  The Kondo model predicts that both the valence at low temperatures and its
temperature dependence scale with the characteristic energy T_K of the Kondo
interaction. Here, we study the evolution of the 4f occupancy with temperature
in a series of Yb Kondo lattices using resonant X-ray emission spectroscopy. In
agreement with simple theoretical models, we observe a scaling between the
valence at low temperature and T_K obtained from thermodynamic measurements. In
contrast, the temperature scale T_v at which the valence increases with
temperature is almost the same in all investigated materials while the Kondo
temperatures differ by almost four orders of magnitude. This observation is in
remarkable contradiction to both naive expectation and precise theoretical
predictions of the Kondo model, asking for further theoretical work in order to
explain our findings. Our data exclude the presence of a quantum critical
valence transition in YbRh2Si2.
