Atomistic origin of lattice softness and its impact on structural and carrier dynamics in three dimensional perovskites

The soft lattices of lead-halide perovskites (LHPs) are responsible for their unique material properties, including polaron formation, defect tolerance, anharmonic vibration, and large electrostrictive response, which result in exotic carrier dynamics and facilitate strain engineering for elevated efficiency and stability. However, the atomistic origin of lattice softness and its impacts on structural and carrier dynamics remain debatable. Based on an ionic interpretation, we discover a straightforward model that the lattice softness of perovskites ABX(3) is mainly dependent on the steric size and valency of their compositional elements (particularly B and X), whereas the previously assumed principal causes-organic molecular rotation, s-p antibonding coupling, and [BX6] octahedral tilt-play auxiliary or secondary roles. The direct correlations between ionic size/valency, lattice softness, structure disorder, and carrier dynamics are comprehensively investigated by first-principles calculations, thus determining the origin of the anomalous temperature-dependent carrier recombination rate in LHPs. Our work manifests a clear answer to the current debates on the soft lattice of LHPs, rationalizes diverse experiments from compositional management (i.e., A-site and B-site mixing) to strain engineering within a clear model, and provides insightful guidance for device engineering and material design for novel optoelectronic applications.