Impact of electron doping on structure and dynamics of synthetic tetrahedrite

Microscopic control of lattice thermal conductivity is critical to the development of thermoelectric materials.~ One route to this control is manipulation of anharmonic lattice dynamics with chemical doping.~ Tetrahedrite compounds, which display an intrinsic lattice anharmonicity, are promising candidates for thermoelectric application, and here we present results of neutron scattering studies on synthetic Cu$_{12-x} $Zn$_x$Sb$_4$S$_{13}$, ($x = 0,~2$).~ The undoped compound exhibits a structural phase transition associated with a metal-insulator transition near 88~K. Doping with Zn results in the stabilization of the structure, and enhancement of a low energy vibrational mode associated with incoherent oscillations of Cu. The low energy mode is localized, and exhibits pronounced softening with temperature.~ The ability to tune the location of this low-energy mode with doping may provide a means to enhance the phonon-phonon scattering that leads to low thermal conductivity in these materials, and a corresponding enhancement of the thermoelectric properties.