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Summary High phonon velocities, i.e., as measured by the speed of sound (vs) lead to high lattice thermal conductivity (κlat), which is detrimental to thermoelectric performance. Conventional wisdom associates vs exclusively with structural features such as average atomic mass but not the number of conducting electrons. Here, we demonstrate vs reduction from electronic doping in eight well-known thermoelectric semiconductors and establish carrier density nH as the main cause for the observed lattice softening by ruling out alternative factors such as changes in density, average atomic mass, and defect formation. In p-type SnTe and n-type La3–xTe4, we find respective decreases of 16% and ∼20% in vs when raising the nH from ∼1019 to 1021 cm–3, which is sufficient to decrease κlat by nearly 50%. Such giant softening effects can account for 25% of the optimized thermoelectric figure of merit (zTmax) in high-performing materials (zTmax > 1) by suppressing total thermal conductivity.

Yb14MnSb11 is a member of a remarkable structural family of compounds that are classified according to the concept of Zintl. This structure type, of which the prototype is Ca14AlSb11, provides a flexible framework for tuning structure-property relationships and hence the physical and chemical properties of compounds. Compounds within this family show exceptional high temperature thermoelectric performance at temperatures above 300 K and unique magnetic and transport behavior at temperatures below 300 K. This review provides an overview of the structure variants, the magnetic properties, and the thermoelectric properties. Suggestions for directions of future research are provided.

The thermoelectric figure of merit reported for n-type Mg3(Sb,Bi)2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg3(Sb,Bi)2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures. Herein, a physics-based model is used to assess the magnitude of the heat capacity over the entire temperature range up to 800 K. The model agrees in magnitude with experimental low-temperature values and reproduces the linear slope observed in high-temperature data. Owing to the large scatter in experimental values of high-temperature heat capacity, the model is likely more accurate (within ±3%) than a measurement of a new sample even for doped or alloyed materials. It is found that heat capacity for the solid solution series can be simply described (for temperatures: 200 K ≤ T ≤ 800 K ) by the polynomial equation: c p [ Jg − 1 K − 1 ] = 3 N R M W ( 1 + 1.3 × 10 − 4 T − 4 × 10 3 T − 2 ) , where 3 N R = 124.71 J mol − 1 K − 1 , M W is the molecular weight [ g mol − 1 ] of the formula unit being considered, and T is temperature in K. This heat capacity is recommended to be a standard value for reporting and comparing the thermal conductivity of Mg3(Sb,Bi)2 including doped or alloyed derivatives. A general form of the equation is given which can be used for other material systems.

Since the report of high zT in Si95Ge5 there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% phosphorus (P) doped n-type Si95Ge5 (SiGe) alloy was reacted with YbH2 (0, 1, 2%). YbP was formed in the SiGe alloy matrix from the reaction between YbH2 and P during the Spark Plasma Sintering (SPS) process. Thermoelectric property measurements were performed on sintered pellets from room temperature to 1273 K. X-ray diffraction patterns were collected from the ground powder samples and confirmed the main phase possessed diamond structured Si95Ge5 (space group: Fd 3 ¯ m) as well as the presence of YbP (space group: Fm 3 ¯ m). The carrier concentration of the sample was controlled by the amount of YbH2 added, removing some of the phosphorus to form YbP. n-type Si95Ge5 alloy samples with higher YbP amounts showed higher electrical resistivity and lower thermal conductivity attributed to loss of the P dopant. Another composite series of n-type Si95Ge5 with YbP were synthesized with additional P compositions (1, 2%). The thermoelectric properties were characterized from room temperature to 1273 K, and the samples possess electrical resistivity, carrier concentrations, and thermal conductivity as expected from the additional P dopant. The presence of YbP lowered lattice thermal conductivity when the sample was appropriately doped. The Seebeck coefficients were measured with both off-axis and uniaxial axis experimental configurations. These results show that the off-axis measurements overestimate the Seebeck coefficients of the Si95Ge5 alloy samples. This is attributed to a cold finger effect and therefore only the uniaxial data are combined for zT calculations. The n-type Si95Ge5 samples with YbP and less than 3% P dopant show similar zT compared with the Si95Ge5 sample with no YbP inclusions and 3% P dopant with a peak zT of 0.6 at 1200 K.

Summary Refractory rare-earth tellurides with the Th 3 P 4 structure type have attracted considerable interest as high-performance thermoelectric materials since the 1980s due to their high dimensionless figure of merit ( ZT ). Extensive work has been conducted on La 3−x Te 4 with peak ZT values greater than 1.1 at 1,273 K. The high ZT of La 3-x Te 4 is in part due to a large peak in the density of states near the Fermi level from the La 5d states. Here, we revisit Pr 3−x Te 4 , for which our electronic structure calculations predict a favorable modification of the density of states by the introduction of praseodymium's 4f electrons. This was experimentally verified by preparing Pr 3−x Te 4 samples with varying Pr vacancy concentrations using a mechanochemical synthesis approach. The thermoelectric properties were measured and a ZT of 1.7 at 1,200 K was achieved with Pr 2.74 Te 4 . The 50% improvement in peak ZT compared with La 3−x Te 4 resulted from an increased effective mass, improved Seebeck coefficient, and lower thermal conductivity.

