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Highly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figure‐of‐merit zT in n‐type polycrystalline Mg_3Sb_2. In this work, discovered is a Mg deficiency near grain boundaries using atom‐probe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The off‐stoichiometry can prevent n‐type dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grain‐boundary resistance and increase zT in Mg_3Sb_2‐based materials.

Bi_2Te_3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg_3Sb_(0.6)Bi_(1.4), with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi_2Te_3. This exceptional performance is achieved by tuning the alloy composition of Mg_3(Sb_(1−x)Bi_x)_2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg_3Bi_2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg_3Sb_(0.6)Bi_(1.4), after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg_3Sb_(0.6)Bi_(1.4) is a significant advancement towards sustainable heat recovery technology.

The charge transport mechanism in a solid is often inferred by observing very simple features like the temperature dependency of electrical conductivity or resistivity. However, comparing complicated physical models to such simple signatures leaves much ambiguity. Because models generally have more parameters than the types of measurements available, inconsistencies can long go unrecognized until the interrelation between different measurements is closely examined. We show that a simple investigation of the thermopower-conductivity relation allows one to phenomenologically characterize transport from experiments; the phenomenologically determined transport function can be compared to physical models to distinguish transport mechanisms and straightforwardly point out inconsistencies in literature models. We highlight two example cases, ceria and strontium titanate, to show that our analysis method can clarify whether the transport mechanism is through hopping or delocalized states. We question previous suggestions about the scattering mechanism in SrTiO_3 and suggest deformation potential scattering on elongated Fermi surfaces as the origin of high-temperature T^2 resistivity.

Thermally activated mobility near room temperature is a signature of detrimental scattering that limits the efficiency and figure-of-merit zT in thermoelectric semiconductors. This effect has been observed dramatically in Mg₃Sb₂-based compounds, but also to a lesser extent in other thermoelectric compounds. Processing samples differently or adding impurities such that this effect is less noticeable produces materials with a higher zT. Experiments suggest that the behavior is related to grain boundaries, but impurity scattering has also been proposed. However, conventional models using Matthissen's rule are not able to explain the dramatic change in the temperature dependency of conductivity or drift mobility which is observed in Mg₃Sb₂-based compounds. We find that it is essential to consider the grain boundary region as an effectively separate phase rather than a scattering center, taking into account the weaker screening in semiconductors compared with classical metals. By modeling a grain boundary phase with a band offset, we successfully reproduce the experimentally observed conductivity versus temperature and thermopower versus conductivity relations, which indicate an improved description of transport. The model shows good agreement with measured grain size dependencies of conductivity, opening up avenues for quantitatively engineering materials with similar behavior. Model estimates predict room for >60% improvement in the room temperature zT of Mg_(3.2)Sb_(1.5)Bi_(0.49)Te_(0.01) if the grain boundary resistance could be eliminated.

Mg₃Sb₂–Mg₃Bi₂ alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg₃Bi₂-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg₃Sb₂–Mg₃Bi₂ and also Mg₃Bi₂-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50% of the improvement. We find that Mg₃Bi₂ alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg₃Bi₂ because the peak zT in unalloyed Mg₃Sb₂ is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg₃Bi₂ alloying, we model the optimum Mg₃Bi₂ content for thermoelectrics to be in the range of 20–30%, consistent with the most commonly reported composition Mg₃Sb_(1.5)Bi_(0.5).

The thermoelectric performance of Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal xfrom 0.2 to 0.01 in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01), leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01); however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material.

The superionic phase transition of Cu2-xSe accompanies drastic changes in transport properties. The Seebeck coefficient increases sharply while the electrical conductivity and thermal diffusivity drops. Such behavior has previously been attributed to critical phenomena under the assumption of a continuous phase transition. However, applying Landau's criteria suggests that the transition should be first order. Using the phase diagram that is consistent with a first order transition, we show that the observed transport properties and heat capacity curves can be accounted for and modeled with good agreement. The apparent critical phenomena is shown to be a result of compositional degree-of-freedom. Understanding of the phase transition allows to explain the enhancement in the thermoelectric figure-of-merit that is accompanied with the transition.