Pressure-induced dimerization and collapse of antiferromagnetism in the Kitaev material $α$-Li$_2$IrO$_3$

Abstract: We present magnetization measurements carried out on polycrystalline and single-crystalline samples of $\alpha$-Li$2$IrO$_3$ under hydrostatic pressures up to 2 GPa and establish the temperature-pressure phase diagram of this material. The N\'eel temperature ($T{\rm{N}}$) of $\alpha$-Li$2$IrO$_3$ is slightly enhanced upon compression with $dT{\rm{N}}/dp$ = 1.5 K/GPa. Above 1.2 GPa, $\alpha$-Li$2$IrO$_3$ undergoes a first-order phase transition toward a nonmagnetic dimerized phase, with no traces of the magnetic phase observed above 1.8 GPa at low temperatures. The critical pressure of the structural dimerization is strongly temperature-dependent. This temperature dependence is well reproduced on the ab initio level by taking into account lower phonon entropy in the nonmagnetic phase. We further show that the initial increase in $T{\rm{N}}$ of the magnetic phase is due to a weakening of the Kitaev interaction $K$ along with the enhancement of the Heisenberg term $J$ and off-diagonal anisotropy $\Gamma$. Our study reveals a common thread in the interplay of magnetism and dimerization in pressured Kitaev materials.