A Case Study of the Thermoelectric Zintl Phases BaCd2Sb2 and BaNa2x Cd2-x Sb2 Reveals Unexpected Behavior of the Sodium Atoms

Abstract

This article details the results from an extensive investigation of the Ba-Na-Cd-Sb quaternary system, where presumably trivial attempts to improve the thermoelectric properties of the Zintl phase BaCd2Sb2 (CaAl2Si2 structure type) by Na-doping lead to the unexpected discovery of the quaternary antimonides with the general chemical formula BaNa2x Cd2-x Sb2 (0 < x < 1). BaCd2Sb2 and ten samples with varied Na contents were synthesized by combining the elements in desired stoichiometric ratios and fusing them together at 1173 K. Their crystal structures were established by single-crystal X-ray diffraction methods, which confirmed that the [Cd2Sb2]2- layers in BaCd2Sb2 are reduced to fragments of lower dimensionality in BaNa2x Cd2-x Sb2. The Na atoms introduced into the structure were found not to be just dopants that create defects on Ba sites, as is often speculated. Instead, the systematic crystallographic studies evidenced expansion of the unit cell with increasing Na content, an effect which is brought about by Na atoms occupying partially an empty site and partially displacing Cd atoms. The aliovalent Na+/Cd2+ substitution and the simultaneous filling of an empty site within the host BaCd2Sb2 structure allow for the retention of the charge-balanced composition; i.e., the resulting quaternaries are also Zintl phases. The expected valence-precise behavior is corroborated by both electronic structure calculations and transport property measurements done on dense pellets that were obtained by spark plasma sintering. Increasing the Na content significantly lowers both electrical resistivity and thermal conductivity, the latter exhibiting glass-like values below 1 W m-1 K-1 at room temperature. The Seebeck coefficients decrease notably upon initial Na-substitution compared to the parent BaCd2Sb2, yet remain relatively stable across the Na-containing compositions, indicating minimal variation in carrier concentration among these samples. These combined enhancements culminate in a promising thermoelectric figure of merit, achieving a peak zT of approximately 0.85 at 700 K for BaNa0.3Cd1.85Sb2, highlighting structural engineering as a highly effective strategy for advancing thermoelectric Zintl phases.