Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb-Eu2 ZnSb2 Alloys.

AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M-site in the planar MX-layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu1-x Zn0.5x Sb solid solution. The transition from a fully-occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn0.5 Sb) leads to non-linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb-sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK-1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn0.5 Sb.

Elastic Moduli: a Tool for Understanding Chemical Bonding and Thermal Transport in Thermoelectric Materials.

The elastic behavior of a material can be a powerful tool to decipher thermal transport. In thermoelectrics, measuring the elastic moduli-directly tied to sound velocity-is critical to understand trends in lattice thermal conductivity, as well as study bond anharmonicity and phase transitions, given the sensitivity of elastic moduli to the chemical bonding. In this review, we introduce the basics of elasticity and explain the origin of high-temperature lattice softening from a bonding perspective. We then review elasticity data throughout classes of thermoelectrics, and explore trends in sound velocity, anharmonicity, and thermal conductivity. We reveal how experimental sound velocities can improve the accuracy of common thermal conductivity models and present a critical discussion of Grüneisen parameter estimates from elastic moduli. Readers will be equipped with tools to leverage elasticity measurements or calculations to accurately interpret thermal transport trends.

Synthesis, crystal and electronic structure of BaLi x Cd13-x (x ≈ 2).

A new ternary phase has been synthesized and structurally characterized. BaLi x Cd13-x (x ≈ 2) adopts the cubic NaZn13 structure type (space group Fm 3 ¯ c, Pearson symbol cF112) with unit cell parameter a = 13.5548 (10) Å. Structure refinements from single-crystal X-ray diffraction data demonstrate that the Li atoms are exclusively found at the centers of the Cd12-icosahedra. Since a cubic BaCd13 phase does not exist, and the tetragonal BaCd11 is the most Cd-rich phase in the Ba-Cd system, BaLi x Cd13-x (x ≈ 2) has to be considered as a true ternary compound. As opposed to the typical electron count of ca. 27e-per formula unit for many known compounds with the NaZn13 structure type, BaLi x Cd13-x (x ≈ 2) only has ca. 26e-, suggesting that both electronic and geometric factors are at play. Finally, the bonding characteristics of the cubic BaLi x Cd13-x (x ≈ 2) and tetragonal BaCd11 are investigated using the TB-LMTO-ASA method, showing metallic-like behavior.

Thermoelectric Properties of Scandium Sesquitelluride.

Rare-earth (RE) tellurides have been studied extensively for use in high-temperature thermoelectric applications. Specifically, lanthanum and praseodymium-based compounds with the Th3P4 structure type have demonstrated dimensionless thermoelectric figures of merit (zT) up to 1.7 at 1200 K. Scandium, while not part of the lanthanide series, is considered a RE element due to its chemical similarity. However, little is known about the thermoelectric properties of the tellurides of scandium. Here, we synthesized scandium sesquitelluride (Sc2Te3) using a mechanochemical approach and formed sintered compacts through spark plasma sintering (SPS). Temperature-dependent thermoelectric properties were measured from 300⁻1100 K. Sc2Te3 exhibited a peak zT = 0.3 over the broad range of 500⁻750 K due to an appreciable power factor and low-lattice thermal conductivity in the mid-temperature range.

Limits of Cation Solubility in AMg2Sb2 (A = Mg, Ca, Sr, Ba) Alloys.

A M 2 X 2 compounds that crystallize in the CaAl 2 Si 2 structure type have emerged as a promising class of n- and p-type thermoelectric materials. Alloying on the cation (A) site is a frequently used approach to optimize the thermoelectric transport properties of A M 2 X 2 compounds, and complete solid solubility has been reported for many combinations of cations. In the present study, we investigate the phase stability of the AMg 2 Sb 2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg 2 + leads to limited solubility when alloyed with larger cations such as Sr or Ba. Phase separation observed in such cases indicates a eutectic-like phase diagram. By combining these results with prior alloying studies, we establish an upper limit for cation radius mismatch in A M 2 X 2 alloys to provide general guidance for future alloying and doping studies.