How unconventional oxidation state Au$^{2+}$ is stabilized in halide perovskite Cs$_4$Au$_3$Cl$_{12}$: a first-principles study of its polaron crystal nature

TL;DR

Cs4Au3Cl12 hosts Au in an unusual $+2$ oxidation state stabilized by a polaron crystal formed from square-planar AuCl4 motifs and Au vacancies. The electronic structure features isolated Au$^{2+}$-derived valence and Au$^{3+}$-derived conduction bands with narrow dispersion, as well as a finite magnetic moment of $0.43\,\mu_B$ on Au$^{2+}$ and a spontaneous polarization of $0.056\,\mathrm{C/m^2}$ arising from Au-site ordering. Redox analyses indicate the Au$^{2+}$ density is saturated and oxidation is exothermic while reduction leads to a metallic state, implying disproportionation is suppressed. Overall, the work demonstrates a general design principle—stabilizing unconventional oxidation states via lattice distortions and polaron physics—that can be extended to other transition-metal compounds, with implications for novel gold magnetism and polaron-transport phenomena.

Abstract

Gold in crystalline compounds is typically only stable in oxidation states Au1+ and Au3+. Even compounds with nominal Au2+ usually disproportionate into Au1+ and Au3+. Recently, Cs4Au3Cl12 was synthesized, where gold took the 2+ state in the bulk. Here, we investigate this compound using first-principles calculations and show that stabilization of the Au2+ ion is through the formation of a polaron crystal. The electronic and phononic structure suggest that the bonding network can be interpreted as a collection of [Au2+Cl4]2- and [Au3+Cl4]1- square planar motifs, and the crystal lacks a smooth pathway for Au2+ to disproportionate into Au1+ and Au3+. The electronic states of Au are contained within each [AuCl4] motif, which allows for the Au2+ state to be localized and isolated electronically. The Au2+-sites form an ordered structure, which is driven by a strong repulsive interaction between [Au2+Cl4]2- motifs due to their lattice distortion. The electron-phonon coupling between Au2+ and Cl explains the stability of Au2+, which suggests this material to be interpreted as a polaron crystal. By considering redox reaction, we show that Cs4Au3Cl12 has the maximal density of Au2+, and further oxidation will induce a delocalized state. Cs4Au3Cl12 has distinctive electronic structure, with a narrow gap, isolated HOMO and LUMO bands strongly localized at the Au-sites, and magnetization at the Au2+-sites making Cs4Au3Cl12 unique among quantum materials. Magnetism in gold is rare, and Cs4Au3Cl12 can be a testbed to explore novel gold chemistry as well as polaron crystal transport. The strategy to stabilize an unconventional oxidation state through engineering of lattice distortions is quite general; therefore, we propose that a similar approach will be applicable to a wide variety of transition metal compounds.

Figures (4)

• Figure 1: (a) Conventional unit cell of Cs4Au3Cl12. (b)$\sim$(e) Cross section along successive (001) planes. (f) Distances to six nearest Cl around (f) Au^2+ and (g) Au^3+.
• Figure 2: (a) Electronic band dispersion. Zero of energy is taken to be the center of the band gap. Projected density of states for (b) Au^2+ and (c) Au^3+. Spin-resolved crystal orbital Hamilton population (COHP) of (d) nearest-neighbor Au^2+-Cl, (e) nearest-neighbor Au^3+-Cl, (f) second nearest-neighbor Au^2+-Cl, and (g) second nearest-neighbor Au^3+-Cl. Spin up and down are plotted with red and green, respectively.
• Figure 3: (a) Phonon-projected density of states for Cs, Au, and Cl. Phonon-projected density of states for (b) Au^2+ and (c) Au^3+. Phonon-projected density of states for (d) in-plane and (e) out-of-plane Au^2+-Cl direction.
• Figure 4: Schematic image of calculated chemical reactions. (a) oxidation, (b) reduction, and (c) reverse reaction of (a).