On the Chemical Origin of the Gap Bowing in (GaAs)1−xGe2x Alloys: A Combined DFT–QSGW Study

Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1−xGe2x series of alloys. We have investigated the structural properties of some (GaAs)1−xGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the order–disorder phase transition

Ab initio MO study of diverse Si<SUB>3</SUB>H<SUB>3</SUB><SUP>+</SUP> isomers

Structures and energies of many Si3H3+ isomers were investigated theoretically at the MP2/6-31G&#8727; level. The global minimum was the classical aromatic planar D3h structure (5). Isodesmic equations indicate the resonance stabilization energy to be half that of the analogous cyclopropenyl cation. The next lowest energy minimum, with a divalent silicon and a bridging hydrogen, also exhibits the 2&#960; aromaticity. Five planar Si3H3+ isomers display cyclic three-center-two-electron (3c-2e) delocalization, and eight minima have 3c-2e Si-H-Si bridged bonds. The planar tetracoordinated silicon and five-coordinated silicon also are represented. Eleven other minima were found within a 46 kcal/mol range. An H-bridged C3v structure, derived from B3H6+, is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb these A3H3+ forms are more stable than the classical structures (5, J. Am. Chem. Soc. 1995, 117, 11361). In contrast to Si3H3+, C3H3+ has only four isomers in the 189 kcal/mol range. The silicon analogues of the C3H3+ acyclic structures, the prop-2-en-1-yl-3-ylidene cation and the 1-propynyl cation, are not favorable

Exotic Structures of Si<SUB>2</SUB>B<SUB>2</SUB>H<SUB>4</SUB>

The potential energy surface of Si2B2H4, explored ab initio at the MP2/6-31G and density functional theory, Becke3LYP/6-311+G&#8727;&#8727; levels, is rich in detail. Twelve minima were found out of the 39 stationary points located. 1,3-Disiladiboretene (1a), with a puckered four-membered ring, is predicted to be the global minimum. Its calculated inversion barrier (1.2 kcal/mol), however, is much lower than that of the analogous 1,3-diboretene (17.0 kcal/mol). The relative energies of two other Si2B2H4 isomers (5 and 7) with B-H-Si bridges are only 2.0 kcal/mol above 1a. The relative energies of 9 and 15a, both with planar pentacoordinate borons, are 7.3 and 27.1 kcal/mol. The acyclic (18) and linear (22a,b) isomers are noncompetitive energetically. Many other Si2B2H4 silaboranes, possessing similar structures and bonding as the corresponding carboranes, are calculated to be saddle points. In general, hydrogen bridging and cyclic &#960;-delocalization stabilize Si2B2H4 isomers, while structures with multiply bonded silicon are unfavorable

Group 14 analogs of the cyclopropenium ion: do they favor classical aromatic structures?

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