Ytterbium valence phase transition in<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>

The recently reported sharp valence phase transition in cubic ${\mathrm{Yb}}_{0.4}$${\mathrm{In}}_{0.6}$${\mathrm{Cu}}_{2}$ at ${T}_{v}$\ensuremath{\simeq}50 K has been studied by a variety of experimental techniques. X-ray diffraction studies from 4.2 to 300 K reveal the cubic Laves phase structure at all temperatures with a jump in the unit-cell size at the transition temperature ${T}_{v}$. Neutron diffraction proves the absence of magnetic order down to 10 K. Magnetic susceptibility data show that the transition temperature ${T}_{v}$ increases when Yb or In are substituted by La, Eu, and Sn and decreases when they are substituted by Y, Lu, and Ga. M\"ossbauer studies of $^{170}\mathrm{Yb}$ show that at 4.2 K the Yb ion is nonmagnetic in a cubic environment, whereas at 60 K it is magnetic. M\"ossbauer studies of $^{119}\mathrm{Sn}$ exhibit changes in both the spectral area and isomer shift at ${T}_{v}$. Electrical resistivity measurements exhibit a large increase in resistivity at ${T}_{v}$. Specific-heat measurements reveal a characteristic increase of ${c}_{p}$ around ${T}_{v}$. Comparison with ${\mathrm{Lu}}_{0.4}$${\mathrm{In}}_{0.6}$${\mathrm{Cu}}_{2}$ behavior yields the increase in entropy due to the valence transition, 13.3(3) J/mole K, in very good agreement with a model calculation of a cubic-split ${\mathrm{Yb}}^{3+}$ and an interconfigurational excitation energy of 220 K. Finally, x-ray-absorption measurements at the Yb ${L}_{\mathrm{III}}$ edge reveal a sudden change in the 4f-electron occupancy at ${T}_{v}$ and temperature-independent valences, v(${L}_{\mathrm{III})=2.9}$ and 2.8, above and below ${T}_{v}$, respectively.