are reported of the equiatomic liquid Zintl alloys NaSn and KPb, over a wide temperature range from 873 to 1873 K. In the crystalline state, these are semiconducting compounds containing tetrahedrally coordinated polyanions (Sn 4 4− , Pb 4 4− ). For liquid KPb, diffraction results indicate the survival of polyanions in the expanded liquid alloy even to the highest temperature measured, as can be inferred from the persistence of the peak of S(Q) at Q ≈ 1 A −1 , which is indicative of intermediate-range order. The position of this peak shifts towards lower Q -values, its height decreases, and its width increases with temperature and correspondingly decreasing density. In liquid NaSn, this so-called prepeak has disappeared at 1873 K. The experimental results are compared with the results of recent ab initio molecular dynamics and computer modelling studies.