Effect of isoelectronic doping on the honeycomb-lattice iridate A2IrO3

We have investigated, experimentally and theoretically, the series (Na_1-xLi_x)2IrO₃. Contrary to what has been believed so far, only for x ≤0.25 does the system form uniform solid solutions where Li preferentially goes to the Ir₂Na planes, as observed in our density functional theory calculations and consistent with x-ray diffraction analysis. 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 Na₃LiIr₂O₆ phase with alternating Na₃ and LiIr₂O₆ planes, and a Li-rich phase close to pure Li₂IrO₃. For x ≤0.25 we observe (1) an increase of c /a with Li doping up to x =0.25, despite the fact that c /a in pure Li₂IrO₃ is smaller than in Na₂IrO₃, and (2) a gradual reduction of the antiferromagnetic ordering temperature T_N and ordered moment. In view of our results showing clear evidence for phase separation for 0.25≤x≤0.6, more detailed studies are needed to confirm the presence or absence of phase separation at the higher doping x ∼0.7, where a continuum quantum phase transition has been proposed previously.