X-ray Absorption Study of the (Y1-xPrx)Ba2Cu3O7-d System,

X-ray absorption spectroscopy has been used to investigate the environment of all four cations in a series of six samples of Y1-xPrxBa2Cu3O7-delta (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). Interatomic distances and their mean square variations, together with some coordination numbers, have been obtained from an analysis of the EXAFS using the multiple-scattering fast curved-wave theory. The environments of the Pa, Cu and Y atoms were found to be invariant with changing composition, the structural parameters being in good agreement with crystallographic data for the end-point materials. The environment of the Pr atom was found to change with composition, both the mean Pr-O distance and its mean square variation varying in a characteristic way. The variations with composition may be closely modelled by the forms RPr-O = (2.39 + 0.16x) Angstrom and sigma(Pr-O)(2) = (40 + 350x(1 - x)) x 10(-4) Angstrom(2). These results have been interpreted in terms of a combination of two Pr-O distances, each invariant with composition, the proportions of these varying linearly with praseodymium content. The consequences of this variation, both for the praseodymium valence and the superconducting properties of the material, are considered.