Statistical thermodynamics study of ErBa2Cu3O6+2x

The oxygen chemical potential in ErBa2Cu3O6+2x has been measured as functio ns of the oxygen content x and temperature by Porschke et al. [J. Phys. Che m. Solids 53, 73 (1991)]. Using, these experimental values, the partial mol ar enthalpy and entropy have been derived. These authors have also made Mon te Carlo simulations in the framework of an asymmetric next-nearest neighbo r interactions (ASYNNI) model. They have shown that this model successfully accounts for the experimental orthorhombic to tetragonal transition in ErB a2Cu3O6+2x, whereas, there is an existing large difference between experime ntal and calculated thermodynamic quantities. In this paper, we present a m odified ASYNNNI model which has already been applied successfully to YBa2Cu 3O6+2x, and which explains the experimental results of Porschke et al. On o ne hand, and following Schleger et al. [Phys. Rev. B 49, 514 (1994)], this model includes entropic effects relative to the electron spin and charge de grees of freedom. On the other hand, it takes into account different format ion energies for the insertion of gaseous oxygen in the oxide. The thermody namic quantities are calculated using the cluster variation method (CVM) in the four- and five-point approximation. (C) 2000 Elsevier Science B.V. All rights reserved.