Silicon
Monoxide at 1 atm and Elevated Pressures:
Crystalline or Amorphous?
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Abstract
The
absence of a crystalline SiO phase under ordinary conditions
is an anomaly in the sequence of group 14 monoxides. We explore theoretically
ordered ground-state and amorphous structures for SiO at <i>P</i> = 1 atm, and crystalline phases also at pressures up to 200 GPa.
Several competitive ground-state <i>P</i> = 1 atm structures
are found, perforce with Si–Si bonds, and possessing Si–O–Si
bridges similar to those in silica (SiO<sub>2</sub>) polymorphs. The
most stable of these static structures is enthalpically just a little
more stable than a calculated random bond model of amorphous SiO.
In that model we find no segregation into regions of amorphous Si
and amorphous SiO<sub>2</sub>. The <i>P</i> = 1 atm structures
are all semiconducting. As the pressure is increased, intriguing new
crystalline structures evolve, incorporating Si triangular nets or
strips and stishovite-like regions. A heat of formation of crystalline
SiO is computed; it is found to be the most negative of all the group
14 monoxides. Yet, given the stability of SiO<sub>2</sub>, the disproportionation
2SiO<sub>(s)</sub> → Si<sub>(s)</sub>+SiO<sub>2(s)</sub> is
exothermic, falling right into the series of group 14 monoxides, and
ranging from a highly negative Δ<i>H</i> of disproportionation
for CO to highly positive for PbO. There is no major change in the
heat of disproportionation with pressure, i.e., no range of stability
of SiO with respect to SiO<sub>2</sub>. The high-pressure SiO phases
are metallic