Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu2IrO3 is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure -induced structural evolution of Cu2IrO3 by powder x-ray diffraction (PXRD) up to -17 GPa and Raman scattering measurements up to -25 GPa. A structural phase transition (monoclinic C2/c -> triclinic P1 over bar ) is observed with a broad mixed phase pressure range (-4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4-300 K show resilient nonmetallic R(T) behavior with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale T0 suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. First-principles density functional theoretical (DFT) analysis shows that monoclinic P21/c phase of Cu2IrO3 is energetically lower than its C2/c phase at ambient pressure and both the structures are consistent with experimental XRD pattern. Our analysis reveals structural transition from P21/c to P1 over bar structure at 7 GPa in agreement with experiment and uncovers the interplay between oxidation states, spin, Ir bond dimerization and their relevance to electronic band gap.