Observation of a halide (F/Cl) stabilized, new perovskite phase in superconducting Y2Ba5Cu7Ox films

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y2Ba5Cu7O20-?'(F/Cl)?'films epitaxially grown on SrTiO3(100). The new phase was formed via fluorination or chlorination introduced during the post-deposition O2furnaceanneal for recrystallization. The structure is determined by x-ray diffraction to be an orthorhombic unit cell [a~b=3.86(1) Å, c=27.24(6) Å] with reflection conditions consistent with a fully twinned A-centered cell. Films containing primarily the new phase show a lower resistivity ?(300 K)~177 ?? cm, a comparable ratio of ?(300 K)/?(100 K)~3.0, and a lower superconducting temperature of 78-74 K when compared to Y1Ba2Cu3O7.