Energy Balance of Mixed-Valent Eu Ions

The interconfigurational mixing and excitation energies, $\Gamma$ and $E_x$, of the Eu ions in ${\mathrm{EuCu}}_2$ ${\mathrm{Si}}_2$ are determined experimentally as functions of pressure and temperature. $\Gamma$ seems to depend only on $E_x$, and the strongly nonlinear $\Gamma \left(E_x\right)$ reflects the intra-ionic multiplet splittings and degeneracies of $4f^6$. The observed pressure and temperature dependence of $E_x$ is analyzed self-consistently in terms of contributions from mixing, elastic, and conduction-electron energies, which arise from the temperature and pressure dependence of the valence.