  Phonon properties of $\mathrm{Mo_3Sb_{7-x}Te_x}$ ($x=0,1.5, 1.7$), a
potential high-temperature thermoelectric material, have been studied with
inelastic neutron and x-ray scattering, and with first-principles simulations.
The substitution of Te for Sb leads to pronounced changes in the electronic
structure, local bonding, phonon density of states (DOS), dispersions, and
phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near
the top of the valence band, resulting in a strong suppression of
electron-phonon screening, and a large overall stiffening of interatomic
force-constants. The suppression in electron-phonon coupling concomitantly
increases group velocities and suppresses phonon scattering rates, surpassing
the effects of alloy-disorder scattering, and resulting in a surprising
increased lattice thermal conductivity in the alloy. We also identify that the
local bonding environment changes non-uniformly around different atoms, leading
to variable perturbation strengths for different optical phonon branches. The
respective roles of changes in phonon group velocities and phonon lifetimes on
the lattice thermal conductivity are quantified. Our results highlight the
importance of the electron-phonon coupling on phonon mean-free-paths in this
compound, and also estimates the contributions from boundary scattering,
umklapp scattering, and point-defect scattering.
