Phase Equilibria and Thermoelectric Properties in the Pb-Ga-Te System in the Vicinity of the PbGa6Te10 Phase

Oleksandr Cherniushok; Raul Cardoso-Gil; Taras Parashchuk; Yuri Grin; Krzysztof T Wojciechowski

doi:10.1021/acs.inorgchem.0c03549

Phase Equilibria and Thermoelectric Properties in the Pb-Ga-Te System in the Vicinity of the PbGa₆Te₁₀ Phase

Inorg Chem. 2021 Feb 15;60(4):2771-2782. doi: 10.1021/acs.inorgchem.0c03549. Epub 2021 Feb 2.

Authors

Oleksandr Cherniushok¹, Raul Cardoso-Gil², Taras Parashchuk³, Yuri Grin², Krzysztof T Wojciechowski¹

Affiliations

¹ Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicza Avenue, Krakow 30-059, Poland.
² Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, Dresden 01187, Germany.
³ Lukasiewicz Research Network-Krakow Institute of Technology, 73 Zakopianska Street, Krakow 30-418, Poland.

PMID: 33528993
DOI: 10.1021/acs.inorgchem.0c03549

Abstract

PbGa₆Te₁₀ is a promising thermoelectric (TE) material due to its ultralow thermal conductivity and moderated values of the Seebeck coefficient. However, the reproducible synthesis of the PbGa₆Te₁₀-based materials for the investigation and tailoring of physical properties requires detailed knowledge of the phase diagram of the system. With this aim, a combined thermal, structural, and microstructural study of the Pb-Ga-Te ternary system near the PbGa₆Te₁₀ composition is presented here, in which polycrystalline samples with the compositions (PbTe)_1-x(Ga₂Te₃)_x (0.67 ≤ x ≤ 0.87) and Pb_yGa₆Te₁₀ (0.85 ≤ y ≤ 1.5) were synthesized and characterized. Differential scanning calorimetry measurements revealed that PbGa₆Te₁₀ melts incongruently at 1007 ± 2 K and has a polymorphic phase transition at 658-693 K depending on composition. Powder X-ray diffraction of annealed samples confirmed that below 658 K, the trigonal modification of PbGa₆Te₁₀ exists (space groups P3₁21 or P3₂21) and above 693 K, the rhombohedral one (space group R32). A homogeneity range was found for Pb_yGa₆Te₁₀, y = 0.9-1.1, based on refined lattice parameters of Pb_yGa₆Te₁₀ in samples annealed at 873 K. The revised version of the PbTe-Ga₂Te₃ phase diagram in the vicinity of the PbGa₆Te₁₀ phase is proposed. Based on the new results of the phase equilibria, the TE properties of the Pb_yGa₆Te₁₀ samples were studied in detail. The deviation from the stoichiometric composition leads to a tuning of the charge transport in Pb_yGa₆Te₁₀, and as a result, the Seebeck coefficient and electrical conductivity were significantly modified over the homogeneity range. The Pb-deficient Pb_0.9Ga₆Te₁₀ sample shows an improved power factor up to 9.5 μW m^-1 K^-2 and a reduced thermal conductivity as low as 0.17 W m^-1 K^-1 due to attuned chemical potential and additional scattering of phonons on point defects. Thus, the ZT parameter for this composition was improved up to ∼0.043 at 773 K, which is almost 4 times higher than that of the stoichiometric specimen. This work shows that the knowledge of phase equilibria and crystal chemistry plays a key role in improving the energy conversion efficiency for new functional TE materials.