Sodium Substitution in Lead Telluride

Sodium is widely used as a substituting element in p-type PbTe-based thermoelectric materials. In this work, two series of polycrystalline samples Pb1-yNayTe1-y/2 (total charge balance) and Pb1-xNaxTe (total charge nonbalance) were examined. Na has limited solubility for both of the series. The MAS Na-23 NMR analysis of Pb1-xNaxTe series (for Pb0.98Na0.02Te 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 Pb1-yNayTe1-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 Pb1-xNaxTe and Pb1-yNayTe1-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.