High-temperature magnetic anomaly via suppression of antisite disorder through synthesis route modification in a Kitaev candidate Cu${}_2$IrO${}_3$

By incorporating inert KCl into the Na${}_2$IrO${}_3$ + 2CuCl $\to$ Cu${}_2$IrO${}_3$ + 2NaCl topochemical reaction, we significantly reduced the synthesis temperature of Cu${}_2$IrO${}_3$ from the 350${}^{\circ }$C reported in previous studies to 170${}^{\circ }$C. This adjustment decreased the Cu/Ir antisite disorder concentration in Cu${}_2$IrO${}_3$ from $\sim$19$\mathrm{\%}$ to $\sim$5$\mathrm{\%}$. Furthermore, magnetic susceptibility measurements of the present Cu${}_2$IrO${}_3$ sample revealed a weak ferromagnetic-like anomaly with hysteresis at a magnetic transition temperature of $\sim$70 K. Our research indicates that the spin-disordered ground state reported in chemically disordered Cu${}_2$IrO${}_3$ is an extrinsic phenomenon, rather than an intrinsic one, underscoring the pivotal role of synthetic chemistry in understanding the application of Kitaev model to realistic materials.