X-ray absorption near-edge structure spectra (XANES) of the Ba1-xKxBiO3 (BKBO) (0.37 ≤ x ≤ 1.0) superconductor have been measured at the Bi LIII edge. To estimate changes of the Bi valence state, and reveal its influence on the superconducting properties of this solid solution, quantitative spectral characteristics of the 2p3/2 → 6s transition in the series comprising different Ba/K ratios were determined. The superconducting transition Tc of the BKBO samples, determined from dc magnetic susceptibility measurements, monotonously decreases from Ba0.63K0.37 BiO3 (Tc = 30.4 K) to Ba0.26K0.74BiO3 (a very low diamagnetic signal was detected at T ∼ 3 K). This corresponds to the increase of Bi valence determined from XANES data in the range +4.36 ≤ v ≤ + 4.74. Further increase of the K content to x > 0.8 results in reappearance of superconductivity with Tc = 9 K. The maximum value of formal Bi valence versus K content corresponds with the minimum in Tc.
Bibliographical noteFunding Information:
The work was supported by the Korea Research foundation (2000-042- D00032) within the Ministry of Education (MOE), Korea and partially by the RFBR (00-03-32472) and INTAS (99-1136). We thank to M.G. Kim and J.M. Lee (PAL) for help in XAS measurements and very valuable discussions.