Silicon monoxide at 1 atm and elevated pressures: crystalline or amorphous?

Area of Science:

• Materials Science
• Computational Chemistry
• Solid State Physics

Background:

• Silicon monoxide (SiO) is anomalous due to its lack of a crystalline phase at ambient conditions.
• Group 14 monoxides exhibit varying stabilities and disproportionation behaviors.

Purpose of the Study:

• To theoretically explore ordered ground-state and amorphous structures of SiO at 1 atm.
• To investigate crystalline SiO phases under high pressures (up to 200 GPa).
• To compute the heat of formation and disproportionation of crystalline SiO.

Main Methods:

• Theoretical calculations of ground-state and amorphous structures at 1 atm.
• High-pressure phase exploration up to 200 GPa.
• Thermodynamic calculations for heat of formation and disproportionation.

Main Results:

• Several competitive ground-state structures for SiO at 1 atm were identified, featuring Si-Si bonds and Si-O-Si bridges.
• Amorphous SiO model showed no segregation into Si and SiO2 regions.
• New crystalline structures evolved at high pressures, including Si triangular nets/strips and stishovite-like regions.
• Crystalline SiO has the most negative heat of formation among group 14 monoxides.
• The disproportionation of SiO to Si and SiO2 is exothermic, consistent with the group 14 monoxide trend.
• High-pressure SiO phases were found to be metallic.

Conclusions:

• SiO exhibits complex structures at ambient and high pressures, challenging its anomalous status.
• Despite a negative heat of formation, SiO is unstable with respect to disproportionation into Si and SiO2.
• Pressure-induced metallicity in SiO phases is a significant finding.