Sodium is widely used as a substituting element in p-type PbTe-based thermoelectric materials. In this work, two series of polycrystalline samples Pb 1−y Na y Te 1−y/2 (total charge balance) and Pb 1−x Na x Te (total charge nonbalance) were examined. Na has limited solubility for both of the series. The MAS 23 Na NMR analysis of Pb 1−x Na x Te series (for Pb 0.98 Na 0.02 Te sample after spark-plasma sintering, SPS) reveals only one Na signal, corresponding to Na atoms coordinated by six Te atoms, indicating substitution of Pb by Na without defects in the Te sublattice. In the Pb 1−y Na y Te 1−y/2 series, clustering of Na atoms with reduced coordination by Te was observed. Additional heat treatment of these samples leads to the reorganization of the Na clusters in PbTe and their equilibration with the homogenized distribution of Na in the whole volume. The maximum ZT values of 1.4−1.6 at 760 K are established for both Pb 1−x Na x Te and Pb 1−y Na y Te 1−y/2 series. Upon long-time annealing at 873 K, reorganization and redistribution of Na atoms lead to the change in carrier concentration and, consequently, the thermoelectric properties for both series.

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The thermoelectric properties of polycrystalline materials on the basis of the solid solution Pb1−xEuxTe prepared by spark-plasma-sintering are characterized. The solid solution undergoes a metal–semiconductor transition in parallel to the p–n transition around 500 K.

As one family of the most investigated thermoelectrics (TE), PbTe-based materials have been developed into state-of-the-art p-type and n-type TE materials. However, there are quite a few studies focusing on the reproducibility of TE properties and microstructure evolution during different heat treatments. In this work, Pb 0.98Àx Na 0.02 Eu x Te (x = 0-0.030) samples were systematically examined after three different kinds of heat treatments: spark plasma sintering (SPS), laser flash measurement (LFA), and long-term annealing. The maximal solubility of Eu (ca. 1.0 atom%) in Pb 0.98Àx Na 0.02 Eu x Te was established at 873 K.The most inhomogeneous samples (samples after SPS) show highest values of figure-of-merit, ZT max , of up to 2.1 at 760 K, due to a large number of micrometer-scale sodium-and europium-rich aggregations in them. After additional heat treatment (LFA measurement or long-term annealing), the ZT max value reduces to 1.6. The distribution of Eu and Na in the samples becomes much more homogeneous, accompanied by increased lattice parameters and decreased carrier concentrations. The long-term annealed samples have the best stable TE properties and good mechanical stability in the cyclic measurements. Surface protection needs to be considered for the temperatures above 773 K in order to avoid material decomposition.

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