  Valence instability is a key ingredient of the unusual properties of f
electron materials, yet a clear understanding is lacking as it involves a
complex interplay between f electrons and conduc- tion states. Here we propose
a unified picture of pressure-induced valence transition in Sm and Yb
monochalcogenides, considered as model system for mixed valent 4f-electron
materials. Using high-resolution x-ray absorption spectroscopy, we show that
the valence transition is driven by the promotion of a 4f electron specifically
into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional
model that the nature of the transition at low pressures is intimately related
to the density of states of the LU band, while at high pressures it is governed
by the hybridization strength. These results set a new standard for the generic
understanding of valence fluctuations in f-electron materials.
