On the room-temperature aging effects in YBa2Cu3O6+δ

The aging effects in YBa2Cu3O6+delta have been investigated on the low-speed quenched samples in contrast to other similar studies where the high-speed quenched samples were examined. In the framework of the investigation, XPS analysis has been conducted for the samples stored after quenching for the specified times tau and a detailed experimental dependence of the YBa2Cu3O6+delta lattice parameters a, b, and c on both delta and tau has been obtained. The regime of low-speed quenching results in a threefold increase in the aging effect magnitude. Particular cases of the c(delta; tau) dependence are well described by a second-order polynomial with applying different coefficients for different tau. The behavior c has been explained on the basis of a simple model of the Coulomb interaction between the CuO2 and CuO delta structural planes, in which the hole charge transfer from CuO delta to CuO2 takes place providing the standard level of T-c in YBa2Cu3O6+delta (this level is characterized by a maximum T-c of 93 K at delta = 0.9). It has been demonstrated that the c(delta; tau) dependence is a good alternative to assessing the hole concentration in YBa2Cu3O6+delta. By analyzing the c(delta; tau) experimental dependence as well as the data on the hole localization in the pairs Cu-O obtained by XPS, the authors have made a conclusion about the nature of observed aging effects. The latter are likely to be related to the transition of the Cu 3d(9)L(-1) electron configuration to Cu 3d(8) for copper in the basal plane of YBa2Cu3O6+delta at RT. (C) 2015 Elsevier B.V. All rights reserved.