M.G. Smith

Tin and antimony valence states in BaSn0.85Sb0.15O3−δ

{sup 119}Sn and {sup 121}Sb Moessbauer spectra indicate the presence of Sb{sup 5+} and Sn{sup (4{minus}x)+} ions down to 1.2 K without evidence for disproportionation into Sn{sup 2+}, Sn{sup 4+} or Sb{sup 3+}, Sb{sup 5+}. It is argued that stabilization of Sb{sup 5+}, which means a lifting of the donor level above E{sub F}, is accomplished by a static oxide-ion displacement from Sn toward Sb in a 180{degree} Sn-O-Sb link. Placement of E{sub F} within an enhanced density of states at the bottom of the Sn-5s {sigma}* band eliminates optical excitations to the conduction band from an impurity band; it is suggested that this situation is a necessary condition for a transparent conductor like In{sub 2}O{sub 3}:Sn. The model also permits rationalization of a low, temperature-independent conductivity.

High-temperature superconductivity in Y1−zCazSr2Cu2.5Fe0.5O6+x

Abstract We report bulk superconductivity with a maximum critical temperature T c of 20 K in single-phase Y 1− z Ca z Sr 2 Cu 2.5 Fe 0.5 O 6+ x (0.05≤ z ≤0.18) synthesized under near ambient oxygen pressure. Materials were characterized by X-ray powder diffraction, iodometric titration, magnetic susceptibility, room-temperature 57 Fe Mossbauer absorption spectroscopy, and transport measurements. With increasing z , the oxidation state of the Fe-Cu-O array remains essentially constant. Five-fold coordinated Fe in the Cu(1)O x planes appear to be intermediate-spin Fe 3+ , all others are present as high-spin Fe 3+ . The concentration of Fe 3+ ions in the Cu(2)O 2 sheets increases gradually with increasing z . We interpret the development of a broad maximum versus z near z =0.12 in T c and the Meissner fraction V f to be due to an increasing transfer of holes from the reservoir in the Cu(1)O x planes to the Cu(2)O 2 sheets as the Coulomb repulsion of the positively charged Y 1− z Ca z plane is reduced.

Electron microscopy of the infinite-layer superconducting oxide phase and related compounds

Abstract The infinite-layer phase existing in the compounds with composition Sr 1−x Re x CuO 2 (Re=Pr, La) synthesized under high pressure has been investigated by high-resolution transmission electron microscopy as well as selected-area electron diffraction techniques. A few stacking faults with a /2 shear vectors occuring in the (1, 0, 0) or (0, 1, 0) planes as well as a local superstructure, caused by ordering of La-atoms, are observed in the infinite-layer phase with composition Sr 0.97 La 0.03 CuO 2 . Two additional phases coexisting in the compound Sr 0.87 Pr 0.13 CuO 2 have also been identified. The first of these is an incommensurate phase with cell parameters a=11.5 A, b=13.0 A and c=4.0A it contains an approximately seven-fold structural modulation along the c -axis. The second phase has cell parameters a=3.86 A, b=11.6 A and c=20.4 A. The remaining compounds investigated also contain up to three phases. A possible structural model for the incommensurate phase is suggested, and the calculated images obtained with this model yield an excellent fit with the experimental observations.

The pressure dependence of Tc in the infinite-layer electron-doped compound Sr0.84Nd0.16CuO2

Abstract We report the effect of nearly hydrostatic pressure in the range 0–20 kbar on the resistively determined superconducting transition temperature T c of the recently discovered infinite-layer electron-doped copper-oxide compound Sr 0.84 ND 0.16 CuO 2 . In contrast to other electron-doped copper oxides, we observe a positive and appreciable change in T c with pressure with value d T c d P =+0.06±0.02 K/kbar . Thus the sign and magnitude of d T c dP are not dominated by the carrier type; in this compound, we suggest they are determined by a pressure-induced enhancement of the interlayer coupling.

Indications for enhanced flux pinning below magnetic ordering of Fe clusters in YBa2 (Cu0.94Fe0.06)3O6+y

Abstract Influence of magnetic ordering of Fe clusters in two different cluster states of YBa 2 (Cu 0.94 Fe 0.06 ) 3 O 6+ y on the magnetic hysteresis (flux pinning related) is demonstrated. In the crystallographically more extended and ordered Fe cluster state, obtained by processing under reducing atmosphere and subsequent reoxygenation [N,O], magnetic order on the Fe cluster occurs near T N =25 K, while T c =81 K and a relatively high magnetic hysteresis, Δ M , strongly decreases above T N . For the conventionally air-cooled materials [O], with T c =69 K, magnetic order of the more disordered clusters occurs near T N =10 K and the relatively lower Δ M again strongly decreases above T N . Flux pinning is therefore enhanced by the onset of magnetic ordering in the Fe clusters.