Cong Wang

The Nd2O3-SrO-CuO system : compounds and phase relations

The subsolidus phase relations of Nd2O3-SrO-CuO ternary system sintered in air at 950-1000-degrees-C have been investigated by X-ray powder diffraction. The system can be divided into 13 three-phase regions and five two-phase regions. In this system there exist at least four compounds and their solid solutions: Nd2SrCu2O6, Nd1+xSr2-xCu2O6 (0 less-than-or-equal-to x less-than-or-equal-to 0.4), Nd2-xSrxCuOy (1.2 less-than-or-equal-to x less-than-or-equal-to 1.5) and NdxSr3-xCu5Oy (0 less-than-or-equal-to x less-than-or-equal-to 1.5). None of these compounds are superconducting above 4.2 K.

CRYSTAL-STRUCTURE AND SUPERCONDUCTIVITY OF NEW PB-BASED 1222-CUPRATE (PB0.5CD0.5)(SR0.9EU0.1)(2)(EU0.7CE0.3)(2)CU2O9+DELTA

A new superconductor (Pb0.5Cd0.5)(Sr0.9Eu0.1)(2)(Eu0.7Ce0.3)(2)Cu2O9+delta has been prepared. The compound has higher T-c values when it is synthesized in Oz than in air. Besides, the O-2-synthesized sample exhibits metallic behaviour, while the air-synthesized sample exhibits semiconducting behaviour in their normal state. It has a typical metal-insulator transition with the increase of the carrier concentration, which is proven by our calculation results of the bond valence sums of the Cu ions in the conducting CuO2 planes. (Pb0.5Cu0.5)(Sr7/8Eu1/8)2(Eu3/4Ce1/4)2Cu(2)O(9) is an analogous compound of (Pb,Cd)-1222. (Pb,Cd)-1222 has higher superconducting transition temperatures than (Pb,Cu)-1222. This difference can be explained from the viewpoint of charge transfer between the charge reservoir (Pb,Cu)O layers and the conducting CuO2 planes. The crystal structure of (Pb0.5Cd0.5)(Sr(0.9)Eu0. I)(2)(Eu0.7Ce0.3)(2)Cu2O9+delta has been refined by the Rietveld method. The refinement results indicate that both (Pb, Cd) and O(3) in the (Pb, Cd)O layer are displaced from their ideal sites.

Oxygen analysis for high-Tc copper oxides : an alternative method for the determination of solid solution limits

The variation of oxygen content with cation substitution for several types of high-T(c) copper oxide solid solutions has been analyzed in terms of both theoretical derivations and experiments. It has been found that for continuous solid solutions the oxygen content varies with substitution in a linear manner over the entire substitution region, while for discontinuous solid solutions the first derivatives of the oxygen content with respect to substitution is discontinuous at the solid solution limit. For the latter, the oxygen content varies in the same way as continuous solid solutions with substitution within the solution limits. However, beyond the solid solution limits, oxygen contents vary with substitution either in a linear manner but with different slope or in a curvy manner, depending on the properties of the component elements of the solid solutions. Therefore, we can determine the solid solution lin-sit or, rather, detect the existence of minor amounts of impurities from the plot of the variation of oxygen content with substitution. The determination of the oxygen content by iodometric titration have verified the above conclusions. The solid solution limits determined by our method are in good agreement with those by powder X-ray diffraction. In some cases, the present method has advantages over conventional powder X-ray diffraction for determining the solid solution limits.

Influence of fluorine on the properties of Nd1.85Ce0.15CuO4-delta n-type superconductor

Abstract The fluorine-doped n-type superconductors Nd 1.85 Ce 0.15 CuO 4− x − y F y (nominal composition) are synthesized with CuF 2 as one of the reactants. Especially, it is found that the superconductivity of Nd 1.85 Ce 0.15 CuO 4 − δ is improved with T c 0 = 27 K, T c 0 = 15.5 K when CuF 2 is taken as a source of atmosphere; the temperature dependence of the resistance exhibits a metallic conducting behavior above T c . Magnetization measurements showed 30% of superconducting phase fractions by means of SQUID magnetometry, which was a better result of superconductivity in (Nd,Ce)CuO bulk samples than when prepared under ambient pressure. Compared with the case of F undoping, F doping not only induces a transition from semiconductivity to metallic conductivity and decreases the normal-state resistivity, but also increases the T c on and T c 0 , and decreases the transition width Δ T c . In particular, the superconductivity of the samples is sensitive to the form of the dopant source and sintering procedures. Especially, the Nd 1.85 Ce 0.15 CuO 3.6 F 0.4 sintered only in air also exhibits superconductivity with T c on = 19.5 K, but does not reach zero resistance, which has not been reported before. X-ray diffraction experiments indicate that fluorine atoms do enter the superconducting phase lattice.

Characterization of a new Pb-based 1222 cuprate Pb(Sr1.2La0.8)Gd2Cu2O9.06

A new compound Pb(Sr(1.2)L(0.8))Gd2Cu2O9.06 has been synthesized and characterized in this letter. The Pb ions in the rock-salt-type monolayer of this compound occur in the form of mixed valence states, i.e., Pb4+ and Pb2+, and the average cationic valence is 3.14, close to that in (Pb0.5Cd0.5)Sr2YCu2O7. It seems to us that the single-Pb-layer Pb-based layered compounds are stable only when the average cationic valence in this layer is close to three. The crystal structure refinement results indicate that the PbO layer is highly distorted, and no Cu ions enter this monolayer. The high distortion of the PbO layer makes the Pb-1222 phase preferentially orientate more readily. This compound does not superconduct. The nonsuperconductivity is not caused by low charge carrier concentration. It may be caused by the Gd2O2 layers, which affect not only the buckling degree of conducting CuO2 planes but the coupling effect between adjacent CuO2 planes as well.