Ilmenite-Type Ca$_x$IrO$_3$ via Topochemical Ion Exchange: Stacking Faults and Low-Temperature Magnetic Anomaly

We report the synthesis of an ilmenite-type polymorph of Ca$_x$IrO$_3$ distinct from the known post-perovskite and perovskite phases, via low-temperature topochemical Ca$^{2+}$/2Na$^+$ exchange from Na$_2$IrO$_3$. Powder X-ray diffraction is indexable in $R\bar{3}$, and whole pattern modelling that includes layer glide faults indicates that the selective broadening can be captured by a first order Markov stacking description based on stochastic switching between two symmetry equivalent lateral stacking steps, with explicit model dependence and an uncertainty of at least several percent. A freezing-like bulk magnetic anomaly is suggested at $T^* \sim 25$ K (defined by the onset of a ZFC/FC bifurcation at $\mu_{0}H = 10$ mT), accompanied by a broad heat capacity feature and Curie-Weiss behavior with a large negative Weiss temperature of $\theta_W \sim -98$ K. The effective moment $\mu_{\rm eff} = 1.68 \mu_{\rm B}$ per Ir is consistent with $J_{\rm eff} = 1/2$ for an Ir$^{4+}$. SEM-EDX suggests an A-site content below unity (Ca/Ir $<$ 1); accordingly, we describe the ion-exchanged product using the nonstoichiometric formula Ca$_x$IrO$_3$. These results identify ilmenite-type CaIrO$_3$ as a honeycomb iridate in which stacking disorder can be quantified (with caveats regarding model and instrument correlations) and related to its low-temperature magnetic behavior.