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

TL;DR

This study synthesizes and characterizes a new layered iridate Ba$_{21}$Ir$_9$O$_{43}$, revealing its unique structure, insulating behavior, and potential for unconventional magnetic states due to Ir ions in triangular-pyramidal environments.

Contribution

The paper reports the synthesis and detailed characterization of a novel layered iridate with a unique structure and discusses the possibility of unconventional spin states of Ir$^{4+}$ ions.

Findings

Insulating behavior with T^{-3} resistivity dependence.

Antiferromagnetic Curie-Weiss behavior with Θ_CW ≈ -90 K.

Magnetic transition observed at 9 K.

Abstract

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.