  Point defects in solids such as vacancy and dopants often cause large thermal
resistance. Because the lattice site occupied by a point defect has a much
smaller size than phonon wavelengths, the scattering of thermal acoustic
phonons by point defects in solids has been widely assumed to be the Rayleigh
scattering type. In contrast to this conventional perception, using an ab
initio Green's function approach, we show that the scattering by point defects
in PbTe and GeTe exhibits Mie scattering characterized by a weaker frequency
dependence of the scattering rates and highly asymmetric scattering phase
functions. These unusual behaviors occur because the strain field induced by a
point defect can extend for a long distance much larger than the lattice
spacing. Because of the asymmetric scattering phase functions, the widely used
relaxation time approximation fails with an error of ~20% at 300K in predicting
lattice thermal conductivity when the vacancy fraction is 1%. Our results show
that the phonon scattering by point defects in IV-VI semiconductors cannot be
described by the simple kinetic theory combined with Rayleigh scattering.
