Evolution of the Crystal and Electronic Structures of the RBa 2 Cu 3 O 6 + δ Cuprates in Annealing

Abstract

Abstract: The crystal structure and the magnetic properties of the HTSC cuprates YBa 2 Cu 3 O 6 + δ , Y 1 ‒ x Ca x Ba 2 Cu 3 O 6 + δ , and Nd 1 + x Ba 2 – x Cu 3 O 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 YBa 2 Cu 3 O 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 CuO 2 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 YBa 2 Cu 3 O 6 + δ cuprate exhibits good quantitative agreement with the experimental data. The second-type changes in the Y 1 – x Ca x Ba 2 Cu 3 O 6 + δ compound (hole doping of CuO 2 planes) turn out to be identical to those in YBa 2 Cu 3 O 6 + δ . However, the differently directed (in time) changes in lattice parameter c in Nd 1 + x Ba 2 – x Cu 3 O 6 + δ indicate that the CuO 2 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. © 2018, Pleiades Publishing, Inc.