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

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.

Ionic N-B-N- and B-N-B-substituted benzene analogues: a theoretical analysis.

Calculations are presented on six-π-electron N-B-N- and B-N-B-substituted benzene rings, [C(3)BN(2)H(6)](+) and [C(3)NB(2)H(6)](-), and their isomers. These compounds display a wide range of thermodynamic stability in those molecules, with N-B-N connectivity favored strongly in the cation, B-N-B in the anion. That stability order is easily understood using the charge distribution in a benzene polarized by heteroatom substitutions or the underlying allyl anion and cation. Deprotonation at N in [C(3)BN(2)H(6)](+) leads to a set of BN-substituted pyridines. The calculations predicted three B-N-substituted pyridines clearly more stable thermodynamically than those synthesized so far. The order of stability of the B-N-B-substituted benzenoid systems, which are as yet not well known experimentally, shows similar features. We investigated in a preliminary way the reactivity and potential stabilization by substitution of the energetically most stable structures and by examining possible escape routes by dimerization. Our study suggests new N-B-N and B-N-B molecules that could be made.