  Identifying and understanding the mechanisms behind strong phonon-phonon
scattering in condensed matter systems is critical to maximizing the efficiency
of thermoelectric devices. To date, the leading method to address this has been
to meticulously survey the full phonon dispersion of the material in order to
isolate modes with anomalously large linewidth and temperature-dependence. Here
we combine quantitative MeV ultrafast electron diffraction (UED) analysis with
Monte Carlo based dynamic diffraction simulation and first-principles
calculations to directly unveil the soft, anharmonic lattice distortions of
model thermoelectric material SnSe. A small single-crystal sample is
photoexcited with ultrafast optical pulses and the soft, anharmonic lattice
distortions are isolated using MeV-UED as those associated with long relaxation
time and large displacements. We reveal that these modes have interlayer shear
strain character, induced mainly by c-axis atomic displacements, resulting in
domain formation in the transient state. These findings provide an innovative
approach to identify mechanisms for ultralow and anisotropic thermal
conductivity and a promising route to optimizing thermoelectric devices.
