H. Faqir

A New High-Pressure Phase of Bismuth Oxide

Abstract A new polymorphic form of bismuth oxide (Bi 2 O 3 ) is synthesized by a high-pressure technique (6 GPa and 880°C, and subsequently slow cooling). This phase can exist under ambient conditions, but it is fairly unstable. The crystal structure is determined to be the A-type rare-earth sesquioxide structure that is isotypic to La 2 O 3 . Hexagonal cell parameters are measured to be a = 3.878(1) A and c = 6.303(1) A. The pressure effect on a polarizable 6s2 lone pair character of Bi 3+ is discussed.

Synthesis and characterization of (Bi, AE)MnO3 (AE=Ca, Sr) system

Abstract The effects of electron lone pairs of trivalent bismuth ions on the crystal structure and magnetic properties were examined in Bi 1− x AE x MnO 3 (AE=Ca, Sr). Polycrystalline perovskite manganites Bi 1− x AE x MnO 3 were synthesized by using a conventional ceramic technique and a high-pressure technique (4 GPa). Perovskites with triclinic, monoclinic, and tetragonal distortions were found in Bi 1− x Sr x MnO 3 from X-ray powder diffraction. In particular superlattice reflections were found in BiMnO 3 by electron diffraction which revealed two kinds of unit cell, i.e. polymorphism. The bismuth lone pair character was reflected in these structures. Both Bi 1− x Ca x MnO 3 and La 1− x Ca x MnO 3 showed the same crystal symmetry and weak ferromagnetism in the Ca-rich region. However, the magnetic moment of Bi 1− x Ca x MnO 3 was larger than that of La 1− x Ca x MnO 3 with the same x value. Their ferromagnetism was suppressed by charge ordering with further substitution of trivalent cations, although the charge ordering temperature of the La-system is higher than that of the Bi system. The difference is probably associated with the Bi lone pair.

Superconductive percolation in Bi-based superconductor/Bi-based insulator composites: case of Bi-2223/Bi-2310 and Bi-2212/BiFeO3

Preparation of Bi-2223/Bi-2310 and Bi-2212/BiFeO3 composites has been performed so as to obtain some active composites where the superconductive properties of Bi-2223 or Bi-2212 can be modified using a property of the insulating phase. In both cases superconductive percolation was obtained, but only in the case of a Bi-2223 (or Bi-2212)/Bi-2310 composite was no (or very weak) chemical reaction observed during sintering. Since the superconductive percolation threshold was obtained in this case for a concentration of Bi-2223 or Bi-2212 lower than 20%, a special composite model had to be considered. The Bi-2212/BiFeO3 system is very interesting due to the ferroic properties of BiFeO3. Superconductive percolation was achieved with this mixture too, but with more difficulties because of a chemical reaction occurring between the two compounds during sintering. A study of the properties of these composites has been performed and will be briefly described in what follows. The good quality of the Bi-2223(or Bi-2212)/Bi-2310 composite enables us to expect some future applications.

Growth of high-quality Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} crystal and characterization

We report growth of high-quality Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystals with dimensions 2{times}1{times}0.03mm{sup 3} with a melt and growth technique. The composition of this crystal is homogeneous and very close to the stoichiometric composition. Structure characterization of single crystal was realized by measuring the (001) x-ray diffraction. The lattice parameters of a typical crystal are a=5.414(2){Angstrom}, b=5.413(2){Angstrom} and c=30.823(10){Angstrom}. The superconducting temperature T{sub c}=95 K with a sharp transition width (10{endash}90{percent} level) between 6 and 10 K was determined from resistivity and dc susceptibility. We have measured the magnetization of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} single crystal under magnetic field applied along the c-axis. The anomalous second peak effect appeared between 20{endash}30 K. We discuss the qualities of these crystals. {copyright} {ital 1997 Materials Research Society.}

Consolidation of Bi-Sr-Ca-Cu-O superconductors by the shock-loading method

Shock compaction experiments were carried out for Bi2223 to prepare a dense, oriented bulk specimen. The shocked specimen exhibited a high degree of crystallographic orientation and a high density, above 96%. The optimal condition of the crystallographic orientation was attained at a shock pressure of 5 GPa and at annealing temperatures between 750 and 800 °C. The post annealing of a specimen shocked at 5 GPa was effective in increasing AM, indicating the introduction of pinning sites. The magnetic torque measurements showed that a high degree of orientation was attained and many pinning centres were induced by the shock loading.

Bi2(Sr, Ln)2CuOz (Ln = Nd, Sm) phases: stability, crystal growth and superconducting properties

Bi 2 (Sr, Ln) 2 CuO z (Ln = Nd, Sm) single crystals were successfully grown by a self-flux method from stoichiometric and (Bi, Cu)-rich melts. Thermal analysis and thermogravimetry were used to determine stability and the melting sequence of Bi 2 (Sr, Ln) 2 CuO z phases in air. As-grown crystals of the ideal Bi 2 (Sr, Ln) 2 CuO z phase, of dimensions 1 x 0.5 x 0.03 mm 3 , exhibit superconducting behaviour with critical temperature T c = 21 K for the Bi 1.9 Sr 1.6 Nd 0.6 CuO z crystal and T c = 14 K for the Bi 1.8 Sr 1.6 Sm 0.6 CuO z crystal. The compositions of these crystals were homogeneous and close to the stoichiometric composition. We report on the growth of Bi 2 Sr 2-x Sm x CuO z single crystals of large dimensions 9 x 3 x 0.03 mm 3 using Bi 2 Sr 1.5 Sm 0.5 CuO z as precursor and Bi 2 CuO 4 as flux.

Crystal growth of (Bi,Pb)2Sr2CaCu2Oz and their superconducting properties

Superconducting single crystal of Bi2Sr2CaCu2Oz was grown from stoichiometric melt composition. The effect of Pb-doping on the superconducting properties was studied. The superconducting transition temperature (Tc0) for Pb-free and Pb-doped Bi-2212 crystals was determined to be in the range of 90–95 K and 80–90 K respectively from resistivity and dc susceptibility measurements. We found that the electrical transport along the c-axis of Bi-2212 is changed from semiconductor-like to metallic by lead doping. In the Pb-doped crystal, the measurement of magnetic response reveals a high volume fraction compared with Pb-free crystal.

Synthesis and structure change of Ba1-xBixMnO3

Abstract Polycrystalline samples of Ba 1− x Bi x MnO 3 were synthesized over a wide Bi concentration range and examined by X-ray powder diffraction, electron diffraction and compositional analysis. The metal composition of the platelet-like crystals was Ba:Bi:Mn=47.8:2.3:49.9 for nominal composition x =0.05 and 0.1. A hexagonal shaped and platelet-like single crystal with the size of 160×150×15 μm 3 was obtained by a melt and growth technique using (Ba 0.9 Bi 0.1 )MnO 3 as precursor and Bi 2 O 3 as flux. The chemical composition of as-grown crystals was nearly the same as the bulk specimen. The relationship between the average ionic radius of the Ba/Bi cations in the A site and Bi content showed that the 9R phase was stabilized for x =0.05.

Phase development of high-temperature Bi-2223 phase and effect of doping on formation rate

Phase formation of Bi-Pb-Sr-Ca-Cu-O (2223) superconducting materials has been studied for samples produced by the thermal co-decomposition and freeze-drying methods under ambient atmosphere. Phase development and effect of doping were investigated by x-ray diffraction and electron microscopy. It was found that the addition of Vanadium enhances 2223 phase substantially depending on the doping amount.