Possibility of an unconventional spin state of Ir${}^{4+}$ in Ba${}_{21}$Ir${}_9$O${}_{43}$ single crystal

We report the synthesis of single crystals of a novel layered iridate Ba${}_{21}$Ir${}_9$O${}_{43}$, and present the crystallographic, transport and magnetic properties of this material. The compound has a hexagonal structure with two iridium oxide layers stacked along the $c$ direction. One layer consists of a triangular arrangement of Ir${}_2$O${}_9$ dimers while the other layer comprises two regular octahedra and one triangular pyramid, forming inter-penetrated triangular lattices. The resistivity as a function of temperature exhibits an insulating behavior, with a peculiar $T^{-3}$ behavior. Magnetic susceptibility shows antiferromagnetic Curie-Weiss behavior with ${\mathrm{\Theta }}_{\mathrm{C}\mathrm{W}}\simeq -$90 K while a magnetic transition occurs at substantially lower temperature of 9 K. We discuss possible valence states and effective magnetic moments on Ir ions in different local environments, and argue that the Ir ions in a unique triangular-pyramidal configuration likely carry unusually large magnetic moments.