First-order valence phase transition in cubic ${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Cu}}_2$

Magnetic-susceptibility measurement of a new Laves-phase cubic system, ${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Cu}}_2$ (0.30≤x≤0.65), exhibit very sharp ${\mathrm{Yb}}^{2+}$→${\mathrm{Yb}}^{3+}$ valence phase transitions at temperatures $T_v$=40–80 K. A simple valence-fluctuation model predicts first-order phase transitions and agrees with the experimental observations. For ${\mathrm{Yb}}_{0.4}$${\mathrm{In}}_{0.6}$${\mathrm{Cu}}_2$ the theoretical fit shows that below $T_v$=46 K, the ${\Gamma }_6$ state of ${\mathrm{Yb}}^{3+}$ is 190 K above ${\mathrm{Yb}}^{2+}$, changing abruptly to -210 K above $T_v$. The present compounds exhibit the sharpest temperature-dependent valence phase transition in any metallic system.