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

Abstract

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 $T^{\ast }\sim 25$ 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 ${\mu }_{\mathrm{eff}}=1.68{\mu }_{\mathrm{B}}$ per Ir is consistent with $J_{\mathrm{eff}}=1/2$ 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.

12 pages, 6 figures, accepted in Journal of Solid State Chemistry