EXAFS Study of the [BaCuO2](2)/[(Ca,Sr)CuO2](n) Artificial Superconducting Superlattices

Polarised extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies have been used to probe the local bonding of the infinite layer compounds CaCuO2 and SrCuO2 and of artificially layered superconducting (BaCuO2)(2)/(CaCuO2)(n) and nonsuperconducting (BaCuO2)(2)/(SrCuO2)(2) superlattices. Fluorescence-yield measurements have been performed at T = 77 K above the Cu and Ba K-edges. The XANES spectrum of CaCuO2 is described in terms of mixing of electronic configurations in the final state. EXAFS results show that the charge reservoir (CR) block of the Ba/Ca superlattice contains 0.6 oxygen vacancies per unit cell randomly distributed in the CuO2 plane sandwiched between the BaO planes and that the two apical oxygens are not in the Ba plane but are shifted toward Cu by 0.21 Angstrom. These oxygens have a pyramidal and distorted octahedral coordination with Cu at the Ba/Ca interface and in the (BaCuO2)(2) block, respectively. The results clarify the origin of the oxygen doping mechanism in these artificially layered compounds. (C) 2000 Elsevier Science B.V. All rights reserved.