Cherniushok Oleksandr, Cardoso-Gil Raul, Parashchuk Taras, Grin Yuri, Wojciechowski Krzysztof T
Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicza Avenue, Krakow 30-059, Poland.
Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, Dresden 01187, Germany.
Inorg Chem. 2021 Feb 15;60(4):2771-2782. doi: 10.1021/acs.inorgchem.0c03549. Epub 2021 Feb 2.
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.
由于其超低的热导率和适度的塞贝克系数值，PbGaTe是一种很有前景的热电（TE）材料。然而，要可重复地合成用于研究和调整物理性质的基于PbGaTe的材料，需要详细了解该体系的相图。出于这个目的，本文对靠近PbGaTe组成的Pb-Ga-Te三元体系进行了热学、结构和微观结构的联合研究，其中合成并表征了组成范围为（PbTe）（GaTe）（0.67≤≤0.87）和PbGaTe（0.85≤≤1.5）的多晶样品。差示扫描量热法测量表明，PbGaTe在1007±2K时发生不一致熔融，并且根据组成在658 - 693K有一个多晶型相变。退火样品的粉末X射线衍射证实，在658K以下，存在PbGaTe的三方变体（空间群321或321），在693K以上，存在菱面体变体（空间群32）。基于在873K退火样品中PbGaTe细化的晶格参数，发现PbGaTe的均匀范围为=0.9 - 1.1。提出了在PbGaTe相附近的PbTe - GaTe相图的修订版。基于相平衡的新结果，详细研究了PbGaTe样品的TE性质。偏离化学计量组成导致PbGaTe中电荷传输的调整，结果，在均匀范围内塞贝克系数和电导率都有显著改变。由于化学势的调整和点缺陷对声子的额外散射，缺Pb的PbGaTe样品显示出高达9.5μW m K的功率因子改善和低至0.17W m K的热导率降低。因此，该组成的ZT参数在773K时提高到约0.043，几乎是化学计量样品的4倍。这项工作表明，相平衡和晶体化学知识在提高新型功能性TE材料的能量转换效率方面起着关键作用。
