Bistructural Pseudocontinuous Solid Solution with Hierarchical Microstructures from Ab initio Study: Application to the Mg2Sn−Mg3Sb2 System

Hetero-structural solid solutions, i.e. those from the parent compounds with different crystal structures, occasionally appear for exploring new materials, but a systematic understanding of the thermodynamics, microstructures and electronic characteristics is still lacking. By alloying cubic (1- x)Mg ₂Sn with hexagonal (0.5 x)Mg ₃Sb ₂ , we show that such a system manifests an overall pseudocontinuous composition distribution with hierarchical microstructures, and the coexistence of n-type/p-type electronic conduction regions. This type of solid solution is defined as a bistructural pseudocontinuous solid solution (BiSS), clearly different from the homogenous iso-structural solid solutions. The Mg _{2-0.5 x}Sn _{1- x}Sb _x phase is found to be stable at the Mg ₂Sn-based phase only in the range of 0 < x ≤ ∼0.47, consistent with experimental observations. Large-scale Monte Carlo simulations reveal the natural occurrence of hierarchical microstructures, which contains primarily both nanoscale Mg-deficiency clusters and a homogeneous matrix showing tunable compositions relevant to alloy composition and annealing temperature. The electronic structures of the BiSS thus manifest a spatially inhomogeneous coexistence of n-type matrix and p-type nanoclusters, implying the importance of selective doping for optimizing thermoelectric performance. Those phenomena could be considered universal in hetero-structural solid solutions. This work offers a general guideline for the design of bistructural and even hetero-structural solid solutions with potential novel functionality.