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Structure, Phonons, and Defects
Published in Yongqing Cai, Gang Zhang, Yong-Wei Zhang, Phosphorene, 2019
Yongqing Cai, Gang Zhang, Yong-Wei Zhang
However, Karttunen et al.26 attempted to improve the stability for phosphorus fullerene by constructing three different types phosphorus fullerenes with tetrahedral, octahedral, and icosahedral structures (Fig. 3.5b–d). Starting from creating triangular faces cut from phosphorene, each polyhedral fullerene can be uniquely defined by a lattice index (h, k). The tetrahedral structures are made of 4 triangular faces, and 4 vertices are formed from joining each of the 3 faces. The octahedral structures consist of 8 triangular faces, with 4 faces joining together at every vertex. The icosahedral fullerenes are composed of 20 triangular faces with 5 faces joining together at every vertex. These fullerenes consist of triangular faces with puckered hexagons and vertices with phosphorus triangles, squares, and hexagons. There are great flexibilities in these structures due to the puckered structure and fullerenes with larger sizes become increasingly stable, approaching phosphorene. Among the above three families of fullerene nanostructures, the tetrahedral structures are the most thermodynamically favorable, followed by octahedral andicosahedral structures (Fig. 3.5f). It is expected that by minimizing the strain arising, the vertices and edges adjoining the triangular faces could lead to potential phosphorous fullerenes. Via DFT and ab initio MP2 calculations, the structural stability of these structures, with respect to dissociation into P4 molecules, is found to increase as a function of their sizes.26 These phosphorus fullerenes are found to be semiconductors with the HOMO–LUMO gaps between 1.6 and 2.9 eV for icosahedral structures,27 comparable to the HOMO–LUMO gap (∼1.7eV) in carbon fullerenes C60.28
Current Interruptions in AC Networks
Published in J.C. Das, Power Analysis Handbook: Short-Circuits in AC and DC Systems, 2017
Molecule of SF6 is octahedral with six fluorine atoms arranged symmetrically around sulfur atom. The molecular shapes can be explained in terms of mutual repulsion of covalent bonds. When two atoms are bonded by covalent bonds, both of them share a pair of electrons. The attraction that one of the atoms exerts on this shared pair of electrons is called electronegativity. Atoms with nearly filled shells of electrons (halogens) tend to have higher electronegativity.
The structure of of cuprate superconductors
Published in J. R. Waldram, Superconductivity of Metals and Cuprates, 2017
The ideal perovskite structure ABX3 is shown in Figure 12.1(a). This structure is cubic. The anion X (typically oxygen) and the cation A (typically Sr or Ba in the case of cuprate superconductors) have relatively large ionic radii and are in contact; they determine the size of the structure. The B cation (Cu for the cuprates) is smaller and occupies some of the interstices of the A–X network. It is coordinated by six anions, forming an octahedron.
Studies on stability and structural aspects of hydrazide-based hypercoordinate silicon(IV) complexes
Published in Journal of Coordination Chemistry, 2020
Pothini Suman, Mohsin Y. Lone, Sannapaneni Janardan, Prakash C. Jha, Akella Sivaramakrishna, Hadley S. Clayton
Silicon displays the ability to form more bonds than the usual four in the presence of donor molecules or ions i.e. formation of five-, six- and even seven-coordinate silicon species for fulfilling the octet rule [1]. Out of two main special theories formulated to explain this behavior, the first one deals with the participation of 3d orbitals of silicon in the expansion of the coordination sphere (Scheme 1) [2]. As a result, this invokes a sp3d hybridization (with trigonal–bipyramidal geometry) with the formation of five-coordinate species, while the hybridization would be sp3d2 (e.g. [SiF6]2− with octahedral geometry and six 2c–2e bonds to the fluorines) in the six-coordinate species. The reduced s-character of the silicon orbitals in the hypercoordinate species can explain their increased Lewis acidity and the transfer of electron density to the ligands. However, quantum chemical calculations suggest that 3d-orbital participation is negligible due to the large energy difference between the relevant p and d-orbitals [3].