OBSERVATION OF TEMPERATURE-DEPENDENT SITE DISORDER IN YBA2CU3O7-SIGMA BELOW 150-DEGREES-C

We have observed annealing effects in both single-crystal and ceramic samples of oxygen-deficient YBa2Cu3O7- that occur at temperatures as low as 0°C. Oxygen stoichiometries were controlled by quenching, from 520°C, samples equilibrated in a controlled O2-N2 atmosphere. For single crystals, transition temperatures with any desired value between 0 and 92 K, and transition widths less than 2 K, were readily obtained with appropriate variation of. After quenching, superconducting transition temperatures of the reduced samples rise, as much as 15 K, when aged in ambient atmosphere for several days; the sample composition does not change with aging. With low-temperature aging, structural changes also occur; e.g., orthorhombicity increases. These effects are attributed to oxygen-vacancy ordering that occurs in the chain region of the structure. The rise in Tc with aging (ordering) is attributed to increased hole doping in the planes that results from an increased population of two-coordinated (monovalent) copper atoms. For samples with reduced stoichiometries, the degree of disorder can be reversibly controlled with secondary quenches in the temperature range 0 120°C. For stoichiometries 7-6.5, an approximate activation energy of 0.96 eV was obtained for the annealing process. This low-temperature annealing behavior, occurring in samples of constant composition, provides a remarkably simple and effective way to study the relationship between superconductivity, structure, and associated electronic properties. © 1990 The American Physical Society.