  Understanding the origin of electron-phonon coupling in lead-halide
perovskites (LHP) is key to interpreting and leveraging their optical and
electronic properties. Here we perform femtosecond-resolved, optical-pump,
electron-diffraction-probe measurements to quantify the lattice reorganization
occurring as a result of photoexcitation in LHP nanocrystals. Photoexcitation
is found to drive a reduction in lead-halide octahedra tilts and distortions in
the lattice, a result of deformation potential coupling to low energy optical
phonons. Our results indicate particularly strong coupling in FAPbBr3, and far
weaker coupling in CsPbBr3, highlighting differences in the dominant machanisms
governing electron-phonon coupling in LHPs. We attribute the enhanced coupling
in FAPbBr3 to its disordered crystal structure, which persists down to
cryogenic temperatures. We find the reorganizations induced by each exciton in
a multiexcitonic state constructively interfere, giving rise to a coupling
strength which scales quadratically with the exciton number. This superlinear
scaling induces phonon-mediated attractive interactions between excitations in
LHPs.
