  Low thermal conductivity is favorable for preserving the temperature gradient
between the two ends of a thermoelectric material in order to ensure continuous
electron current generation. In high-performance thermoelectric materials,
there are two main low thermal conductivity mechanisms: the phonon anharmonic
in PbTe and SnSe and phonon scattering resulting from the dynamic disorder in
AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron
scattering. Using neutron scattering and ab initio calculations, we report here
a mechanism of static local structure distortion combined with
phonon-anharmonic-induced ultralow lattice thermal conductivity in
{\alpha}-MgAgSb. Since the transverse acoustic phonons are almost fully
scattered by the compound's intrinsic distorted rocksalt sublattice, the heat
is mainly transported by the longitudinal acoustic phonons. The ultralow
thermal conductivity in {\alpha}-MgAgSb is attributed to its atomic dynamics
being altered by the structure distortion, which presents a possible
microscopic route to enhance the performance of similar thermoelectric
materials.
