Pressure-Induced Metal-like Transport and Magnetoresistance in a Au²⁺-Au³⁺ Halide Perovskite

The Cs₄Au^IIAu^III ₂Cl₁₂ perovskite (1), featuring AuCl₄ trimers separated by vacancies, enables the first high-pressure study of Au^2+/3+mixed-valence. Our computational analysis of the gold frontier orbitals suggests that the Au²⁺→Au³⁺ intervalence charge transfer (IVCT) occurs across the vacancies. Computational structures indicate that these vacancies rapidly shrink with pressure and the Au²⁺ and Au³⁺ coordination spheres become very similar at the phase transition to nearly cubic symmetry at ca. 15 GPaenabling facile IVCT. Although the activation energy of conductivity of 0.73(4) meV and far-infrared absorption indicate a small but nonzero bandgap, ambient thermal energy drives the IVCT, affording metallic properties: prominent infrared reflectivity and transport values of 10² S·cm^-1. This prompted us to perform the first high-pressure studies of magnetoresistance (MR) and Hall effect in halide perovskites. At 16 GPa, the MR increases by 9.3% at 2 K and 9 T; this value is maintained up to 27 GPa, when a local distortion drives electronic localization. By globally fitting the MR and Hall resistance to a two-carrier model we quantify how the carrier densities and mobilities evolve with pressure. Thus, metal-like transport and MR in 1 is driven by a pressure-induced transition from localized to partially delocalized mixed-valence.