Structure transition and zigzag magnetic order in Ir/Rh-substituted honeycomb lattice RuCl3

We report magnetization and neutron diffraction studies on crystal and magnetic structures of Ir- and Rh-substituted honeycomb lattice $\alpha$-RuCl${}_3$. The iridium or rhodium atoms are distributed at the Ru site with little structural modification. Both systems undergo a room-temperature monoclinic $C2\mathrm{/}m$ to low-temperature trigonal $R\bar{3}$ phase transformation with a large recoverable hysteresis. At low temperature, a zigzag spin order is observed with the same characteristic wavevector $(0,0.5,1)$ as in the parent $\alpha$-RuCl${}_3$. Detailed magnetic structure refinement reveals an ordered moment of $0.32(5){\mu }_{\mathrm{B}}\mathrm{/}\mathrm{R}\mathrm{u}$ and a upper boundary of canting angle of $15(4{)}^{\circ }$ away from the basal plane at 5~K for the 10% Ir-substituted $\alpha$-RuCl${}_3$, which is different from the 0.45-0.73~${\mu }_{\mathrm{B}}\mathrm{/}\mathrm{R}\mathrm{u}$ and ${32}^{\circ }$-${48}^{\circ }$ canting angle reported in the parent compound $\alpha$-RuCl${}_3$. The observation of unchanged RuCl${}_6$ local octahedral environment, reduced magnetic moment size and canting angle highlights the potential to study quantum spin liquid behavior through non-magnetic ion doping.