Competition between spin-orbit coupling, magnetism, and dimerization in the honeycomb iridates: α-Li2IrO3 under pressure

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate alpha-Li2IrO3 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 ab 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)MO(3) with A = Li, Na and M = Ir, Ru.