DEPENDENCE OF HOLE CONCENTRATION ON OXYGEN VACANCY ORDER IN YBA2CU3O7-DELTA - A CHEMICAL VALENCE MODEL

Using a simple chemical model, the hole concentration is calculated for YBa2Cu3O7-delta as a function of oxygen stoichiometry-delta. For the calculations, it is assumed that 2-coordinated Cu is monovalent while 3- and 4-coordinated Cu's are nominally divalent. Because monovalent Cu's bind more electrons than do divalent Cu's, the degree of hole doping of the conduction band depends on the oxygen vacancy configuration. We examine extreme ordering conditions and apply the chemical valence model to an extended phase field of the double-cell (Ortho II) structure. For delta > 0.2, the dependence of the calculated hole concentration on delta, obtained for the (defect) double-cell structure, is similar to the observed dependence of T(c) on delta suggesting a simple relationship between T(c) and hole concentration. If T(c) is assumed to depend quadratically on hole concentration (as observed for other oxide superconductors), then the calculated T(c) versus delta corresponds closely with experiment. Low temperature aging behavior is also simulated in the model.