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Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration0.2 have been found in thesystem. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range ofup to0.8. However, the variation of the coherence temperatureand the superconducting critical temperaturewith nominal Yb concentrationfor bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function ofto determine the actual compositionof bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, and transmission X-ray absorption edge spectroscopy measurements. The actual compositionis found to be about one-third of the nominal concentrationup to0.5, and resolves the discrepancy between the variation of the physical properties ofsingle crystals and thin films with Yb concentration. [ABSTRACT FROM AUTHOR]