STRUCTURE AND SUPERCONDUCTIVITY OF HGBA2CUO4+DELTA

We have used neutron powder diffraction to investigate the defect stru cture of HgBa2CuO4+delta. An interstitial oxygen defect in the Hg plan e is the primary doping mechanism. A superconducting transition temper ature, T(conset), of 95 K is achieved when approximately 0.06 oxygen a toms per formula unit are incorporated at this site by annealing the s ample at 500-degrees-C in pure oxygen. Annealing in argon at 500-degre es-C lowers the oxygen content in this site to approximately 0.01 and results in a T(c) of 59 K. The neutron powder diffraction data give ev idence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorpo ration of additional oxygen (approximately 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 o f the compound is the same at room temperature and superconducting tem peratures.