Electron and phonon scattering in the high-temperature thermoelectric La₃ Te_4-z M_z (M=Sb,Bi )

In this work, scattering mechanisms in the highly efficient thermoelectric material La_3-x Te₄ are investigated by controlling the carrier concentration via anion substitution in the nominally vacancy-free compositions La₃ Te_4-z Sb_z and La₃ Te_4-z Bi_z. Through a comparison of the lattice thermal conductivity κL in samples with and without Sb/Bi, this work reveals that La vacancies scatter phonons very efficiently and provide a ∼100% reduction in κL at 575 K. The addition of Sb or Bi leads to a significant reduction in the band gap, which is observed in the temperature-dependent transport data as well as first-principles calculations. Despite this significant change to the band structure, the transport parameters of the conduction band are only slightly modified. Also, an increase in the Hall mobility is observed at high T and z, which is caused by a reduction in either the La-vacancy concentration or the electron's effective mass. A slight increase in thermoelectric efficiency is observed for nominal La₃ Te_3.35 Sb_0.65 at high T. Thus, the net result is a system with large thermoelectric efficiency and a tunable band gap, thereby enabling a clear example to examine the effect of band gap on thermoelectric properties.