Ytterbium valence phase transition in YbxIn1-xCu2

The recently reported sharp valence phase transition in cubic Yb0.4In0.6Cu2 at Tv50 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 Tv. Neutron diffraction proves the absence of magnetic order down to 10 K. Magnetic susceptibility data show that the transition temperature Tv 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 Yb170 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 Sn119 exhibit changes in both the spectral area and isomer shift at Tv. Electrical resistivity measurements exhibit a large increase in resistivity at Tv. Specific-heat measurements reveal a characteristic increase of cp around Tv. Comparison with Lu0.4In0.6Cu2 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 Yb3+ and an interconfigurational excitation energy of 220 K. Finally, x-ray-absorption measurements at the Yb LIII edge reveal a sudden change in the 4f-electron occupancy at Tv and temperature-independent valences, v(LIII)=2.9 and 2.8, above and below Tv, respectively.