Competition between spin-orbit coupling, magnetism, and dimerization in the honeycomb iridates: $\alpha$-Li${}_2$IrO${}_3$ under pressure

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $\alpha$-Li${}_2$IrO${}_3$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_c$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir–Ir dimers. These observations are independently predicted and corroborated by our $\mathit{\text{ab}}\mathit{\text{initio}}$ density functional theory calculations where we find that the appearance of Ir–Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_c$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_2$$M$O${}_3$ with $A$= Li, Na and $M$ = Ir, Ru.