First-order valence phase transition in cubic YbxIn1-xCu2.

Magnetic-susceptibility measurement of a new Laves-phase cubic system, Yb/sub x/In/sub 1-x/Cu/sub 2/ (0.30 ..Yb/sup 3 +/ valence phase transitions at temperatures T/sub v/ = 40--80 K. A simple valence-fluctuation model predicts first-order phase transitions and agrees with the experimental observations. For Yb/sub 0.4/In/sub 0.6/Cu/sub 2/ the theoretical fit shows that below T/sub v/ = 46 K, the GAMMA/sub 6/ state of Yb/sup 3 +/ is 190 K above Yb/sup 2 +/, changing abruptly to -210 K above T/sub v/. The present compounds exhibit the sharpest temperature-dependent valence phase transition in any metallic system.