Orbital antibonding band

The structure of CaB contains bonding bands typical of the boron sublattice and capable of accommodating 20 electrons per CaB formula, and separated from antibonding bands by a relatively narrow gap (from 1.5 to 4.4 eV) . The B atoms of the B(, octahedron yield only 18 electrons thus a transfer of two electrons from the metal to the boron sublattice is necessary to stabilize the crystalline framework. The semiconducting properties of M B phases (M = Ca, Sr ", Ba, Eu, Yb ) and the metallic ones of M B or M B5 phases (Y, La, Ce, Pr, Nd ", Gd , Tb , Dy and Th ) are directly explained by this model . The validity of these models may be questionable because of the existence and stability of Na,Ba, Bft solid solutions and of KB, since they prove that the CaB -type structure is still stable when the electron contribution of the inserted atom is less than two . A detailed description of physical properties of hexaborides involves not only the bonding and antibonding B bands, but also bonds originating in the atomic orbitals of the inserted metal . ... 

Fig. 2-12. Electron energy band formation of silicon crystals from atomic frontier orbitals number of silicon atoms in crystal r = distance between atoms rg = stable atom-atom distance in crystals, sp B8 = bonding band (valence band) of sp hybrid orbitals sp ABB = antibonding band (conduction band) of sp hybrid orbitals.

Most metal oxides are ionic crystals and belong to either the class of semiconductors or insulators, in which the valence band mainly comprises the frontier orbitals of oxide ions and the conduction band contains the frontier orbitals of metal ions. In forming an ionic metal oxide ciTstal from metal ions and oxide ions, as shown in Fig. 2-21, the crystalline field shifts the frontier electron level of metal ions to higher energies to form an antibonding band (the conduction... 

The different shift mechanisms may be understood in more detail by considering the effect of the magnetic field on the populations and energies of the different crystal orbitals (Figure 7a). Transfer of electron density via the 90° interaction arises due to a direct delocalization of spin density due to overlap between the half-filled tzg. oxygen jt, and empty Li 2s atomic orbitals (the delocalization mechanism. Figure 7b).This overlap is responsible for the formation of the tzg (antibonding) molecular orbital in a molecule or the tzg crystal orbital (or band) in a solid. No shift occurs for the 180° interaction from this mechanism as the eg orbitals are empty. 

As shown in Figure 5.6, the dz2 and pz orbitals can hybridize to form a o-bonding band and a o-antibonding band. The dxz and dyz orbitals hybridize with the pA and pv orbitals to produce a jr-bonding band and a jt-antibonding band. The dxy and dxi y2 do not hybridize with any p-orbitals and so produce a metallic 6-band in the gap between the hybridized covalent orbitals. 

The material becomes semiconducting if electrons are injected into the unoccupied antibonding orbitals (conduction band) or are removed from the occupied bonding orbitals (valence band). At a surface, bonds between Si atoms are broken. Within the idealized hybridization scheme, the surface atom has a free sp% orbital... 

IP s can be useful in two ways. First, they give an idea of the availability of the halogen lone pairs for molecular associations. Second, they run parallel to the frequency of the lowest ultraviolet absorption band. The latter are, in these molecules, of the (C—type in which an electron is excited from a halogen lone pair orbital to an orbital antibonding in the C—Xbond [24] [9]. This frequency measures the height above the ground state of the lowest empty orbital available for charge transfer complex formation in which the fluorocarbon is the electron acceptor. 

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