To explore the conversion mechanism, we performed more indepth
analyzes on the evolution of the constituent phases during
our new magnesiothermic reduction process. A rapid cooling
system enabled the fast quenching of the samples at controlled
reaction times so that precise phase analysis is possible. The
high-angle X-ray diffraction (XRD) analysis results shown in
Figure 3 a indicate that Mg 2 Si and MgO were formed by the
initial 10-min-long magnesiothermic reduction process. Mg 2 Si
is an undesirable byproduct because it should be removed by
the fi nal acid-washing process, leading to an eventual loss of Si.
However, further increasing of the reduction time signifi cantly
decreased the fraction of Mg 2 Si and SiO 2 , while the amounts
of Si and MgO increased with time. According to the Rietveld
analysis of the XRD patterns in Figure 3 b and Supporting
Information Figure S5, the mass fractions of SiO 2 and Mg 2 Si
in the fi nal product (25 min of reduction) fell to ≈1.18% and
≈0.11%, respectively, and Si and MgO made up 98.71% of the
mass of the fi nal product, which demonstrates an unprecedentedly
high conversion yields with an extremely small loss of Si,
in contrast to the case of atmospheric-pressure process shown
in Supporting Information Figure S6. These results show that
Mg 2 Si is a transient intermediate phase that is dominantly
formed during the initial stage of the conversion process and
almost completely disappears in the later stages. To the best of
our knowledge, this phenomenon has not been reported in previous
studies