PHYSICAL REVIE& B VOLUME 44, NUMBER 13 1 OCTOBER 1991-I Local structure of BaBi„Pb, „O3 determined by x-ray-absorption spectroscopy J. B. Boyce Xerox Palo Alto Research Center, Palo Alto, California 94304 F. G. Bridges Department of Physics, Uniuersity of California, Santa Cruz, California 95064 T. Claeson Physics Department, Chalmers University of Technology, S-412 96 Goteborg, Su)eden T. H. Geballe Department of Applied Physics, Stanford Uniuersity, Stanford, California 94305 G. G. Li Department of Physics, University of California, Santa Cruz, California 95064 A. W. Sleight Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003 (Received 26 April 1991) X-ray-absorption fine-structure (XAFS) spectroscopy was employed to study the near-neighbor envi- ronments of the Bi and Pb atoms in the perovskite-structured material BaBi Pb, 0„ for x =1, 0. 75, 0 0.5, and 0. 25. For the semiconducting end member, BaBi03, two Bi-0 distances, separated by 0. 18 A, could be distinguished, consistent with diffraction results. After alloying with Pb, two Bi-0 distances are still observed in the semiconducting alloys (x =0.75 and 0.50) as well as in the metallic and supercon- ducting phase (x =0.25). In all the alloys, the two Bi-0 distances and their separations decrease some- what with increasing Pb concentration. The weighted average of the Bi-0 and Pb-0 distances is con- sistent with half the pseudocubic cell edge (V' ) determined from diffraction. The different Bi-0 bond lengths are accompanied by charge-density fluctuations which may enhance local, breathing-type pho- non modes and thereby increase the coupling between electron and phonon states. For the metallic and superconducting phase, these fluctuations are consistent with a local charge-density-wave picture and the presence of a pseudogap. These results may have implications for the superconductivity, with a rela- tively high T„in these oxides. I. INTRODUCTION Two different Bi sites and two distinctly different Bi-0 distances have been found in BaBi03 by neutron diffraction. ' This has led to a suggestion of valence disproportionation, Ba2(Bi +Bi +)O6, giving the pre- ferred valences of 3+ and 5+ for Bi instead of the mean value of 4+, which is unfavorable for Bi and not general- ly found in chemical compounds. However, x-ray photo- emission spectroscopy ' and photoemission ' measure- ments of the electronic structure, as well as band- structure calculations, do not give any appreciable amount of charge transfer. In a previous study, ' we have shown that x-ray-absorption fine-structure (XAFS) measurements were able to separate two distinctly diff'erent Bi-0 distances, but the near-edge structure gave no clear evidence for a mixture of 3+ and 5+ valences. Heald et al. " could find no evidence of any significant amount of disproportionation into two Bi valence states and concluded that the valence is 4+. Contrary to these conclusions, there is a claim' of two Bi valences based on broadened absorption spectra similar to those observed in the previous work mentioned above. ' '" There are indi- cations of charge-density ordering of holes on 0 sites in- stead of on Bi sites. '" This is analogous to the impor- tance of holes on the 0 for the high-T, copper oxides. In BaBi03, the Bi-0 bond is more covalent than ionic in character. As a result, it should be emphasized that the fluctuating charge would be located somewhere between the Bi and 0 ions, with a possible preference towards the 0 site. Hence, 3+ and 5+ for Bi should be interpreted as formal charges, not absolute charges. The question is what happens when Pb is alloyed into BaBi03. The perovskite-type lattice remains' for all Pb compositions in BaBi Pb, „03. It has a monoclinic dis- tortion for x ) 0. 9, orthorhombic for 0. 35 &x &0. 9 and x &0. 1, and tetragonal for 0. 1&x &0. 35. The tetrago- nal phase corresponds to the superconducting phase, and those with x ) 0. 35 are semiconducting. One might ex- pect the end member compound BaBi03 to be a good metal with a half-filled conduction band. However, it is a 44 6961 1991 The American Physical Society