Many monumental breakthroughs in p-type PbTe thermoelectrics are driven by optimizing a Pb 0.98 Na 0.02 Te matrix. However, recent works found that x > 0.02 in Pb 1−x Na x Te further improves the thermoelectric figure of merit, zT, despite being above the expected Na solubility limit. We explain the origins of improved performance from excess Na doping through computation and experiments on Pb 1−x Na x Te with 0.01 ≤ x ≤ 0.04. High temperature X-ray diffraction and Hall carrier concentration measurements show enhanced Na solubility at high temperatures when x > 0.02 but no improvement in carrier concentration, indicating that Na is entering the lattice but is electrically compensated by high intrinsic defect concentrations. The higher Na concentration leads to band convergence between the light L and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. This results in a high temperature zT nearing 2 for Pb 0.96 Na 0.04 Te, ∼25% higher than traditionally reported values for pristine PbTe-Na. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high zT Na-doped materials. A starting matrix of simple, high performing Pb 0.96 Na 0.04 Te synthesized following our guidelines may be superior to Pb 0.98 Na 0.02 Te for continued zT optimization in p-type PbTe materials.

Defect chemistry is critical to designing high performance thermoelectric materials. In SnTe, the naturally large density of cation vacancies results in excessive hole doping and frustrates the ability to control the thermoelectric properties. Yet, recent work also associates the vacancies with suppressed sound velocities and low lattice thermal conductivity, underscoring the need to understand the interplay between alloying, vacancies, and the transport properties of SnTe. Here, we report solid solutions of SnTe with NaSbTe2 and NaBiTe2 (NaSn m SbTe m+2 and NaSn m BiTe m+2, respectively) and focus on the impact of the ternary alloys on the cation vacancies and thermoelectric properties. We find introduction of NaSbTe2, but not NaBiTe2, into SnTe nearly doubles the natural concentration of Sn vacancies. Furthermore, DFT calculations suggest that both NaSbTe2 and NaBiTe2 facilitate valence band convergence and simultaneously narrow the band gap. These effects improve the power factors but also make the alloys more prone to detrimental bipolar diffusion. Indeed, the performance of NaSn m BiTe m+2 is limited by strong bipolar transport and only exhibits modest maximum ZTs ≈ 0.85 at 900 K. In NaSn m SbTe m+2 however, the doubled vacancy concentration raises the charge carrier density and suppresses bipolar diffusion, resulting in superior power factors than those of the Bi-containing analogues. Lastly, NaSbTe2 incorporation lowers the sound velocity of SnTe to give glasslike lattice thermal conductivities. Facilitated by the favorable impacts of band convergence, vacancy-augmented hole concentration, and lattice softening, NaSn m SbTe m+2 reaches high ZT ≈ 1.2 at 800–900 K and a competitive average ZTavg of 0.7 over 300–873 K. The difference in ZT between two chemically similar compounds underscores the importance of intrinsic defects in engineering high-performance thermoelectrics.

Drastic effects of phase equilibrium on semiconductor doping efficiency are demonstrated in n-type PbTe.

mobilities it favors, because such tetrahedral structures are more tightly packed and usually have stiffer lattices than the rocksalt materials. They present high phonon frequencies and velocities giving rise to very high thermal conductivity. [9,10] Typical compound semiconductors in this class include the zincblende-, wurtzite-, and the chalcopyrite-type materials. [11][12][13][14] The AMQ 2 (A = Cu, Ag; M = Al, Ga, In, Tl; Q = S, Se, Te) ternary diamondoid compounds are a large family of relatively wide band gap (E g > 1 eV) semiconductors that possess various unique transport properties, and have many important applications in photovoltaic cells, [15] nonlinear optics, [16] and thermoelectricity. [11,13] Recently, a thermoelectric figure of merit (ZT) beyond 1.6 has been reported in Cu 1−x Ag x InTe 2 [17] and (Cu 1−x Ag x )(In 1−y Ga y ) Te 2 diamondoid compounds. [18,19] These performance advances have drawn intense interest in fundamental understanding of the electronic and heat transport properties of the diamondoid compounds in greater detail.The ternary diamondoid compounds (chalcopyrites) derived from the diamond structure can be considered as a double sphalerite cell (M'Q) stacked along the c-axis, where the divalent M' cation is replaced by monovalent A and trivalent M, see Figure 1a. Among compositions with the identical crystal structure, the Ag-based diamondoid compounds exhibit a much lower intrinsic lattice thermal conductivity than the Cu-based Typically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid compound AgGaTe 2 is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd 3 orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic-optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m −1 K −1 at 850 K in AgGaTe 2 . The findings explain the underlying reason for the unexpectedly low thermal conductivities of silver-based compounds compared to copper-based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.

Grain boundaries critically limit the electronic performance of oxide perovskites. These interfaces lower the carrier mobilities of polycrystalline materials by several orders of magnitude compared to single crystals. Despite extensive effort, improving the mobility of polycrystalline materials (to meet the performance of single crystals) is still a severe challenge. In this work, the grain boundary effect is eliminated in perovskite strontium titanate (STO) by incorporating graphene into the polycrystalline microstructure. An effective mass model provides strong evidence that polycrystalline graphene/strontium titanate (G/STO) nanocomposites approach single crystal‐like charge transport. This phenomenological model reduces the complexity of analyzing charge transport properties so that a quantitative comparison can be made between the nanocomposites and STO single crystals. In other related works, graphene composites also optimize the thermal transport properties of thermoelectric materials. Therefore, decorating grain boundaries with graphene appears to be a robust strategy to achieve “phonon glass–electron crystal” behavior in oxide perovskites.