Within the field of thermoelectric materials for energy conversion magnesium silicide, Mg2Si, is an outstanding candidate due to its low density, abundant constituents and low toxicity. However electronic and thermal tuning of the material is a required necessity to improve its Figure of Merit, zT. Doping of Yb via reactive YbH2 into the structure is performed with the goal of reducing the thermal conductivity. Hydrogen is released as a by-product at high temperatures allowing for facile incorporation of Yb into the structure. We report on the properties of Yb- and Bi-doped Mg2Si prepared with MgH2 and YbH2 with the focus on the synthetic conditions, and samples’ microstructure, investigated by various electron microscopy techniques. Yb is found in the form of both Yb3Si5 inclusions and Yb dopant segregated at the grain boundary substituting for Mg. The addition of 1 at% Yb concentration reduced the thermal conductivity, providing a value of 30 mW/cm K at 800 K. In order to adjust carrier concentration, the sample is additionally doped with Bi. The impact of the microstructure on the transport properties of the obtained material is studied. Idealy, the reduction of the thermal conductivity is achieved by doping with Yb and the electronic transport is adjusted by doping with Bi. Large grain microstructure facilitates the electronic transport. However, the synthetic conditions that provide the optimized microstructure for electrical transport do not facilitate the additional Yb dopant incorporation. Therefore, the Yb and Bi containing sample with the optimized microstructure provides a zT=0.46 at 800 K.

Yb14MnSb11 is the most efficient bulk p-type thermoelectric material for high temperature applications. Materials with Y and Sc substitutions in Yb14MnSb11 were made both in Sn-flux and by ball milling. These small 3+ rare earth (RE) cations were introduced with the goal of providing chemical pressure on the structure. The RE3+ cation is smaller than Yb2+ and also donates one additional electron to this p-type semiconductor. In Yb14−xRExMnSb11 (RE = Sc, Y) the maximum x was about 0.5. X-ray diffraction experiments on the single crystals obtained from Sn-flux showed that Sc preferentially substitutes for Yb(1) and Yb(3), and decreases the size of the unit cell by about 0.3%. Y substitutes on all Yb sites and increases the size of the unit cell by about 0.2%. Samples with Yb14−xRExMnSb11 (x~0.3) were prepared via powder metallurgy and spark plasma sintering for transport and thermal conductivity measurements. Electron microprobe of the Sc-substituted sample showed small regions (≤1 μ m) containing greater amounts of Sc, and X-ray powder diffraction of the ball milled Sc sample could be fitted as phase pure Yb14−xScxMnSb11. Y-substituted samples showed larger regions of excess Y in electron microprobe, and small amounts of Yb4Sb3 in X-ray powder diffraction. The Sc sample has slightly reduced carrier concentration over optimized Yb14MnSb11, while the Y samples have even lower carrier concentrations. These carrier concentrations lead to comparable resistivity to Yb14MnSb11 in the Sc-substituted material, and higher resistivities in the Y-substituted material. All materials had similar Seebeck coefficients that slightly exceed Yb14MnSb11 at high temperatures, with the Sc-substituted sample having the highest despite having a higher carrier concentration. Sc-substituted samples also had a slightly higher thermal conductivity over the Y-substituted samples, which had comparable thermal conductivity to Yb14MnSb11. The zT values of the Sc and Y substituted samples are similar (zT1000 K~0.8), however below that of Yb14MnSb11 due to the compensation of Seebeck and resistivity.

Bulk nanocrystalline Mg2Si thermoelectric materials were synthesized and consolidated in a one-step process through a solid-state reaction between magnesium hydride and silicon, using the spark plasma sintering (SPS) method. The hydrogen produced in the process alleviates the problem of the oxidation of Mg. The samples were reactively sintered at temperatures in the range 723–823 K and under a uniaxial pressure in the range of 71–164 MPa in 5 min. Powder X-ray diffraction (XRD) analysis showed the products to be pure Mg2Si. The grain size of the consolidated samples was less than 500 nm, as determined by transmission electron spectroscopy (TEM). Residual nano-pores were observed by scanning electron microscopy at grain boundaries; their presence is believed to be the consequence of hydrogen evolution during the reactive sintering. The effect of synthesis temperature and pressure on crystallite size, density, and transport properties was determined. The results showed that use of MgH2 instead of Mg in the onestep method prevents the formation of MgO. The addition of 1 at.% Bi as a dopant improved the power factor significantly. Samples with 1 at.% Bi had a ZT of 0.6 at 775 K.

BaZn2Sn2 (space group P4/nmm, a = 4.7459(5) A, c = 11.330(2) A, Z = 2) crystallizes in the CaBe2Ge2 structure type with a polyanionic framework comprising alternately stacked PbO-like {ZnSn4/4} and anti-PbO-like {SnZn4/4} layers along the c-axis. BaZn2Sn2 samples were obtained by either direct solid state reaction of the elements or from a Sn-flux method in very high yield with very small amount of β-Sn as the secondary phase. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The chemical compositions were determined to be off-stoichiometric with Zn/Sn ratio lower than 1.0 and Sn2 atoms in the crystal structure were found to be either loosely bonded or not bonded which might lead to an incomplete charge balance. Electrical and thermal transport measurements have been performed in the temperature range 300–773 K. BaZn2Sn2 displays the electrical resistivity of a metal (or semimetal) along with very low Seebeck coefficients and relatively high thermal conductivity.

Samples with the type-I clathrate composition Ba{sub 8}Ni{sub x}Si{sub 46-x} have been synthesized and their structure and thermoelectric properties characterized. Microprobe analysis indicates the Ni incorporation to be 2.62{