Ytterbium valence phase transition in Yb/sub x/In/sub 1-//sub x/Cu2

The recently reported sharp valence phase transition in cubic Yb/sub 0.4/In/sub 0.6/Cu2 at T/sub 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/sub v/. Neutron diffraction proves the absence of magnetic order down to 10 K. Magnetic susceptibility data show that the transition temperature T/sub v/ increases when Yb or In are substituted by La, Eu, and Sn and decreases when they are substituted by Y, Lu, and Ga. Moessbauer studies of /sup 170/Yb show that at 4.2 K the Yb ion is nonmagnetic in a cubic environment, whereas at 60 K it is magnetic. Moessbauer studies of /sup 119/Sn exhibit changes in both the spectral area and isomer shift at T/sub v/. Electrical resistivity measurements exhibit a large increase in resistivity at T/sub v/. Specific-heat measurements reveal a characteristic increase of c/sub p/ around T/sub v/. Comparison with Lu/sub 0.4/In/sub 0.6/Cu2 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 Yb/sup 3+/ and an interconfigurational excitation energy of 220 K. Finally, x-ray-absorption measurements at the Yb L/sub III/ edge reveal a sudden change in the 4f-electron occupancy at T/sub v/ and temperature-independent valences, v(L/sub III/) = 2.9 and 2.8, above and below T/sub v/, respectively