Lattice dynamics in the thermoelectric Zintl compound Yb14MnSb11

The density of phonon states in the thermoelectric material Yb${}_{14}$MnSb${}_{11}$ has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb${}_{14}$MnSb${}_{11}$ contrasts with the low energy of all phonons ($l$25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu${}_{14}$MnSb${}_{11}$ compound and of the thermoelectric Zintl phase Zn${}_{4}$Sb${}_{3}$ in order to compare with related thermodynamic properties of Yb${}_{14}$MnSb${}_{11}$ and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases.