Sr$_3$Ir$_2$O$_7$F$_2$: Topochemical conversion of a relativistic Mott state into a spin-orbit driven band insulator

TL;DR

This study demonstrates a topochemical fluorine insertion transforming Sr$_3$Ir$_2$O$_7$ from a Mott insulator into a nonmagnetic band insulator driven by spin-orbit coupling, supported by experimental and computational evidence.

Contribution

It reports the first topochemical conversion of Sr$_3$Ir$_2$O$_7$ into a new phase Sr$_3$Ir$_2$O$_7$F$_2$ with a different electronic state, highlighting the role of fluorine insertion and spin-orbit effects.

Findings

Ir$^{4+}$ oxidized to Ir$^{5+}$

Transition from antiferromagnetic Mott insulator to nonmagnetic band insulator

Fluorination process driven by flat insertion energy

Abstract

The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent to the Ir metal ions of this compound.