Professor Paolo G. Radaelli OSI

Abstract:

A neutron powder diffraction study of three n-alkanes, octane, nonane and pentadecane, down to 2 K is presented. The temperature dependence of the octane diffraction pattern reveals a solid state phase transition between 40 and 55 K, which involves a doubling of the unit cell in the b direction, the space group remaining P1̄. Confirmation of the phase transition, which results in a doubling of the number of crystallographically inequivalent methyl groups, is sought in the published NMR, tunnelling data and neutron scattering, vibrational data. Density functional theory and force field techniques are used to simulate spectroscopic data based on the measured structures. While no unequivocal evidence is found in spectroscopic data, the published data does not rule out the existence of energetically inequivalent methyl groups. Indeed close inspection of the spectroscopic data for other n-alkanes suggests that the phase transition may be common to many alkanes. © 2001 Elsevier Science B.V.

Abstract:

Microstructural stability for nickel-base superalloys were studied to investigate between lattice stability, diffusion and thermal treatments. The volume fraction of precipitates, phase composition and lattice mismatch between precipitates and matrix were found. Neutron scattering experiments were carried out using a diffractometer. An individual diffraction spectrum of sufficient resolution was obtained in one minute interval.

Abstract:

Magnetite is a classic example of a mixed-valent transition metal oxide, in which electronic conductivity and ferromagnetism result from electron hopping between octahedrally coordinated Fe²⁺ and Fe³⁺ states. Below the 122 K Verwey transition, the conductivity falls by a factor of ∼100 and a complex monoclinic (or triclinic) superstructure of the high temperature cubic spinel arrangement is adopted. This is assumed to be the result of Fe²⁺/Fe³⁺ charge ordering on the octahedral sites, but this has not been confirmed crystallographically, as single crystal refinements have been hampered by the extensive twinning that accompanies the Verwey transition. We have used very highly resolved powder diffraction data to attempt Rietveld refinements of the low temperature structure. The powder sample was prepared by grinding a single crystal of stoichiometric magnetite. Data were collected at 90 K on instruments HRPD at the ISIS neutron source, UK, and BM16 at the European Synchrotron Radiation Facility, France. The very high resolution of these data enables the monoclinic distortion to be observed, and the structure has been refined on the supercell proposed by Iizumi et al (Acta Cryst. B38, 2121 (1982)) with Pmca pseudosymmetry, giving parameters a = 5.94443(1), b = 5.92470(2), c = 16.77518(4) Å, β = 90.236(1)°. The mean octahedral site Fe-O distances differ from each other significantly, but the maximum difference between values is only 20% of that expected for ideal Fe²⁺/Fe³⁺ charge ordering.

Abstract:

We report the synthesis under pressure, the critical temperature and the structural characteristics of YSr2Cu3Ow. The compound does not form at normal pressure, but, once formed, can be retained at normal pressure in a metastable state. The synthesis was carried out in a multi-anvil apparatus at 30 kBars and 1050°C. The as-prepared sample was found to become superconductor with a Tc of ≈ 63K. Tc increased up to ≈ 70 K after annealing in oxygen at 290°C. The refinement of the neutron diffraction data indicated that the as prepared YSr2Cu3Ow is tetragonal (a=3.7855 Å and c=11.386 Å) and the oxygen content, w, is ≈ 6.84.