Lattice dynamics and structural transition of the hyperhoneycomb iridate ${\hat{I}}^2$-Li${}_2$IrO${}_3$ investigated by high-pressure Raman scattering

Abstract

We report a polarized Raman scattering study of the lattice dynamics of ${\hat{I}}^2$-Li${}_2$IrO${}_3$ under hydrostatic pressures up to 7.62 GPa. At ambient pressure, ${\hat{I}}^2$-Li${}_2$IrO${}_3$ exhibits the hyperhoneycomb crystal structure and a magnetically ordered state of spin-orbit entangled Jeff = 1/2 moments that is strongly influenced by bond-directional (Kitaev) exchange interactions. At a critical pressure of ~ 4.1 GPa, the phonon spectrum changes abruptly consistent with the reported structural transition into a monoclinic, dimerized phase. A comparison to the phonon spectra obtained from density functional calculations shows reasonable overall agreement. The calculations also indicate that the high-pressure phase is a nonmagnetic insulator driven by the formation of Ir-Ir dimer bonds. Our results thus indicate a strong sensitivity of the electronic properties of ${\hat{I}}^2$-Li${}_2$IrO${}_3$ to the pressure-induced structural transition.

16 pages, 7 figures, 5 tables, Supplemental Material included