Oxide electronics

Abstract:

Neutron Rietveld refinement of a synthetic low silica ferrierite in deuterium form was performed in the Immm space group. This space group is different from that of as-synthesized form (which is P21/n), and its cell volume (V=1977Å3) is slightly smaller (V=1984Å3). These differences do not cause noticeable modifications in the framework. Two Brønsted acid sites were recognized. The first was on framework oxygen O4, which bridges T1 and T3 tetrahedron cations, and headed towards the center of the ferrierite cage. The other was on the framework oxygen O6, which bridges two T4 tetrahedron cations, and headed towards the 10-ring channel parallel to [001], not far from the center of the 8-ring of the ferrierite cage facing towards the channel. Both deuterium sites were occupied in about 15% of cases. Three other extraframework sites, at great distances from the extraframework oxygens and weakly occupied, were recognized in D-ferrierite, and were attributed to residual ND4 groups. © 1999 Elsevier Science B.V.

Abstract:

Few recent examples are discussed where neutron diffraction had a major impact in rapidly developing areas of materials science and that illustrate advances in experimental technique and data analysis. Emphasis is put on imaging structural information across a multidimensional parameter space. High-speed diffractometers with low background have enabled impressive structural studies on small samples previously thought to be beyond the capability of neutron diffraction. Precision measurements of structural parameters have provided critical tests of theoretical models for the behavior of novel materials, and extreme sample environments have opened new experimental frontiers. The impact of neutron diffraction on condensed-matter science is highlighted.

Abstract:

The structural, magnetic and transport phase diagrams of the manganese perovskites (A1-xA'xMnO3) are characterized by several phenomena, including high-temperature polaronic behavior, charge-orbital and magnetic ordering and colossal magnetoresistance (CMR). These properties can be tuned by changing the doping level, the electronic bandwidth, and the A-site disorder. To demonstrate this, the recent x-ray synchrotron and neutron diffraction data on the crystallographic and magnetic modulation in La0.33Ca0.67MnO3 are presented.

Abstract:

Neutron powder diffraction measurements of perdeutero durene in the temperature range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunneling and librational dynamics. Ab initio methods and atom-atom potentials are used to determine rotational single particle and coupling potentials. Tunneling splittings and librational bands are calculated by numerical solution of Schrödinger's equation for a system of many coupled methyl groups. High-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolving an inconsistency with earlier NMR work. Quantum molecular dynamics provide a stringent test of the numerical methods and the data are ultimately well reproduced. These results are also discussed in the context of optical measurements of dye molecules in a host lattice of durene. © 1999 American Institute of Physics.

Abstract:

The crystallographic structure and the magnetic order of the distorted perovskite La1-xSrxCoO3 (0.10≤x ≤0.30) has been studied by neutron diffraction, high-resolution electron microscopy, and magnetic-susceptibility measurements. The results give direct evidence for an inhomogeneous distribution of the Sr2+ ions and the segregation of the material into hole-rich ferromagnetic regions and a hole-poor semiconducting matrix at lower values of x. The holes introduced by Sr doping are attracted to the Sr2+ ions where they stabilize to lowest temperatures an intermediate-spin state at neighboring trivalent cobalt. The antibonding e electrons so stabilized increase the mean unit-cell volume and are delocalized over the cobalt atoms of the cluster where they couple the localized t5 configurations ferromagnetically. Long-range ferromagnetic order between clusters is realized even for Sr doping as low as x=0.10. The transition to a spin glass state is observed only for Sr concentrations smaller than 0.10. The volume of a hole-rich cluster grows in a magnetic field, and the origin of the large negative magnetoresistance observed near Tc for 0.15 ≤ x ≤ 0.25 appears to be due to a growth of the clusters to a percolation threshold. For x=0.30, the σ* band of the intermediate-spin state below Tc is at the threshold of a transition from itinerant to polaronic conduction and, above Tc, the system transforms smoothly to a cluster state. © 1999 The American Physical Society.