Oxide electronics

Abstract:

We have used neutron powder diffraction to investigate the defect structure of HgBa2CuO4+δ. An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, Tc onset, of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a Tc of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures. © 1993.

Abstract:

Two powder samples of electrochemically oxidized La2CuO4+δ (nominally δ0.08 and 0.12) and one single crystal (δ0.1), with superconducting critical temperatures of 32, 42, and 40 K, respectively, were studied using neutron diffraction. All samples appear to be single phase, both at room temperature and at low temperature (10-18 K), as evidenced by sharp Bragg peaks, indicating that these samples have compositions beyond the phase-separated region of the phase diagram. A detailed analysis of the Bragg reflections demonstrated that the basic crystallographic structure of all samples has Fmmm symmetry, with the excess oxygen located between adjacent LaO layers. However, a number of low-intensity peaks in the powder data suggested the existence of a very large superstructure. The satellites could be clearly identified in the single-crystal data, allowing the propagation vectors of the modulation to be determined. Rietveld refinements of the average structure, based on the main Bragg peaks, are presented here for samples prepared with this technique. © 1993 The American Physical Society.