  The pyrochlore iridates, A$_2$Ir$_2$O$_7$, show a wide variety of structural,
electronic, and magnetic properties controlled by the interplay of different
exchange interactions, which can be tuned by external pressure. In this work,
we report pressure-induced phase transitions at ambient temperature using
synchrotron-based X-ray diffraction (up to ~ 20 GPa) and Raman-scattering
measurements (up to ~ 25 GPa) of the pyrochlore series
(Sm$_{1-x}$Bi$_x$)$_2$Ir$_2$O$_7$ ($x =$ 0, 0.02, and 0.10). Our Raman and
X-ray data suggest an iso-structural transition in Sm$_2$Ir$_2$O$_7$ at Pc ~
11.2 GPa associated with the rearrangement of IrO6 octahedra in the pyrochlore
lattice. The transition pressure decreases to ~ 10.2 and 9 GPa for $x =$ 0.02
and 0.10, respectively. For all the samples, the linewidth of three phonons
associated with Ir-O-Ir (A1g and Eg) and Ir-O (T$_2$g$_4$) vibrations show
anomalous decrease up to Pc, due to decrease in electron-phonon interaction.
