Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

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Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb14MnSb11, clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.

For high temperature thermoelectric applications, Yb14MnSb11 has a maximum thermoelectric figure of merit (zT) of ∼1.0 at 1273 K. Such a high zT is found despite a carrier concentration that is higher than typical thermoelectric materials. Here, we reduce the carrier concentration with the discovery of a continuous transition between metallic Yb14MnSb11 and semiconducting Yb14AlSb11. Yb14Mn1‐xAlxSb11 forms a solid solution where the free carrier concentration gradually changes as expected from the Zintl valence formalism. Throughout this transition the electronic properties are found to obey a rigid band model with a band gap of 0.5 eV and an effective mass of 3 me. As the carrier concentration decreases, an increase in the Seebeck coefficient is observed at the expense of an increased electrical resistivity. At the optimum carrier concentration, a maximum zT of 1.3 at 1223 K is obtained, which is more than twice that of the state‐of‐the‐art Si0.8Ge0.2 flown by NASA.

Lanthanum telluride ͑La 3−x Te 4 ͒ has been synthesized via mechanical alloying and characterized for thermoelectric performance. This work confirms prior reports of lanthanum telluride as a good high-temperature thermoelectric material, with zT ϳ 1.1 obtained at 1275 K. The thermoelectric performance is found to be better than that of SiGe, the current state-of-the-art high-temperature n-type thermoelectric material. Inherent selfdoping of the system allows control over carrier concentration via sample stoichiometry. Prior hightemperature syntheses were prone to solute rejection in liquid and vapor phases, which resulted in inhomogeneous chemical composition and carrier concentration. The low-temperature synthesis provides homogeneous samples with acceptable control of the stoichiometry, and thus allows a thorough examination of the transition from a heavily doped degenerate semiconductor to a nondegenerate semiconductor. The effect of carrier concentration on the Hall mobility, Seebeck coefficient, thermal and electrical conductivity, lattice thermal conductivity, and thermoelectric compatibility are examined for 0.03Յ x Յ 0.33.