On the room-temperature aging effects in YBa2Cu3O6+δ

• New model for the lattice parameter c.
• The hole concentration in CuO2 is related with c(δ).
• Holes shift from oxygen to copper at RT.

The aging effects in YBa2Cu3O6+δ 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 τ and a detailed experimental dependence of the YBa2Cu3O6+δ lattice parameters a, b, and c on both δ and τ has been obtained. The regime of low-speed quenching results in a threefold increase in the aging effect magnitude. Particular cases of the c(δ; τ) dependence are well described by a second-order polynomial with applying different coefficients for different τ. The behavior c has been explained on the basis of a simple model of the Coulomb interaction between the CuO2 and CuOδ structural planes, in which the hole charge transfer from CuOδ to CuO2 takes place providing the standard level of Tc in YBa2Cu3O6+δ (this level is characterized by a maximum Tc of 93 K at δ = 0.9). It has been demonstrated that the c(δ; τ) dependence is a good alternative to assessing the hole concentration in YBa2Cu3O6+δ. By analyzing the c(δ; τ) experimental dependence as well as the data on the hole localization in the pairs CuO 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 3d9L−1 electron configuration to Cu 3d8 for copper in the basal plane of YBa2Cu3O6+δ at RT.