Competition between spin-orbit coupling, magnetism, and dimerization in the honeycomb iridates: $\alpha$-Li$_{2}$IrO$_{3}$ under pressure

TL;DR

This study uncovers a pressure-induced structural transition in honeycomb iridate -LiIrO3, driven by a complex interplay of spin-orbit coupling, magnetism, and covalent bonding, leading to Ir--Ir dimer formation and magnetic collapse.

Contribution

It provides the first combined experimental and theoretical analysis of pressure effects on -LiIrO3, revealing the mechanisms behind dimerization and magnetic collapse.

Findings

Pressure induces a monoclinic to triclinic transition at 3.8 GPa.

Ir--Ir dimers form due to interplay of magnetic, correlation, and spin-orbit effects.

Magnetic collapse occurs at the transition point.

Abstract

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 \textit{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}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru.