We have performed bulk-sensitive measurements of the temperature dependence of the Yb valence in the hexagonal ${\text{YbCu}}_{5-x}{\text{Al}}_x\left(0\le x\le 2\right)$ and cubic ${\text{YbCu}}_5$ Kondo compounds using x-ray absorption spectroscopy in the partial fluorescence yield and resonant x-ray emission spectroscopy. A large difference in the valence is found between the cubic $\left(\sim 2.96\right)$ and hexagonal $\left(\sim 2.5\right)$ $x=0$ systems, which evidences a strong structural dependence of the Yb electronic properties. For all studied compounds, a temperature dependence of the valence on the order of 0.02 is measured and found consistent with the magnetic susceptibility. This crossover from the low-temperature state having a higher mixed valence to a high-temperature local-moment behavior is analyzed within the Anderson impurity model (AIM). A good agreement is obtained between our data and the AIM. Evidence is brought for valence fluctuation to occur near the quantum critical point at $x=1.5$ and near the antiferromagnetic phase transition at $x=2.0$.

• Received 11 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.035120