Poorer is better: towards robust, high performance Mg₂(Si,Sn) thermoelectric material by avoiding excess Mg

Mg₂(Si,Sn)-based semiconductors constitute promising thermoelectrics (TE), in particular as n-type materials. These are usually synthesized under Mg-excess to compensate for losses of Mg during synthesis and achieve the high carrier concentration required for optimal performance. However, this usage of excess Mg leads to loosely bound Mg in the material which is easily lost during operation, leading to a fast and massive degradation of the TE performance. In this work, we introduce Mg-poor n-type Mg₂(Si,Sn), avoiding excess and loosely bound Mg. We find that (1) employing relatively large nominal Mg deficiency leads nevertheless to single-phase, Mg-poor Mg₂(Si,Sn) by a self-adjustment of the composition during sintering; and (2) that despite showing a lower dopant efficiency, Sb can be employed to achieve the required optimum carrier concentration, resulting in a figure of merit of zT = 1.2 +/- 0.2 at 700 K, comparable to Mg-rich samples. This is confirmed by a comparison of Mg-rich and Mg-poor samples in a single parabolic band model which reveals similar microscopic material parameters like weighted mobility and scattering constants. Finally, we compare Mg-poor synthesized samples with initially Mg-rich ones that experienced Mg loss. Despite similar global compositions we identify grain boundary scattering to be more pronounced in Mg-depleted samples, marking one of the fundamental reasons for the performance degradation of synthesized Mg-rich samples. Overall this work highlights the importance of grain boundaries on the performance of TE materials and the successful application of thermodynamic degrees of freedom to address fundamental challenges in TE material systems while retaining promising TE performance.