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Lanthanum telluride (La_3Te_4) is an n-type high-performance thermoelectric material in the high temperature range, but its mechanical properties remain unknown. Since we want robust mechanical properties for their integration into industrial applications, we report here quantum mechanics (QM) simulations to determine the ideal strength and deformation mechanisms of La_3Te_4 under pure shear deformations. Among all plausible shear deformation paths, we find that shearing along the (0 0 1)/〈100〉 slip system has the lowest ideal shear strength of 0.99 GPa, making it the most likely slip system to be activated under pressure. We find that the long range La–Te ionic interactions play the predominant role in resisting shear deformation. To enhance the mechanical strength, we suggest improving the long ionic La–Te bond stiffness to strengthen the ionic La–Te framework in La_3Te_4 by a defect-engineering strategy, such as partial substitution of La by Ce or Pr having isotypic crystal structures. This work provides the fundamental information to understand the intrinsic mechanics of La_3Te_4.

The superionic phase transition of Cu_(2-x)Se 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.

The Zintl phase Sr5In2Sb6 is isostructural with Ca5In2Sb6-a promising thermoelectric material with a peak zT of 0.7 when the carrier concentration is optimized by doping. Density functional calculations for Sr5In2Sb6 reveal a decreased energy gap and decreased valence band effective mass relative to the Ca analog. Chemical bonding analysis using the electron localizability indicator was found to support the Zintl bonding scheme for this structure type. High temperature transport measurements of the complete Ca(5-x)Sr(x)In2Sb6 solid solution were used to investigate the influence of the cation site on the electronic and thermal properties of A5In2Sb6 compounds. Sr was shown to be fully miscible on the Ca site. The higher density of the Sr analog leads to a slight reduction in lattice thermal conductivity relative to Ca5In2Sb6, and, as expected, the solid solution samples have significantly reduced lattice thermal conductivities relative to the end member compounds.

Optical absorption edge measurements are performed on I doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy. The Burstein–Moss shift, an increase in the absorption edge (optical band gap) with increasing doping level, is explored. The optical gap increases on the order of 0.1 eV for doping levels ranging from 3 × 10^(18) to 2 × 10^(20) cm^(−3), relevant doping levels for good thermoelectric materials. Chemical potential is estimated from transport measurements—specifically, Hall effect and Seebeck coefficient—using a single band Kane model. In heavily doped semiconductors, it is well-known that the band gap shrinks with increasing doping level. This effect, known as band gap renormalization, is fit here using an n^(1/3) scaling law which reflects an electron–electron exchange interaction. The renormalization effect in these samples is shown to be more than 0.1 eV, on the same order of magnitude as the band gap itself. Existing models do not explain such large relative changes in band gap and are not entirely self-consistent. An improved theory for the renormalization in narrow gap semiconductors is required.

The thermoelectric transport properties of Lu-doped Yb_(14)MnSb_(11) Zintl compounds have been investigated. The electrical conductivity increased and the Seebeck coefficient decreased with increasing Lu doping content, due to an increased carrier concentration. The carrier mobility behavior was in good accordance with the single parabolic band model. It is suggested that Lu doping simply shifts the Fermi energy with the band structure minimally altered. The lattice thermal conductivity roughly decreased via Lu doping. The maximum dimensionless figure of merit ZT of the Lu-doped samples was improved by ∼30% at 670 K, compared with that without doping.