Structural systematics around high T c superconductors YBa 2Cu 3O 6+ x

Abstract

Starting from the ABO 3 perovskite structure a structural scheme is developed based on extended oxygen defects and on the yttrium/barium ordering. The YBa 2Cu 3O 9− n series, with n = 1, 2,and 3, was derived from a “YBa 2Cu 3O 9” tri-perovskite through systematic rationalization of the oxygen loss. For each n value two possible polymorphs, (I) and (II), are selected. The structure of the YBa 2Cu 3O 6+ x series ( 0 ɤ x ɤ 2), containing the high T c superconducting phases, results from A. D. Wadsley's (“Nonstoichiometric Compounds” (L. Mandlecorn, Ed.), Academic Press, New York (1964)) intergrowth defects combining these polymorphs. For 0 ɤ x ɤ 1 such models imply the possibly simultaneous presence of Cu(I), Cu(II), and Cu(III) oxidation states; for x ≥ 1 the presence of Cu(I) is no longer necessary. Following this structural scheme, theoretical powder patterns are calculated. They are in agreement with the experimental ones of YBa 2Cu 3O 6+ x phases prepared by standard procedures involving different annealing times. Conductivity measurements show that superconductivity above liquid nitrogen temperature is associated with a high oxygen content, i.e., x > 0.66.