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

PbGaTe 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 PbGaTe-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 PbGaTe composition is presented here, in which polycrystalline samples with the compositions (PbTe)(GaTe) (0.67 ≤ ≤ 0.87) and PbGaTe (0.85 ≤ ≤ 1.5) were synthesized and characterized. Differential scanning calorimetry measurements revealed that PbGaTe 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 PbGaTe exists (space groups 321 or 321) and above 693 K, the rhombohedral one (space group 32). A homogeneity range was found for PbGaTe, = 0.9-1.1, based on refined lattice parameters of PbGaTe in samples annealed at 873 K. The revised version of the PbTe-GaTe phase diagram in the vicinity of the PbGaTe phase is proposed. Based on the new results of the phase equilibria, the TE properties of the PbGaTe samples were studied in detail. The deviation from the stoichiometric composition leads to a tuning of the charge transport in PbGaTe, and as a result, the Seebeck coefficient and electrical conductivity were significantly modified over the homogeneity range. The Pb-deficient PbGaTe sample shows an improved power factor up to 9.5 μW m K and a reduced thermal conductivity as low as 0.17 W m K 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.