Anomalous pressure dependence of phonon line widths in metallic pyrochlore iridates (Eu _1-x Bi _x ) _2 Ir _2 O _7 : crossover from incoherent to coherent metal

Pyrochlore iridate (Eu _2 Ir _2 O _7 ) 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 (Eu _1-x Bi _x ) _2 Ir _2 O _7 ( x = 0, 0.02, 0.035 and 0.1) up 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^1_2g , E_g and T^2_2g . We attribute these results to the decreasing contribution of electron–phonon coupling in the incoherent metal till P *.