Professor Paolo G. Radaelli OSI

Abstract:

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network. The CuIr2S4 thiospinel is another example, well known for the presence of a metal-insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal-insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition-a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.

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.