Pressure-induced anomalous insulating behavior in frustrated iridate La_3Ir_3O_(11)

The geometrically frustrated iridate La₃Ir₃O₁₁ with strong spin-orbit coupling and fractional valence was recently predicted to be a quantum spin liquid candidate at ambient conditions. Here, we systematically investigate the evolution of structural and electronic properties of La₃Ir₃O₁₁ under high pressure. Electrical transport measurements reveal an abnormal insulating behavior rather than metallization above a critical pressure P_c ~38.7 GPa. Synchrotron x-ray diffraction (XRD) experiments indicate the stability of the pristine cubic KSbO₃-type structure up to 73.1 GPa. Nevertheless, when the pressure gradually increases across P_c, the bulk modulus gets enhanced and the pressure dependence of bond length d_Ir-Ir undergoes a slope change. Consistent with the XRD data, detailed analyses of Raman spectra reveal an abnormal redshift of Raman mode and a change of Raman intensity around P_c. Our results demonstrate that the pressure-induced insulating behavior in La₃Ir₃O₁₁ can be assigned to the structural modification, such as the distortion of IrO₆ octahedra. These findings will shed light on the emergent abnormal insulating behavior in other 5d iridates reported recently.