First-order valence phase transition in cubic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Yb</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">In</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Magnetic-susceptibility measurement of a new Laves-phase cubic system, ${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cu}}_{2}$ (0.30\ensuremath{\le}x\ensuremath{\le}0.65), exhibit very sharp ${\mathrm{Yb}}^{2+}$\ensuremath{\rightarrow}${\mathrm{Yb}}^{3+}$ 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 ${\ensuremath{\Gamma}}_{6}$ 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.