Tim Williams

Phase diagram of LaTiO x : from 2D layered ferroelectric insulator to 3D weak ferromagnetic semiconductor

LaTiOx compounds are structurally related to perovskites and there are two known phases. The first,x=3.50, is a 2D layered-type ferroelectric. The second,x=3.00, is a weak ferromagnet with a 3D orthorhombic distorted perovskite structure. 20 samples with varying oxygen stoichiometry between these end members were prepared by floating zone melting, and then characterized by means of X-ray powder diffraction, electron microscopy, thermogravimetric analysis, resistivity and magnetic measurement. A phase diagram is established which displays the following physical and structural properties. A structural phase boundary atx=3.20 separates a new series of 2D layered structures from the 3D orthorhombic one. The former series represents the first conducting titanium oxides with a 2D layered structure to be reported. Atx=3.10 a phase boundary exists between a metallic and a weak ferromagnetic state where the magnetic transition temperatureTc can be sensitively tuned by the oxygen stoichiometryx. Samples withTc between 100 K and 130 K exhibit a metal-semiconductor transition whereas samples with higherTc, up to 149 K, are semiconductors between room temperature and 4.2 K.

Structural study on high-Tc superconductor Bi2−x(Ca, Sr)3Cu2+xO9−y

The crystal structure of the high-Tc Bi2-x(Ca, Sr)3Cu2+xO9-y with x=0.2 and y=0.78 was studied by means of neutron and X-ray powder diffraction. Since the unit cell parameters were relatively large for powder diffraction analysis, an averaged structure was refined, assuming an orthorhombic sub-cell (a=5.39 A, b=5.39 A and c=30.37 A) which corresponds to 1/5 of the unit cell: This is a derivative of Bi4Ti3O12. The double perovskite unit interleaved by the double bismuth layers is B-type, i.e. a CuO6 octahedron is surrounded by eight alkaline earth metal ions. The pyramidal CuO5 configuration is found in this oxide. The copper valency is estimated at about 2.3 on the basis of the determined chemical composition.

Oxygen vacancy ordering in superconducting Nd2−xCexCuO4−y

Abstract We report transmission electron microscopy results which show that an appreciable fraction of superconducting ceramic and single-crystal samples of the Nd 2− x Ce x CuO 4− y T phase shows evidence for oxygen vacancies in the [Cu-O] planes which in some cases are ordered, either commensurately or incommensurately with the cation sublattice. As T phase samples displaying a 100% Meissner signal are not yet available we cannot determine unambigously whether or not the oxygen loss is associated with superconduction but our results strongly suggest that supercell formation accompanying vacancy ordering occurs in over-reduced material which is no longer superconducting.

Electron Channelling Effect in an Al–Fe–Cu Quasicrystal

Strong electron channelling effect has been observed in a quasicrystal of Al-Cu-Fe alloy which has a near-perfect icosahedral symmetry and low phason density. When the electron beam was set parallel with one of the high symmetry zone axes such as the 5-fold and 2-fold, the intensities of the characteristic X-ray of Fe and Cu relative to that of Al were much stronger than under non-channelling conditions. This result directly indicates that both Fe and Cu atoms occupy their own definite atomic sites different from those of the host element Al.

A New Series of Compounds in the T* Structure: La2-x-zLnzSrxCuO4-y (Ln=Eu, Gd, Tb and Dy)

We report the synthesis of a new series of layered copper-oxide compounds La2-x-zLnzSrxCuO4-y (Ln=Eu, Gd, Tb and Dy), which are isostructural to the T*-phase of superconductive Nd-Ce-Sr-Cu-O. Annealing in high-pressure oxygen atmosphere improves the metallic conductivity of the samples. However, we have not been able to induce bulk superconductivity in these new compounds.