Oxide electronics

Abstract:

Two series of three specimens for a total of six compositions were prepared by the sol-gel method and the precursors were pyrolised at 1000°C under inert atmosphere of Argon after the hydrolysis/condensation process. The Neutron Diffraction patterns of these products are typical of amorphous materials. The radial distribution function (RDF) of these glasses suggests that the main network is composed, as in silica glass, of inter-connected tetrahedral units where some of the di-valent oxygen atoms have been substituted for tetravalent carbon atoms. A concomitant structural study was conducted on the same specimens after pyrolysis at 1200, 1400 and 1500°C respectively, in order to follow phase separation phenomena induced at high temperature. After pyrolysis at 1500°C the patterns show the formation of β-SiC and, additionally, for the composition with low carbon content, the appearance of microcrystalline cubic silicon, with a consistent fraction of the matrix still amorphous. The specimens with high carbon content display clearly in the RDF the C-C distances typical of graphitic-like structures.

Abstract:

The crystal structure of Fe(3)O(4) below the 122 K Verwey transition has been refined using high-resolution x-ray and neutron powder diffraction data. The refinements give direct evidence for charge ordering (CO) over four independent octahedral Fe sites, two with a charge of +2.4 and the other two of +2.6. CO schemes consistent with our model do not meet the widely accepted Anderson condition of minimum electrostatic repulsion. Instead we propose that CO is driven primarily by a [001] electronic instability, which opens a gap at the transition through a charge density wave mechanism.

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.