Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu_1-xBi_x)₂Ir₂O₇: crossover from incoherent to coherent metal

Pyrochlore iridate (Eu2Ir2O7) shows an interesting metallic phase, termed as incoherent metal, above the metal-insulator transition temperature, with high resistivity and positive temperature coefficient of resistivity (TCR). On increasing the pressure, there is a crossover at pressure P* to a coherent metal phase with positive TCR. We have investigated this crossover using high-pressure Raman spectroscopy and X-ray diffraction for a series of samples (Eu1-xBix)(2)Ir2O7 (x = 0, 0.02, 0.035 and 0.1) up to similar to 20 GPa. X-ray diffraction data show a clear change in the pressure dependence of Ir-Ir bond distance at P*. The line width of the A(1g) phonon shows an anomalous decrease with pressure up to P*, without significant change in the line width of the other three Raman modes with symmetries T-2g(1), E-g and T-2g(2). We attribute these results to the decreasing contribution of electron-phonon coupling in the incoherent metal till P*.