Bandgap closure and reopening in ${\mathrm{CsAuI}}_3$ at high pressure

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound ${\mathrm{CsAuI}}_3$, where Au adopts ${\mathrm{Au}}^{\mathrm{I}}$ and ${\mathrm{Au}}^{\mathrm{III}}$ valency. Raman spectroscopy shows softening with pressure of the vibration modes in the ${\mathrm{Au}}^{\mathrm{III}}$-${\mathrm{I}}_4$ square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds ${\mathrm{CsAuCl}}_3$ and ${\mathrm{CsAuBr}}_3$. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of ${\mathrm{CsAuI}}_3$, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.

Figure 1

(a) Ambient pressure crystal structure of ${\mathrm{CsAuI}}_3$ at 300 K, explicitly showing the linear square planar coordination of ${\mathrm{Au}}^{\mathrm{I}}{\mathrm{I}}_2$ and ${\mathrm{Au}}^{\mathrm{III}}{\mathrm{I}}_4$ respectively: I(1) as ${\mathrm{I}}^{-}$ associated with square planar ${\mathrm{Au}}^{\mathrm{III}}{\mathrm{I}}_4$, and I(2) as ${\mathrm{I}}^{-}$ associated with linear ${\mathrm{Au}}^{\mathrm{I}}{\mathrm{I}}_2$. (b) Crystal structure shown in alternating elongated and compressed ${\mathrm{AuI}}_6$ octahedra, more clearly revealing the distorted perovskite structure. (c) Raman spectra of ${\mathrm{CsAuI}}_3$ at ambient condition with different light polarization. The solid line represents parallel polarization, while the dashed line corresponds to cross-polarization. Schematics of the $A_{1g}$ and $B_{1g}$ vibrational modes are also presented.

Figure 2

Raman spectra of ${\mathrm{CsAuI}}_3$ during (a) compression and (b) decompression. Black represents tetragonal phase; red: orthorhombic; and blue: disordered. Details of the experimental geometry are described in the main text.

Figure 3

Raman modes of ${\mathrm{CsAuI}}_3$ compared with mass-corrected modes of ${\mathrm{CsAuCl}}_3$ and ${\mathrm{CsAuBr}}_3$ as a function of pressure. The original Raman frequencies of ${\mathrm{CsAuCl}}_3$ and ${\mathrm{CsAuBr}}_3$ are from Ref. [5]. Dotted lines represent modes from Au-Cl units; solid lines from Au-Br units. Unfilled and filled squares are for the $B_{1g}$ and $A_{1g}$ modes of ${\mathrm{Au}}^{\mathrm{III}}$-${\mathrm{I}}_4$, and filled triangles are for the $A_{1g}$ mode of ${\mathrm{Au}}^{\mathrm{I}}$-${\mathrm{I}}_2$ in the tetragonal phase. Filled circles stand for Raman modes in the iodide orthorhombic phase. The grey region indicates the pressure range of the first-order tetragonal-orthorhombic transition.

Figure 4

(a) Mid-IR transmittance and (b) ${\mathrm{CsAuI}}_3$-diamond reflectivity below 5.6 GPa. The arrow in (a) points to the onset of the finite transmission signal. The arrow in (b) indicates the Drude-like mode observed for pressure at 5.6 GPa. (c) Fitted gap of tetragonal ${\mathrm{CsAuI}}_3$ and ${\mathrm{CsAuBr}}_3$ as a function of pressure (both obtained in this study). (d) Reflectivity spectrum at 5.6 GPa fitted with the Drude-Lorentzian model. The grey region is where diamond absorbs strongly.

Figure 5

(a) Mid-IR transmittance and (b) ${\mathrm{CsAuI}}_3$-diamond reflectivity above 5.6 GPa. The arrow in (a) points to the finite transmission through low wave numbers at 6.2 GPa. The grey region is where diamond absorbs strongly.

Figure 6

Far-IR absorption for tetragonal (a) and orthorhombic (b) structure as a function of pressure. Inset: Optical density of the ${\mathrm{CsAuI}}_3$ sample at 400 ${\mathrm{cm}}^{-1}$ as a function of pressure; dashed lines are guides to the eyes.