  The valence of Ru was analyzed for two RuSr2(Gd0.75Ce0.25)2Cu2O10 samples
with different oxygen contents by Ru LIII edge x ray absorption near-edge
structure (XANES) spectroscopy. For the sample as-synthesized in 1 atm O2 the
DC magnetization data measured in an applied field of 5 Oe showed a clear
branching of the zero-field-cooled (ZFC) and field-cooled (FC) curves at around
100 K, a cusp at 85 K and a diamagnetic transition around 30 K in the ZFC part.
A further confirmation for the superconductivity at 30 K was obtained from a
resistance vs. temperature measurement. Annealing the as-synthesized sample in
100-atm O2 atmosphere at 420 oC increased the diamagnetic transition
temperature from 30 K to 43 K. According to a thermogravimetric analysis, the
oxygen content increased accordingly by ca. 0.15 oxygen atoms per formula unit.
Quantitative analysis of the XANES spectra using Sr2RuO4 (RuIV) and Sr2GdRuO6
(RuV) as reference materials revealed a valence value of +4.74 and +4.81 for Ru
in the as-synthesized and the 100-atm O2-annealed sample, respectively. The
obtained result suggests that the valence of Ru in Ru-1222 is affected by the
change in oxygen content.
