Synthesis, Crystal Structure, and High-Temperature Phase Transition of the Novel Plumbide Na₂MgPb

A hitherto unknown sodium magnesium plumbide, Na₂MgPb, was synthesized by heating the constituent elements. Na₂MgPb crystallizes in a hexagonal unit cell with the Li₂CuAs-type structure (P6₃/mmc, Z = 2, a = 5.110(2) Å, c = 10.171(4) Å at 293 K). The compound furthermore displays polymorphism: high-temperature powder XRD measurements revealed that hexagonal Na₂MgPb (dubbed the “α” phase) transforms to another hexagonal phase (β) which is existent at 493–553 K, and the β phase changes to a cubic structure (γ) at 533–633 K further. The molar volume of γ-Na₂MgPb is approximately 9% and 13% smaller than the molar volumes of the α phase and the β phase, respectively (at 543 K). The electrical resistivity of Na₂MgPb is 0.39 mΩ at 300 K; it rises with increasing temperature from 300 to 491 K, and then drops at 491 and 523 K. These abrupt changes in resistivity may be attributed to the α → β and β → γ phase transitions, respectively. To gain further insight into the structure of cubic γ-Na₂MgPb, putative models with regular Heusler-type (Cu₂MnAl-type) and inverse Heusler-type (Li₂AgSb-type) arrangements were probed using first-principles computations based on density functional theory (DFT). These computations indicate that, for the cubic γ phase, an inverse Heusler-type structure is distinctly more stable than the alternative regular Heusler type (at 0 K); beyond that, ab initio thermochemistry was successfully used to verify the stability ordering (α-Na₂MgPb being favorable at low temperature, γ-Na₂MgPb at high temperature), albeit the theoretically predicted transition temperature of 900 K which is higher than observed in experiment.