Effect of isoelectronic doping on honeycomb lattice iridate A_2IrO_3

We have investigated experimentally and theoretically the series (Na${}_{1-x}$Li${}_x$)${}_2$IrO${}_3$. Contrary to what has been believed so far, only for $x\le 0.25$ the system forms uniform solid solutions. For larger Li content, as evidenced by powder X-ray diffraction, scanning electron microscopy and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na${}_3$LiIr${}_2$O${}_6$ phase with alternating Na${}_3$ and LiIr${}_2$O${}_6$ planes, and a Li-rich phase close to pure Li${}_2$IrO${}_3$. For $x\le 0.25$ we observe (1) an increase of $c\mathrm{/}a$ with Li doping up to $x=0.25$, despite the fact that $c\mathrm{/}a$ in pure Li${}_2$IrO${}_3$ is smaller than in Na${}_2$IrO${}_3$, and (2) a gradual reduction of the antiferromagnetic ordering temperature $T_N$ and ordered moment. The previously proposed magnetic quantum phase transition at $x\approx 0.7$ may occur in a multiphase region and its nature needs to be re-evaluated.