Band-like Carriers in a Soft, Anharmonic Lattice: Lead-Halide Perovskites
Lead-halide perovskites APbX3 present a striking dichotomy: they exhibit band-like electronic transport despite their exceptionally soft and anharmonic lattices. Optically and electrically, they resemble conventional direct-gap semiconductors, exhibiting light carrier masses and steep absorption onsets, whereas their lattices display liquid-like dynamics, including overdamped octahedral motions, quasielastic Raman central peaks, and exceptionally low thermal conductivities. Solution-processed films nevertheless sustain micrometre-scale carrier diffusion at defect densities that would severely suppress transport in conventional semiconductors. Here, we argue that these apparently disparate properties emerge from a common microscopic framework: a soft, strongly anharmonic, and polar [PbX3]- framework, strongly influenced by the Pb 6s2 lone pair, which simultaneously shapes the antibonding orbital character of the band edges, the magnitude and multiple timescales of the dielectric response, and the slow relaxational dynamics that dress every charge carrier. We therefore invert the conventional order and develop the lattice before the electronic structure, because the nominally cubic phase is better viewed as a thermally fluctuating ensemble of locally symmetry-broken configurations rather than a single geometry. Within this framework, we discuss excitons, Frohlich large polarons in the intermediate-coupling regime, carrier transport, defect tolerance, dimensional reduction in two-dimensional and nanocrystalline derivatives, and symmetry-breaking phenomena. We critically assess three contested issues - defect tolerance, ferroelectricity, and the interpretation of the T-3/2 mobility law - and identify seven open questions together with the key measurements needed to resolve them