  Superconductivity can be realized in Eu-containing pnictides by application
of chemical (internal) and physical (external) pressure, the intrinsic physical
mechanism of which attracts much attention in physics community. Here we
present the experimental evidence for the valence change of europium in
compounds of EuFe2As1.4P0.6 exposed to ambient pressure and EuFe2As2 to high
pressure by x-ray absorption measurements on L3-Eu edge. We find that the
absorption spectrum of EuFe2As1.4P0.6 at ambient pressure shows clear spectra
weight transfer from a divalent to a trivalent state. Furthermore, application
of pressure on EuFe2As2 using a diamond anvil cell shows a similar behavior of
valence transition as EuFe2As1.4P0.6. These findings are the first observation
of superconductivity mechanized by valence change in pnictides superconductors
and elucidate the intrinsic physical origin of superconductivity in
EuFe2As1.4P0.6 and compressed EuFe2As2.
