Shift of x-ray-photoelectron core levels inBi2Sr2Ca1−xYxCu2Oy: An explanation by bond-valence-sum calculation

Shifts of the binding energies of x-ray-photoelectron core levels are observed in the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Y}}_{\mathit{x}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ system when x is varied. It is pointed out that this is due to the change in the chemical environment associated with the changes of the interatomic distances. We show that the so-called bond-valence-sum calculation is useful for the explanation of the shift; and propose that this method is generally applicable in explaining or predicting the shift of core levels in any system with a variable composition.