  We report the temperature dependence of the Yb valence in the geometrically
frustrated compound $\mathrm{YbB_4}$ from 12 to 300 K using resonant X-ray
emission spectroscopy at the Yb $L_{\alpha_1}$ transition. We find that the Yb
valence, $v$, is hybridized between the $v=2$ and $v=3$ valence states,
increasing from $v=2.61\pm0.01$ at 12 K to $v=2.67\pm0.01$ at 300 K, confirming
that $\mathrm{YbB_4}$ is a Kondo system in the intermediate valence regime.
This result indicates that the Kondo interaction in $\mathrm{YbB_4}$ is
substantial, and is likely to be the reason why $\mathrm{YbB_4}$ does not order
magnetically at low temperature, rather than this being an effect of geometric
frustration. Furthermore, the zero-point valence of the system is extracted
from our data and compared with other Kondo lattice systems. The zero-point
valence seems to be weakly dependent on the Kondo temperature scale, but not on
the valence change temperature scale $T_v$.

Strongly Correlated Electrons