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

The recently reported sharp valence phase transition in cubic ${\mathrm{Yb}}_{0.4}$ ${\mathrm{In}}_{0.6}$ ${\mathrm{Cu}}_2$ at $T_v$≃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. Mössbauer 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. Mössbauer 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($L_{\mathrm{III})=2.9\mathrm{}}$ and 2.8, above and below $T_v$, respectively.