Synthesis, Crystal Structure, and High-Temperature Phase Transition of the Novel Plumbide Na<sub>2</sub>MgPb

A hitherto unknown sodium magnesium plumbide, Na<sub>2</sub>MgPb, was synthesized by heating the constituent elements. Na<sub>2</sub>MgPb crystallizes in a hexagonal unit cell with the Li<sub>2</sub>CuAs-type structure (<i>P</i>6<sub>3</sub>/<i>mmc</i>, <i>Z</i> = 2, <i>a</i> = 5.110(2) Å, <i>c</i> = 10.171(4) Å at 293 K). The compound furthermore displays polymorphism: high-temperature powder XRD measurements revealed that hexagonal Na<sub>2</sub>MgPb (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-Na<sub>2</sub>MgPb 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 Na<sub>2</sub>MgPb 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-Na<sub>2</sub>MgPb, putative models with regular Heusler-type (Cu<sub>2</sub>MnAl-type) and inverse Heusler-type (Li<sub>2</sub>AgSb-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, <i>ab initio</i> thermochemistry was successfully used to verify the stability ordering (α-Na<sub>2</sub>MgPb being favorable at low temperature, γ-Na<sub>2</sub>MgPb at high temperature), albeit the theoretically predicted transition temperature of 900 K which is higher than observed in experiment.