STRUCTURAL SYSTEMATICS AROUND HIGH-TC SUPERCONDUCTORS YBA2CU3O6+X

Starting from the ABO3 perovskite structure a structural scheme is developed based on extended oxygen defects and on the yttrium/ barium ordering. The YBa2Cu3O9-n series, with n=1, 2, and 3, was derived from a 'YBa2Cu3O9' 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 YBa2Cu3O6+x series (0&lex&le2), containing the high Tc superconducting phases, results from A. D. Wadsley's growth defects combining these polymorphs. For 0&lex&le1 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 YBa2Cu3O6+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.