  Eu2Ir2O7, a candidate Weyl semimetal, shows an insulator-to-metal transition
as a function of Bi substitution at the Eu site. In this work, we investigate
the (Eu_1-xBi_x)2Ir2O7 series via Hard X-ray Photoemission Spectroscopy
(HAXPES), where substitution of larger Bi3+ for Eu3+ is reported to result in
an anomalous lattice contraction (20 %) for low Bi doping (3.5 %). Using
HAXPES, we confirm that all the cations retain their nominal valence state
throughout the series. The asymmetric nature of Bi core-level spectra for
compositions in the metallic region indicates that Bi contributes to the
density of states at the Fermi energy in this doping range. The valence band
spectra shows that the Bi 6s peak is unaltered throughout the series and is
situated deep within the valence band. based on these observations we argue
that Bi 6p - Ir 5d hybridization drives the insulator-to-metal transition.
