Oxidation thermodynamics of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{y}}$: Evidence for the presence of localized oxygen p holes

An analysis is presented of the high-temperature thermodynamic data of the oxidation reaction of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{y}}$ in the orthorhombic phase (y≳0.6). From the independence of the reaction enthalpy on temperature and oxygen content we argue that a localized ionic picture is appropriate. We then analyze a series of models, assuming that the vacancy-oxygen occupation of chain sites is random along the b axis, and that the charge compensation associated with oxygen take-up occurs by oxidation of ${\mathrm{Cu}}^{+}$ in the ${\mathrm{CuO}}_{\mathit{y}}$ chain fragments and by creation of holes on oxygen ions. The results demonstrate that in the chains the oxygen ions determine the valency of the neighboring Cu ions. Most importantly, the agreement obtained with the experimental data provides evidence for the presence of localized p holes; the bandwidth of any hole band is estimated to be less than 0.1 eV. The results further indicate that if oxygen p holes are present both in the chains and in the ${\mathrm{CuO}}_2$ planes, the holes in the chains have their spins coupled to the Cu spins, while the holes in the ${\mathrm{CuO}}_2$ planes go into bound states centered on copper sites by forming a singlet with the ${\mathrm{Cu}}^{2+}$(${\mathit{d}}^9$) spin.