ORIGIN OF THE BA CORE-LEVEL BINDING-ENERGY DIFFERENCE BETWEEN TETRAGONAL AND ORTHORHOMBIC YBA2CU3O7-DELTA

Sintered bulk samples of the orthorhombic, superconducting, phase of YBa2Cu3O7-delta exhibit a congruent-to 1.5 eV Ba core-level decrease compared to the tetragonal, nonsuperconducting phase. This is also observed for thin-film material in the two phases. Using ab initio cluster calculations, we show that shifts of this magnitude are explainable by isotropic contraction of the Ba-O bond length in the 0.01- to 0.02-angstrom range, values compatible with the known lattice-parameter changes between the phases. Finally, even in situ cleaved good-quality single crystals of the orthorhombic phase show a chemically shifted Ba component for some regions of the surface, which may be due to Ba-O bond-length contractions at the surface-vacuum interface.