  The structural, electronic, and optical properties of pyrochlore-type
Pb$_{2}$Ir$_{2}$O$_{6}$O'$_{0.55}$, which is a metal without spatial inversion
symmetry at room temperature, were investigated. Structural analysis revealed
that the structural distortion relevant to the breakdown of the inversion
symmetry is dominated by the Pb-O' network but is very small in the Ir-O
network. At the same time, gigantic second-harmonic generation signals were
observed, which can only occur if the local environment of the Ir 5$d$
electrons features broken inversion symmetry. First-principles electronic
structure calculations reveal that the underlying mechanism for this phenomenon
is the induction of the noncentrosymmetricity in the Ir 5$d$ bands by the
strong hybridization with O' 2$p$ orbitals. Our results stimulate theoretical
study of inversion-broken iridates, where exotic quantum states such as a
topological insulator and Dirac semimetal are anticipated.
