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 $\Theta_\mathrm{CW} \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. 
L. Yang, M. Jeong, A. Arakcheeva, I. \v{Z}ivkovi\'c, B. N\'afr\'adi,   A. Magrez, A. Pisoni, J. Jacimovic, V. M. Katukuri, S...
cond-mat.str-el cond-mat.mtrl-sci