Optical signature of the pressure-induced dimerization in the honeycomb iridate α−Li2IrO3

We studied the effect of external pressure on the electrodynamic properties of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Li}}₂{\mathrm{IrO}}₃$ single crystals in the frequency range of the phonon modes and the Ir $d\ensuremath{-}d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure ${P}_c=3.8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. With increasing pressure an overall decrease in spectral weight of the Ir $d\ensuremath{-}d$ transitions is found up to ${P}_c$. Above ${P}_c$, the local (on-site) $d\ensuremath{-}d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The nonlocal (intersite) Ir $d\ensuremath{-}d$ transitions show a monotonic blueshift and decrease in spectral weight. The changes observed for the nonlocal excitations are most prominent well above ${P}_c$, namely, for pressures $\ensuremath{\ge}12$ GPa, and only small changes occur for pressures close to ${P}_c$. The profile of the optical conductivity at high pressures ($\ensuremath{\sim}20$ GPa) appears to be indicative of the dimerized state in iridates.