Lattice dynamics and structural transition of the hyperhoneycomb iridate β−Li2IrO3 investigated by high-pressure Raman scattering

We report a polarized Raman scattering study of the lattice dynamics of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ under hydrostatic pressures up to 7.62 GPa. At ambient pressure, $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ exhibits the hyperhoneycomb crystal structure and a magnetically ordered state of spin-orbit entangled ${J}_{\mathrm{eff}}$ = 1/2 moments that are strongly influenced by bond-directional (Kitaev) exchange interactions. At a critical pressure of $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}4.1$ 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 $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ to the pressure-induced structural transition.