Please note the unusual time!

General ILL Seminar organized by College 3

Tuesday, 7th February 2012, 9:00

Seminar room ILL1

Guillaume Pignol

LPSC Grenoble

The accelerating expansion of the universe has been recently discovered and confirmed. It is one of the most puzzling observations of modern cosmology: 70% of the energy budget of the universe today has to be attributed to a completely unknown type of "Dark Energy". One theoretical route to address this problem is to assume the existence of a cosmological scalar field, the quintessence, with nontrivial dynamics. If this quintessence field is coupled to ordinary matter particles, it would also mediate a fifth fundamental force displaying very unusual "chameleon like" properties. I will discuss how to probe such a force using bouncing ultracold neutrons, in particular using the GRANIT instrument.

GENERAL ILL SEMINAR College 6

Wednesday, 8 February 2012 11:00 am

ILL, Room 427 (Seminar Room, ILL 04, fourth floor)

J. Kozaily

Institut Laue Langevin (ILL)

CEMHTI site HT, Orléans, France

Because of their special properties as glass-forming systems, molten silicates play an important role in the geology of the Earth’s crust and mantle and are also of industrial interest for nuclear waste treatment. Research in these areas requires fundamental information on the microscopic structure and dynamics of silicate melts, but such measurements are hampered by the very high melting points of these systems. By extending the technique of aerodynamic levitation to inelastic neutron scattering, and also making use of inelastic synchrotron x-ray scattering, we have obtained results on the microscopic dynamics of silicates both above the melting point and in the supercooled regime. In particular, we have determined the temperature evolution of the viscosity and diffusion coefficient of calcium aluminosilicates, and thereby quantified the decrease in fragility of this glass-forming system as a function of increasing silica content. In parallel with our dynamical studies, we have performed x-ray and neutron diffraction experiments on the same compositions at the same temperatures in order to examine the local chemical order pertinent to the observed dynamical properties.

Keywords : Calcium aluminosilicates, Liquids, Levitation, QENS, Diffraction

M. BRUNELLI College 6 Secretary

External visitors may ask for a site access to Brigitte Dubouloz (dubouloz@ill.fr).

GENERAL ILL SEMINAR organised by College VII

Tuesday, February 14, 2012, 15h00
Seminar Room 1rst floor, ILL 4

Dr. Alexandra Friedrich
Institut für Geowissenschaften, Goethe-Universität Frankfurt am Main, Germany

Transition-metal carbides, nitrides and borides have a high potential for industrial applications as they generally show higher hardness, lower compressibility and higher melting points than the pure metals. The formation of new materials with high density and hence potential for high hardness is promising at extreme pressures and temperatures [1]. Rhenium has a high density of valence electrons and forms ultra-incompressible, hard Re2C and ReB2 [1]. No bulk rhenium nitrides have been known so far.
Reaction of rhenium and nitrogen was achieved in the diamond anvil cell at pressures up to 32 GPa via double-sided laser heating at temperatures up to 3000 K [2]. Two phases of rhenium nitrides were synthesized at different (p,T)-conditions. Samples were analysed in situ using powder synchrotron x-ray diffraction at the ALS (Berkeley, beamline 12.2.2). Both phases are ultra-incompressible with bulk moduli of ~400 GPa. Recovered samples were characterized using synchrotron x-ray Laue microdiffraction at the ALS beamline 12.3.2 and micro-Raman spectroscopy.
The crystal structures and ideal stoichiometries of both phases (Re3N and Re2N) were obtained from density functional theory (DFT)-based calculations and confirmed by Rietveld refinement of the rhenium positions. From the comparison of the Raman spectra of recovered samples with the results of lattice-dynamical calculations using DFT by our group [3] and by Deligoz et al. [4] the structural models of both rhenium nitrides could be confirmed with respect to the nitrogen atom positions. Further, the mode-Grüneisen parameters were obtained from pressure-dependent Raman-spectroscopic measurements and DFT-based calculations.
References:
[1] A. Friedrich, B. Winkler, E.A. Juarez-Arellano, L. Bayarjargal, Materials, 4, 1648, 2010.
[2] A. Friedrich, B. Winkler, L. Bayarjargal, W. Morgenroth, E.A. Juarez-Arellano, V. Milman, K. Refson, M. Kunz, K. Chen, Phys. Rev. Lett. 105, 085504, 2010.
[3] A. Friedrich, B. Winkler, K. Refson, V.Milman, Phys. Rev. B 82, 224106, 2010.
[4] E. Deligoz, K. Colakoglu, H.B. Ozisik, Y.O. Ciftci, Solid State Commun. 151, 1122, 2011.

M. Zbiri
(College VII Secretary)
Participants who have no badges allowing entrance to the ILL-ESRF site are kindly requested to give their names
to Laurence Tellier (tellier@ill.fr).