Fabrication of nanocrystalline Mg3X2 (X = Bi, Sb) with supersaturated solid solubility by mechanical alloying

Mechanical alloying plus hot-pressing was employed to prepare nanocrystalline Mg3X2 (X = Bi, Sb) compounds that were characterized by microstructural examinations and dc electrical resistance measurements. The results indicated that Mg3Sb2+y compounds with mean grain size ∼30 nm and with solute Sb atoms as much as 5 at.% in the powder specimens were successfully prepared. By comparison, Mg3+xBi2 compound with solute Mg atoms as much as 13 at.% could be fabricated with similar grain sizes. Hot-pressing at 673 K (for 20–50 min) only led to a limited grain growth (<∼10 nm) for both compound systems. However, experiments showed that hot-pressing could further facilitate alloying of the elements in Mg–Bi system. In contrast, elementary Sb phase formed gradually in the Mg3Sb2+y system with supersaturated solute Sb atoms during hot-pressing, presumably due to its high ionicity and large number of intragranular imperfections, as revealed by large lattice strain. dc resistance measurements indicated that a semimetal–metal transition occurred in Mg3+xBi2 system as solute Mg x reached ∼0.72; while in Mg3Sb2+y system a transition of its conduction behavior from logarithmic law to Mottʹs T−1/4 law was observed at T < ∼190 K as solute Sb content y > ∼0.097, suggesting that phonon-assisted hopping occurred in the Mg3Sb2+y at the low temperatures.