Non-Stoichiometric <i>fcc</i>-Base GdO<i><sub>x</sub></i> Precipitations in a Mg–Zn–Gd Alloy

We have found that, during in-situ scanning transmission electron microscopy observations, heating of a Mg97Zn1Gd2 (at%) alloy at 623 K leads to dynamic precipitations of face-centered-cubic (fcc)-based Gd nanoparticles. With the aid of density-functional theory (DFT) calculations, the observed lattice constant of 5.32 Å, which is larger than that expected for pure fcc-Gd of 5.06 Å, is likely to be due to oxygen atoms inserted at tetrahedral interstitial sites with essentially a fractional occupation. Systematic DFT calculations show that the fcc-base GdOx can be stable configurations in a wide composition range, where the O atoms behave like a solute up to x ∼ 1.5 to provide continuously the non-stoichiometric GdOx solid-solution phase.