Lattice dynamics and structural transition of the hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ investigated by high-pressure Raman scattering

TL;DR

This study uses high-pressure Raman scattering to explore lattice dynamics and structural transitions in $eta$-Li$_2$IrO$_3$, revealing a pressure-induced phase change from a hyperhoneycomb to a dimerized monoclinic structure affecting its magnetic and electronic properties.

Contribution

It provides the first detailed Raman scattering analysis of pressure-induced structural changes in $eta$-Li$_2$IrO$_3$, linking phonon spectra to a transition into a nonmagnetic dimerized phase.

Findings

Abrupt phonon spectrum change at ~4.1 GPa indicating structural transition.

High-pressure phase is a nonmagnetic insulator with Ir-Ir dimers.

Reasonable agreement between experimental spectra and DFT calculations.

Abstract

We report a polarized Raman scattering study of the lattice dynamics of $\beta$-Li$_2$IrO$_3$ under hydrostatic pressures up to 7.62 GPa. At ambient pressure, $\beta$-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 $\beta$-Li$_2$IrO$_3$ to the pressure-induced structural transition.