W.-C. Pilgrim

High-pressure vessel for elastic and inelastic x-ray diffraction experiments for liquids over a wide temperature range

A new x-ray scattering environment is presented which can be employed at high temperatures up to 2000 K and high pressures up to 150 bar. This device is especially designed for angle-dispersive scattering experiments. It consists of a thick closed-end aluminum cylinder equipped with a steel flange. Incident and scattered radiation passes through continuous Be windows which cover an angle range up to 55°. The high potential of this scattering setup is demonstrated by representative example experiments at high temperatures and high pressures, which could be carried out in combination with a single-crystal sapphire cell. Inelastic-, elastic-, and anomalous scattering experiments using synchrotron radiation and energy dispersive static measurements employing a conventional tube device are reported.

Anomalous X-ray scattering studies on glassy GexSe1-x across the stiffness threshold composition x = 0.20

Anomalous x-ray scattering experiments on glassy GexSe1−x have been carried out at energies close to the Ge and Se K edges at concentrations close to the onset- and at the mid-point of the rigidity percolation threshold (x = 0.195 and 0.23). The total structure factors S(Q) show rapid variations in both the position and intensity of the prepeak around 11 nm−1, while it stays almost unchanged in the remaining Q range. The differential structure factors ΔiS(Q) comprise characteristic features. Comparing them suggests that the prepeak purely originates from the Ge-Ge correlation. The present structural results strongly support the chemically-ordered continuous-random-network model for the Ge-Se glasses. The origin of the prepeak was discussed from a comparison between the SGeGe(Q) of g-GeSe2 obtained by Petri et al. and S(Q) of a-Ge. The concentration dependence of the prepeak is consistent with the Raman data, which reveal the diminishing of short Se chain connections between the Ge(Se1/2)4 tetrahedra near the stiffness transition composition.

The metal-non-metal transition in compressed metal vapours

Knowledge of the properties of hydrogen and helium and their mixtures, at temperatures and pressures prevailing in the giant planets is of considerable interest for planetary modelling. In the light of the unfavourable outlook for reliable measurements under these extreme conditions effort has been spent investigating the high-temperature high-pressure properties of fluid metals which are experimentally accessible in the laboratory and which might serve as models for compressed fluid hydrogen. The main emphasis of the paper is on the density dependence of the dynamic structure factor of liquid rubidium which reveals that a monoatomic-molecular transition occurs in the metal-non-metal transition region of the expanded liquid analogous to that suggested to occur in shock compressed hydrogen. Additional emphasis is on new results of the phase behaviour of dilute mixtures of helium in the near critical metal mercury.

The metal-non-metal transition and the dynamic structure factor of expanded fluid alkali metals

This paper discusses the electrical and optical properties and, in particular the neutron scattering measurements of the dynamic structure factor in the metal-non-metal transition range. This transition occurs when the dense liquid evaporates to the dilute vapour or when the fluid is expanded by heating to its liquid-vapour critical point. The shape of S(Q, omega ) changes considerably on approaching the transition from the high-density liquid side, indicating a change in the interparticle interaction and the molecular structure. S(Q, omega ) of rubidium in the density range between the melting point density and three times the critical density is characterized by the existence of well defined acoustic-phonon-like collective density excitations at high momentum transfer, whereas S(Q, omega ) at a density of about twice the critical density is consistent with excitations of an optic-type mode in which two species tend to move in opposite directions.

Sub-picosecond dynamics in liquid Si

We are the first group to succeed in measuring the dynamic structure factor S(Q,ω) of liquid Si close to melting using high-resolution inelastic x-ray scattering. The spectra clearly demonstrate the existence of propagating short wavelength modes in the melt with a Q–ω relation similar to those in other liquid metal systems. A specific variation of the quasi-elastic line shape with increasing Q is observed close to the structure factor maximum. This observation is related to the onset of atomic correlations on the sub-picosecond timescale in the vicinity of a metal-to-insulator transition. Such observations have been made previously only in computer simulations of metallic systems with increasing covalent character. Our data provide the first experimental evidence for these ultrashort density correlations.

