Phase transformation in magnesium hydride induced by ball milling

The synthesis of magnesium hydride by reactive ball milling leads to the formation of a metastable orthorhombic gamma-MgH2 phase along with tetragonal beta-MgH2. The structures of the beta-MgD2 and gamma-MgD2 phases of ball milled nanocrystalline MgD2 were studied by neutron powder diffraction. The ball-milled beta-MgD2 and gamma-MgD2 structures are distorted compared to the same phases synthesized by high-pressure and high temperature. Two Mg-D bond lengths are significantly distorted in beta-MgD2 while in the gamma-MgD2 phase, only one bond length is largely affected. The micro-stress associated with the strain fields and deformations caused by the ball milling is most likely the driving force of this phase transformation.