Surface and bulk valence of Y_0.9Yb_0.1Al₂ and La_0.9Yb_0.1Al₂

Photoemission and susceptibility measurements have been made on alloys of 10% YbAl2 with 90% YAl2 or LaAl2. Similar to YbAl2, Yb in the YAl2 host is intermediate valent in the bulk consistent with a single ion picture. Yb in the LaAl2 host has a valence of 2.0-2.1 in the bulk. In both cases the surface is completely divalent and the surface 4f emission is shifted to higher binding energies relative to the bulk 4f emission.