Coherent dynamic scattering law of divalent liquid Mg

The dynamic structure factor S ( Q ,ω) of liquid ( l -) Mg was measured at 700°C using high-resolution inelastic X-ray scattering. Based on the high-quality experimental data presented in this paper, we discuss the particle dynamics in dense l -Mg using the generalized hydrodynamic theory for phonon excitations. The particle motion in this liquid is influenced by collective longitudinal modes, similar to other simple liquid metals such as l -alkali metals. The Q dependence of quasielastic line width can be interpreted based on an existing theory modified from the de Gennes formalism.

Quasielastic lineshape of liquid Sn studied by inelastic X-ray scattering

Abstract Dynamic structure factors S(Q,ω) of liquid Sn were measured at 320 and 1000 °C in the Q range from 2 to ∼30 nm−1 using inelastic X-ray scattering (IXS). The obtained spectra clearly demonstrate the existence of longitudinal propagating modes. A positive dispersion is found in the low Q region, where the mode velocity is about 12% and 6% larger than the hydrodynamic speed of sound at 320 and 1000 °C, respectively. The quasielastic lineshape contains a Gaussian contribution near the Q position of the first maximum in S(Q), which has not yet been reported in simple liquid metals like liquid alkalis. With increasing temperature, the Gaussian contribution increases and can be observed in a wider Q range. The quasielastic lineshape is discussed by connecting with the formation of instantaneous clusters in the melt.

X-ray diffraction measurements on expanded fluid Rb

Abstract An investigation of the density dependence of the structure in liquid rubidium was performed along the liquid vapour coexistence line up to temperatures of 1573 K and 50 bar, and corresponding densities between 1.45 and 0.87 g/cm 3 . All experiments were carried out at the beamline ID30 of the European Synchrotron Radiation Facility (ESRF) in Grenoble. The diffraction pattern could be measured using the combination of a newly developed high-temperature high-pressure scattering setup and monochromatized X-rays of about 50 keV with an imaging plate detector. Although the duration of a single experiment took only approximately 20 s, an extreme improvement in scattering statistics and an extended Q-range could be achieved compared with a previous neutron scattering result on the same system.

Density dependent single particle motion in liquid sodium

Abstract In a series of quasielastic neutron scattering experiments on liquid sodium the density dependence of the single particle motion was investigated for a variety of states along the liquid vapor co-existence curve. The results are in excellent agreement with computer simulation studies on hard sphere systems and can – on a quantitative basis – be discussed in the light of mode-coupling theory. A distinct universal deviation for the density dependence of the diffusion constant is found, which can be understood on a purely microscopic basis: at high density, the particles motion is hindered by a coupling to longitudinal density fluctuations, while at elevated temperatures and intermediate densities a coupling to transverse shear modes sets in, promoting single particle dynamics relative to Brownian Diffusion. At even higher temperatures and low liquid densities mode-coupling effects die out and the system gradually returns to simple binary collision dynamics.

Rigidity transitions and intermediate structures of Ge–Se glasses – An anomalous X-ray scattering study

Abstract Anomalous X-ray scattering experiments on glassy GexSe1−x have been carried out at energies close to the Ge and Se K edges in a wide concentration range from x=0.15 to 0.333 across the rigidity percolation threshold. The total structure factors S(Q) show considerable variations in both the position and intensity of the prepeak around 10 nm−1, while in the remaining Q range, it stays almost unchanged. The differential structure factors ΔiS(Q) comprise characteristic features. Comparing them suggests that the prepeak purely originates from the Ge–Ge correlation. The threshold is characterized by the percolation of two-Se-linked Ge(Se1/2)4 tetrahedra distributed all over the network as proposed by Feltz et al.