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 P = 1 atm, and crystalline phases also at pressures up to 200 GPa. Several competitive ground-state P = 1 atm structures are found, perforce with Si-Si bonds, and possessing Si-O-Si bridges similar to those in silica (SiO2) 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 SiO2. The P = 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 SiO2, the disproportionation 2SiO (s) → Si(s)+SiO2(s) is exothermic, falling right into the series of group 14 monoxides, and ranging from a highly negative ΔH 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 SiO2. The high-pressure SiO phases are metallic. © 2014 American Chemical Society.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): CHE-0910623
Acknowledgements: We thank Saudi Aramco for supporting K.A. in pursuing his degree. Our work at Cornell was supported by NSF Research Grant CHE-0910623 and DMR-0907425. Work at U. Texas at Arlington was supported by NSF Research Grant DMR- 0907117. Computational facilities provided by KAUST (King Abdullah University of Science and Technology) Supercomputing Laboratory, EFree (an Energy Frontier Research Center funded by the Department of Energy, award no. DEC0001057 at Cornell), the XSEDE network (provided by the National Center for Supercomputer Applications through Grant TG-DMR060055N), the IIT Kanpur Computer Centre High Performance Computing facility, and Cornell’s NanoScale Facility (supported by the National Science Foundation through Grant ECS-0335765) are gratefully acknowledged. We thank Andreas Hermann for useful discussions.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.