Observation of a Halide (F/Cl) Stabilized, New Perovskite Phase in Superconducting Y sub 2 Ba sub 5 Cu sub 7 O sub x Films

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y sub 2 Ba sub 5 Cu sub 7 O sub (20-delta') (F/Cl) sub delta" films epitaxially grown on SrTiO sub 3 (100). The new phase was formed via fluorination or chlorination introduced during the post-deposition O sub 2 furnace anneal for recrystallization. The structure is determined by X-ray diffraction to be a tetragonal unit cell (a`b = 3.86(1)angstroms, c = 27.24 (6)angstroms) with a space group partially assigned as P4 sub 2. Films containing primarily the new phase show a lower resistivity rho (300K) ` 177 micron OMEGA-cm, a comparable ratio of rho(300K)/rho(100K) ` 3. 0, and a lower superconducting temperature of 78-74K when compared to Y sub 1 Ba sub 2 Cu sub 3 O sub 7.