Pressure-induced phase transition in pyrochlore iridates (Sm₁₋ₓBiₓ)₂Ir₂O₇ ($x =$ 0, 0.02, and 0.10): Raman and X-ray diffraction studies

The pyrochlore iridates, A₂Ir₂O₇, 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 (Sm₁₋ₓBiₓ)₂Ir₂O₇ ($x =$ 0, 0.02, and 0.10). Our Raman and X-ray data suggest an iso-structural transition in Sm₂Ir₂O₇ 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 (T₂g₄) vibrations show anomalous decrease up to Pc, due to decrease in electron-phonon interaction.