EXAFS study of the [BaCuO2](2)/[(Ca,Sr)CuO2](n) artificial superconductingsuperlattices

Polarised extended X-ray absorption fine structure (EXAFS) and X-ray absorp tion near edge structure (XANES) spectroscopies have been used to probe the local bonding of the infinite layer compounds CaCuO2 and SrCuO2 and of art ificially layered superconducting (BaCuO2)(2)/(CaCuO2)(n) and nonsupercondu cting (BaCuO2)(2)/(SrCuO2)(2) superlattices. Fluorescence-yield measurement s have been performed at T = 77 K above the Cu and Ba K-edges. The XANES sp ectrum of CaCuO2 is described in terms of mixing of electronic configuratio ns in the final state. EXAFS results show that the charge reservoir (CR) bl ock of the Ba/Ca superlattice contains 0.6 oxygen vacancies per unit cell r andomly 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 octahedr al coordination with Cu at the Ba/Ca interface and in the (BaCuO2)(2) block , respectively. The results clarify the origin of the oxygen doping mechani sm in these artificially layered compounds. (C) 2000 Elsevier Science B.V. All rights reserved.