Bond ordering and phase transitions in Na₂IrO₃ under high pressure

The Kitaev model of spin-1/2 on a honeycomb lattice supports degenerate topological ground states and may be useful in topological quantum computation. Na₂IrO₃ with a honeycomb lattice of Ir ions has been extensively studied as a candidate for the realization of this model, due to the effective J_eff = 1/2 low-energy excitations produced by the spin-orbit and crystal-field effect. As the eventual realization of the Kitaev model has remained evasive, it is highly desirable and challenging to tune the candidate materials toward such an end. It is well known that external pressure often leads to dramatic changes in the geometric and electronic structure of materials. In this work, the high-pressure phase diagram of Na₂IrO₃ is examined by first-principles calculations. It is found that Na₂IrO₃ undergoes a sequence of structural and magnetic phase transitions, from a magnetically ordered phase with space group C2/m to two bond-ordered nonmagnetic phases. The low-energy excitations in these high-pressure phases can be well described by the J_eff = 1/2 states.