Different valence states of Tm in YB6 and YbB6

We have investigated the electronic structures of Y1-xTmxB6 (x = 0, 0.25) and Yb1-xTmxB6 (x = 0, 0.2) by means of hard x-ray photoemission spectroscopy (HAXPES) at h ν = 5.95 keV. The Tm 3d HAXPES revealed that the valence of Tm in YB6 is ∼2.6 at 300 K and ∼2.5 at 20 K, indicating strong valence fluctuation. In contrast, Tm in YbB6 is almost trivalent independent of temperature. Yb in YbB6 is almost divalent and the Yb2+ state is stabilized by the Tm doping.