Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase,designated as β“phase,induced to form by electron beam irradiation in a Mg-9.8 wt.％Sn alloy.This phase is spherical in three dimensions,having a DO19 structure with the lattice param-eters of a=0.642 nm,c=0.521 nm and space group of P63/mmc.Its chemical formula is Mg3Sn,like the β\'metastable precipitate phase.The orientation relationship between the β”phase and the α-Mg matrix is such that[2(1)(1)0]β“//[2(1)(1)0]α and(0001)β”//(0001)α.Its formation involves solely the ordering of Sn atoms in the solid solution magnesium matrix.First-principles calculations indicate that the formation of the β\"phase is energetically favored.