Jinrong Min

The effects of chemical substitutions on the (Pb, Cd)-1212 superconducting cuprates

The effects of different substitutions on (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 have been investigated. For (Pb0.5Cd0.5)Sr2(Y1-xCax)Cu2O7 (0.0 less-than-or-equal-to x less-than-or-equal-to 1.0), an anomalous variation of lattice constants with x is observed, and a sharp increase in T(c) occurs when x increases from 0.4 to 0.5. For (Pb0.5Cd0.5)Sr2-y(Y0.5+yCa0.5)Cu2O, (-0.2 less-than-or-equal-to y less-than-or-equal-to 0.3), as yttrium content increases the phase purity increases, and both the a and c lattice constants decrease. The superconducting properties deteriorate as y deviates from 0.0. For (Pb0.5Cd0.5)Sr2-z(Y0.5Ca0.5+z)Cu2O7 (-0.2 less-than-or-equal-to z less-than-or-equal-to 0.2), both the a and c lattice constants decrease as z increases. The superconducting properties deteriorate as z deviates from 0.0, although the effect is very small. From the T(c) results obtained in the three series, the conclusion can be reached that the yttrium content in the (Y, Ca) sites is a key factor in controlling the superconducting properties of the (Pb, Cd)- 1 212 phase.

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.

THE CRYSTAL-STRUCTURE OF (PB0.5CD0.5)SR2(Y1-XCAX)CU2O7

The crystal structures of (Pb0.5Cd0.5)Sr2YCu2O7, (Pb0.5Cd0.5)Sr2(Y0.6Ca0.4)Cu2O7 and (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7 have been refined by the Rietveld method for the X-ray diffraction data. The refinement results indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a complicated occupation in the (Pb, Cd)O layers, i.e., other cations, such as Sr2+, Cu2+ and Ca2+, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotonically with the calcium content, and why there exist broad superconducting transitions in the system (Pb0.5Cd0.5)Sr2(Y1-xCax)Cu2O7. The refinement results also indicate that, as the calcium content increases, the O(2) atoms move close to the CuO2 planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transfer from the charge reservoir layer (Pb, Cd)O to the CuO2 planes. This result is consistent with the bond valence sums of the Cu ions in the CuO2 planes.

A new family of Pb-based 1222 cuprates Pb(Sr,La)2Ln2Cu2Oz (LnGd, Dy, Eu, and Pr)

Abstract A series of new Pb-based 1222 cuprates of the general formula Pb(Sr,La) 2 Ln 2 Cu 2 O z (LnGd, Dy, Eu, and Pr) has been reported in this paper. In this kind of compounds, cations in the rock-salt-type monolayer completely consist of Pb ions, but the iodometric titration results indicate that the Pb ions in the rock-salt-type monolayer actually exist in the form of mixed valence states, i.e., Pb 4+ and Pb 2+ , and the proportion of Pb 4+ and Pb 2+ is close to 1:1. The Pb-1222 phases Pb(Sr 2− x La x )Ln 2 Cu 2 O z are very sensitive to sintering temperature and atmosphere, as well as the Sr content. For example, pure Pb-1222 can be only obtained at 920°C in air for the composition Pb(Sr 1.2 La 0.8 )Gd 2 Cu 2 CO z . All these Pb-1222 phases are not superconducting.

Synthesis and superconducting properties of (Pb0.5Cd0.5)Sr2(Y0.5Ca0.5)Cu2O7

The effect of synthesis conditions on (Pb0.5Cd0.5)Sr-2(Y0.5Ca0.5)Cu2O7 has been investigated. For the (Pb,Cd)-1212 phase, quenching is not necessary. Quenching the (Pb,Cd)-1212 phase exclusively suppresses its superconductivity, although quenching is favorable for the (Pb,Cu)-1212 phase in removing the extra oxygen of the (Pb,Cu)O layers. On the other hand, annealing the (Pb,Cd)-1212 phase in flowing nitrogen leads to a phase decomposition from Pb-1212 phase to Pb-3212 phase. In this paper, we report the optimal synthesis condition for the (Pb,Cd)-1212 phase by annealing the (Pb,Cd)-1212 phase at 970 degrees C in O-2 for 2 h.

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.

The crystal structure and superconductivity of (Ln = Pr, Sm, Eu, Gd and Dy)

In this paper we report the crystal structure and superconductivity of (Pb0.5Cd0.5)(Sr(0.9)Ln(0.1))(2)(Ln(0.7)Ce(0.3))(2)Cu2O9 (Ln = Pr, Sm Eu, Gd and Dy). The lattice parameters of these compounds decrease nearly linearly with the decrease of the average ionic radius of the rare-earth element (Ln, Ce), and the superconducting transition temperatures of these compounds increase as the ionic radius of the rare-earth element Ln decreases from Pr to Gd. (Pb0.5Cd0.5)(Sr0.9Gd0.1)(2)(Gd0.7Ce0.3)(2)Cu2O9 has the highest T-c-values (T-c,T-onset = 49 K, T-c,T-zero = 38 K) which, to our knowledge, are also the highest T-c-values reported for any 1222 phase. The superconductivity of the system is affected by many factors: (i) the coupling effect between CuO2 planes; (ii) the charge-carrier concentration in the CuO2 planes; and (iii) the degree of buckling of CuO2 planes.

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.