Bond Orbital Approximation Research Articles

The Magnetic Susceptibilities of Some Liquid Chalcogenides

AbstractThe magnetic susceptibilities for seven binary alloy systems (SnTe, SnSe, InTe, InSe, AgTe, AgSe and AgS) are measured as a funtion of temperature and composition. The diamagnetic susceptibilities of these systems fall rapidly on melting except for the InSe system, and decrease gradually with increasing temperature in the liquid state. The change of magnetic susceptibility at the melting point is considerably large in SnTe and SnSe, while rather small in Ag2X (X: Te, Se, and S). Diamagnetic maxima appeared around the stoichiometric composition (SnTe, SnSe, In2Te3, In2Se3, and Ag2X) and correspond well to minima in the electrical conductivity reported previously. These data suggest that electrons tend to localize at these compositions. Relationships between the degree of electron localization and the chemical bonding are discussed using the bond orbital approximation for the diamagnetism of semiconductors. The result indicates that the electron localization is stongly influenced by the bonding nature of anions.

Surface states on unrelaxed GaAs (110) in the bond-orbital model

We have used the bond-orbital model of Pantelides and Harrison to compute eigenstates for a twelve layer slab of GaAs (110). Dispersion for each of the states is presented as a function of surface wave vector. Also, probability distributions for electrons in each of the surface states are given. These results are in good agreement with those of Joannopoulos and Cohen who did not make the bond-orbital approximation, thus helping to establish the usefulness of this method for surface calculations.

The magnetic susceptibility of antimony and bismuth chalcogenides in the liquid state

AbstractThe magnetic susceptibilities of liquid SbTe, BiTe, SbSe, and BiSe systems are measured by the Faraday method using a torsion balance. The decrease in diamagnetic susceptibility at the melting point is relatively large in liquid Bi2Te3 and Sb2Te3. On the other hand, it is considerably small in liquid Sb2Se3. This corresponds to the temperature change in the electrical conductivity reported previously. The value of susceptibility for liquid SbTe alloys is considerably different from the data measured by a previous author. It is clarified from the composition dependence of susceptibility that both liquid Te systems behave like metals rather than semiconductors in a wide range of composition. The composition dependence of susceptibilities in the liquid Se systems showes that in the Se‐rich region the short‐range bonding remains unchanged when melting, and these alloys take rapidly a metallic character with increasing metal content beyond the composition of Sb2Se3 or Bi2Se3. These experimental results are discussed using the bond orbital approximation.

Surface reconstruction on semiconductors

Reconstruction of semiconductor surfaces is considered in a tight-binding framework using the bond orbital approximation to simplify the total energy calculation. A Jahn-Teller type of distortion, based upon a dehybridization of the dangling hybrids, is found to dominate and to lead to distortions larger than those customarily proposed at surfaces. This result, together with experimental knowledge of the surfaces, leads to specific forms for the reconstruction expected on the principal symmetry surfaces of both polar and nonpolar semiconductors. The forms found for (111) and (110) silicons surfaces are among forms which have been proposed earlier, but a canted ridge structure for the (100) surface is new. The reconstruction is large enough to drop the occupied dangling hybrid state deep into the valence band, suggesting that observed structure near the top of the valence band is due to back bonds, in agreement with Pandey and Phillips. The reconstruction also tends to increase the work function above the unreconstructed value on all surfaces. The effect of adsorption of atoms on the reconstruction as a function of the valence of the adsorbate, is summarized. It is found that while the energy to form a vacancy on a silicon (111) surface is very large, silicon atoms outside the nominal surface should be quite stable. This suggests that the 7 × 7 reconstruction is a pattern of add-atoms rather than (and topologically unrelated to) a pattern of vacancies.

Electronic structure, spectra, and properties of 4:2-coordinated materials. I. Crystalline and amorphousSiO2andGeO2

A two-parameter tight-binding theory of the electronic structure of 4:2-coordinated materials is proposed. The parameters, a covalent and a polar energy, are fitted to the optical absorption spectra. The valence energy bands and density of states are calculated. In terms of these a consistent interpretation of all the observed photoemission and x-ray-emission spectra of ${\mathrm{SiO}}_{2}$ is obtained. The x-ray-absorption spectra are also analyzed. A bond-orbital approximation allows a simple calculation of the refractive index (or dielectric constant) of the various allotropic forms of silica and germania. Finally, the variation in total energy and charge distribution with local distortion is analyzed in order to study structural stability, elastic rigidity, and the effective charges (including dynamic contributions) which determine the piezoelectric constants and infrared absorption intensities.

Bond orbital analysis of the hydrogen bond in the linear water dimer

The hydrogen bond energy of the isolated linear water dimer has been investigated with the use of the bond orbital approximation. This approach provides a simple description of the relative contributions from electrostatic, overlap repulsion, and charge transfer effects. The basis set used is a minimum basis of Slater atomic orbitals. The hydrogen bond energy obtained is in reasonable agreement with the values obtained from SCF calculations.

The Bond Orbital Approximation for π-electron System in Benzene

The electronic energies of π-electron system in benzene molecule have been calculated by the bond orbital (BO) method for singlet states. Configuration interactions (CI) have not been taken into account. All the levels other than 1 B 2 u and 1 E 2 g calculated by the present method are higher than the levels calculated by the MO method without CI. In the MO method, however, the order of levels is changed and coincides with the observed order, by including CI. In the BO method the order of levels would not be changed by including CI, since the BO method without CI already gives the correct order.