Evidence for a valence transition in UxY1-xSb

Electrical, optical and lattice-constant measurements on single crystalline U_xY_1-xSb (x=0, 0.01, 0.03, 0.08, 0.15, 0.20, 0.30, 0.70 and 1) reveal a nonmagnetic U 5f² singlet groundstate for x ? 0.15, a lowering of the f³→f²d¹ transition energy with decreasing x for x > 0.15 and a marked anomaly of the lattice parameter for x ? 0.15. These results as well as the anomalous concentration dependence of the U moment can be described by a valence transition of U at a concentration of about 15% U (x = 0.15).