Electron and phonon scattering in the high-temperature thermoelectric La_3Te_(4−z)M_z (M=Sb,Bi)

Abstract

In this work, scattering mechanisms in the highly efficient thermoelectric material La_(3−x)Te_4 are investigated by controlling the carrier concentration via anion substitution in the nominally vacancy-free compositions La_3Te_(4−z)Sb_z and La_3Te_(4−z)Bi_z. Through a comparison of the lattice thermal conductivity κ_L in samples with and without Sb/Bi, this work reveals that La vacancies scatter phonons very efficiently and provide a ~100% reduction in κ_L 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 La_3Te_(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.

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