SIMPLE ELECTRONIC MODEL FOR OXYGEN NONSTOICHIOMETRY IN LA2-XSRXCUO4-Y

We have calculated the loss in electrostatic and covalent energy when an O-2 ion is removed from the apical or plane positions in La2-xSrxCuO4-y. The Sr+2 ions are assumed to replace randomly La+3 ions. The electrostatic energy at an O position is calculated for each configuration of its nearest La, Sr ions, taking the rest of the ions into account by means of a self-consistent calculation of the average Madelung potential. The covalent energy is included by means of a simple expression which fits the dependence of the energy of the t - J model with the number of holes. We obtain a qualitative agreement with the observed dependence of the total number of vacancies y versus the Sr content x.