Pressure-Induced Structural Evolution with a Turnover Point in the Honeycomb Iridate Na₂IrO₃

The tunability of the quasi-to-dimensional honeycomb lattice in iridate Na2IrO3 plays a crucial role in the realization of the Kitaev quantum spin liquid state, which has potential applications in topological quantum communication and computation. Here, we unveil a continuous structural modification, indicated by changes in the relative reflection intensities and d-spacings in Na2IrO3 under high pressures in the X-ray diffraction experiments. The temperature-dependent resistance at various pressures demonstrates a turnover point at similar to 30 GPa, below which the Na2IrO3 semiconductor evolves toward the metal with a decrease in the activation gap upon compression and above which the gap increases under further compression, consistent with the X-ray diffraction measurements. These experimental results suggest a transformation from the original C2/m phase to the high-pressure phase with a change in the transport behavior. Based on the X-ray diffraction and density functional theory calculations, here, we propose that the high-pressure phase adopts a monoclinic structure with the P2(1)/m symmetry. Compared with the structure of the original C2/m phase, the P2(1)/m structure was found to have dimerization of the Ir-Ir bonds owing to the continuous shrinkage of the lattice under high pressures, which could be beneficial for the realization of the Kitaev quantum spin liquid state in Na2IrO3.