Evolution of the Crystal and Electronic Structures of the RBa2Cu3O6 + δ Cuprates in Annealing

Abstract

The crystal structure and the magnetic properties of the HTSC cuprates YBa₂Cu₃O_{6 + δ}, Y_1 ‒ xCa_xBa₂Cu₃O_{6 + δ}, and Nd_{1 +x}Ba_{2 –x}Cu₃O_{6 + δ} (x = 0.2) with the structure of a layered perovskite are studied. The well-known aging effect detected in these HTSC materials during storage under standard conditions, namely, an increase in their critical temperature T_c and a decrease in lattice parameter c in time, is investigated. Using YBa₂Cu₃O_{6 + δ} as an example, we show that the dependence of c on the oxygen content undergoes the following changes in time: (1) the negative slope of the dependence with respect to axis δ increases and (2) nonlinearity appears and grows in time according to a quadratic law. The first type of changes is explained by an increase in the valence of copper ions, which is accompanied by a decrease in their radius. The second type is explained by the electrostatic interaction of structural CuO₂ planes due to the accumulation of electron holes, which come from CuO_δ planes, in them. The calculation of the second type of changes in parameter c in the YBa₂Cu₃O_{6 + δ} cuprate exhibits good quantitative agreement with the experimental data. The second-type changes in the Y_{1 –x}Ca_xBa₂Cu₃O_{6 + δ} compound (hole doping of CuO₂ planes) turn out to be identical to those in YBa₂Cu₃O_{6 + δ}. However, the differently directed (in time) changes in lattice parameter c in Nd_{1 +x}Ba_{2 –x}Cu₃O_{6 + δ} indicate that the CuO₂ planes in the neodymium cuprate are doped by electrons at the initial stage of aging. However, when these planes are saturated with holes in time, the type of doping changes from n to p without a noticeable change in the crystal structure, which has been considered to be impossible for layered cuprates to date.