Q. Y. Tu

Structural and superconducting properties of La2CuO4+δ chemically oxidized by NaClO

Abstract A series of La 2 CuO 4+ δ samples chemically oxidized in NaClO aqueous solution at room temperature with different oxidation time of 2–120 h have been prepared. The X-ray powder diffraction analyses indicate that all the oxidized samples are orthorhombic and single phase, and show an enhanced orthorhombic distortion with the oxidation time, compared with that of the as-prepared sample. The magnetic susceptibility measurements indicate that all the oxidized samples are bulk superconductors with an onset transition temperature at 45 K. The excess oxygen contents of the oxidized samples determined by iodometric titration increase with the oxidation time, which versus the oxidation time obeys the law of the second order exponential decay. The grain size observed by scanning electron microscopy has no obvious changes for the sample before and after oxidation. All the results indicate that NaClO is indeed an efficient oxidizing agent for La 2 CuO 4 .

The bond ionicity of MB2 (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y)

The bond ionicity in biborides MB2 (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and?Y) has been studied by using the complex chemical bond theory based on a generalization of the Phillips-Van Vechten-Levine scheme. The ionicity of M-B bonds decreases in the following order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V and?Ti. The Mg-B bond in MgB2 has the largest value of ionicity of 96.8% among these examined diborides. Our calculations support the recent results of band structure calculations that Mg atoms are fully ionized so that they can donate valence electrons to the system. The observed superconductivity loss in the solid solution Mg1-xAlxB2 with x>0.1 can be understood in terms of the ionicity and the number of valence electrons.

A NOTE ON THE AlB2 TYPE STRUCTURE

In this paper, the crystal structures of a series of diborides are analyzed based on the hard ball model. We show that the packing efficiency has a close relation with the lattice parameter ratio c/a and the closest packing is achieved at c/a = 1.074. For the compounds with c/a 1.074 the packing is loose along the c-direction and the B–B bond is stretched. The structures will be unstable as the c/a value deviates from 1.074. MgB2 has the largest value of c/a among the examined diborides, except for lanthanide diborides and YB2.

Hydrothermal oxidation : a new chemical oxidation method to dope oxygen in La2CuO4+δ

A new oxidation method, chemical oxidation under hydrothermal conditions, is introduced to insert oxygen into La2CuO4 compounds. X-ray powder diffraction and magnetic susceptibility measurements show that the oxidized La2CuO4+δ with orthorhombic space group becomes a superconductor with a superconducting transition up to 42 K, which indicates the method is effective in inserting oxygen into La2CuO4. In addition, the oxidation mechanism and the unique properties of the closed system are discussed.

Study on the structure and superconductivity of La2CuO4+δ chemically oxidized by NaClO at 50 °C

A series of La2CuO4+δ samples chemically oxidized at 50 °C with different oxidation times using NaClO as the oxidizing agent have been prepared. All the oxidized samples are single-phase bulk superconductors with an onset transition temperature at 42-45 K measured by dc magnetic susceptibility. X-ray powder diffraction indicates that all the oxidized samples are orthorhombic, and show an enhanced orthorhombic distortion with the oxidation time, compared with that of the starting material. The excess oxygen contents of the oxidized samples determined by iodometric titration are large and increase with the oxidation time. The fit to the experimental data reveals that the excess oxygen content versus the oxidation time obeys the law of the second-order exponential decay. No obvious changes in the grains of the samples before and after oxidation have been observed by scanning electron microscopy.

STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF La2CuO4+δ OXIDIZED BY KMnO4 UNDER HYDROTHERMAL CONDITIONS

A series of samples of La2CuO4+δ chemically oxidized in KMnO4 solution under hydro-thermal conditions have been prepared. A large orthorhombic distortion and the increased unit cell volume of the oxidized sample have been observed by means of X-ray powder diffraction, compared with that of the starting material. Selected-area electron diffraction investigations reveal that two kinds of incommensurate modulation structures exist in the oxidized samples. Magnetic susceptibility measurements indicate that the oxidized samples are multiphase with two superconducting phases, one is dominant with onset of superconducting transition temperature at 43 K or 40 K, and the other at 19 K. Up to now, a Tc of 43 K is the highest value achieved in KMnO4-treated samples. The excess oxygen content determined by iodometric titration increases with the oxidation temperature. Average grain sizes observed by scanning electron microscope have no obvious changes for the samples before and after oxidation.