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

The topochemical transformation of single crystals of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ into ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ is reported via fluorine insertion. Characterization of the newly formed ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ phase shows a nearly complete oxidation of ${\mathrm{Ir}}^{4+}$ cations into ${\mathrm{Ir}}^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled $J_{\mathrm{eff}}=1/2$ 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 in the Ir metal ions of this compound.

Figure 1

Illustration of the crystallographic unit cells of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$. ${\mathrm{IrO}}_6$ octahedra are shaded, and purple spheres denote Sr atoms. Green spheres denote the positions of intercalated ${\mathrm{F}}^{-}$ anions. The long axis of the parent monoclinic unit cell expands by $16\%$ upon incorporation of two F atoms per formula unit into the crystal matrix.

Figure 2

${}^{19}\mathrm{F}$ spin echo NMR spectra illustrating a dominant F environment with a chemical shift of $\delta =25.1$ ppm in a measurement of crushed ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ crystals. Asterisks denote spinning sidebands associated with this peak. A weak secondary signal is observed at $\delta =88$ ppm beneath the center band and the sideband of the primary site, which matches the shift expected for a ${\mathrm{SrF}}_2$-like F environment. The black line denotes data for ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$, and the red line denotes data for the reference ${\mathrm{SrF}}_2$.

Figure 3

Illustration of the ${\mathrm{IrO}}_6$ octahedra, Ir-O bond lengths, and Ir-O-Ir bond angles determined via (a) and (b) x-ray/neutron diffraction and (c) and (d) DFT structural relaxation calculations.

Figure 4

(a) The dc charge transport measured as a function of temperature for both ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$. (b) Optical conductivity measured at room temperature for both ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$. The $\alpha , \beta$, and $\gamma$ transitions are the expected transitions for the $J_{\mathrm{eff}}=1/2$ Mott state of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ as described in the text, and the “A” peak is labeled as the dominant excitation in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$.

Figure 5

(a) X-ray absorption data showing the measured $\mu \left(E\right)$ at both the Ir $L_3$ and $L_2$ absorption edges. (b) Inverse susceptibility ${(\chi -{\chi }_0)}^{-1}$ plotted as a function of temperature. The temperature-independent term in the susceptibility ${\chi }_0$ (comprising the Van Vleck paramagnetic and background terms) has been removed to isolate the local moment response. The resulting Curie-Weiss fit is shown as the black line through the data with values detailed in the text. The inset shows the field dependence of the magnetization collected at 2 K. The solid line is a fit to a $J=1/2$ Brillouin function with a weak linear background as described in the text.

Figure 6

(a) Fluorine absorption energy [Eq. (1)] and (b) insertion energy [Eq. (2)] of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_x$. This shows that fluorine incorporation will proceed spontaneously until $x=2$, in agreement with the synthesis results.

Figure 7

Band structure of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7{\mathrm{F}}_2$ (cell defined such that the $z$ axis is aligned out of the iridium oxide planes). Occupied valence bands are shown in blue, and unoccupied conduction bands are shown in orange.