Molecular Cluster Model Research Articles

A molecular cluster model of the electronic structure of IV and III-V covalent semiconductors: Application to GaAs

A molecular cluster model of the tetrahedrally coordinated semiconductors is discussed within the framework of the self-consistent-field multiple-scattering method. A method to avoid the dangling bond effects is discussed and the model is particularly investigated with respect to gallium arsenide. It is shown that the model leads to a fairly consistent description of the bulk electronic structure.

Calculation of angular dependence of photoemission for the Al(100) + O system using a simple molecular orbital cluster model

The angular dependence of photoemission from oxygen chemisorbed on the (100) face of aluminum is calculated using a molecular cluster model. The cluster contains five aluminum atoms with one oxygen atom located in the four-fold site; two A1O distances are considered. The calculations employed the Xα scattered wave formalism and are the first results to be obtained for a chemisorption problem in which both initial and final states are based on a cluster model.

Molecular cluster model of covalent semiconductors

A multiple-scattering-cluster model of IV and III-V covalent semiconductors is proposed. The feasibility of the model is tested by carrying out calculations on GaAs. The valence band width, the energy band gap and the location of the cationic and anionic d levels obtained are in fairly good agreement with the experimental results for the bulk crystal.

Molecular cluster studies of binary alloys: LiAl

The electronic structure of the ordered Zintl ($B32$) phase of LiAl has been studied in a molecular-cluster model within the framework of the Hartree-Fock-Slater theory. ${\mathrm{Li}}_{5}$${\mathrm{Al}}_{4}$ and ${\mathrm{Al}}_{5}$${\mathrm{Li}}_{4}$ clusters were embedded in a potential field representative of the alloy environment; energy levels and wave functions were obtained by self-consistent iteration. Density-of-states and charge-density results are used to interpret NMR and electrical-conductivity studies. The case of a single Li vacancy was also treated, and discussed in the light of positronannihilation data.

The neutral vacancy in silicon and diamond: Generalized valence bond studies

Self-consistent ab initio generalized valence bond and configuration interaction calculations are presented for molecular cluster models of the electronic states of the neutral vacancy in diamond and silicon. In each case the ground state of the vacancy is found to be 1E (T d designation) with the 3T 1 and 5A 2 states slightly higher (0.18 eV and 0.60 eV for Si and 0.33 eV and 1.2 eV for C). The first excited singlest state is 1T 2 in each case. The 1E- 1T 2 excitation energy is foundto be 1.7 ± 0.7 eV for diamond and 1.3 ± 0.5 eV in silicon, the range being due to uncertainty in the model used for estimating the polarization effects due to the remainder of the infinite crystal.

Molecular cluster theory of CO chemisorption on a nickel(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of carbon monoxide on a (100) transition metal surface are presented. Energy levels and charge distribution for the CO: Ni 5 cluster in C 4v symmetry are obtained, and the variation of binding energies with height of the CO molecule above the surface of nickel is studied in detail. Comparison is made with experimental binding energy spectra and with the multiple-scattering results of Batra and Bagus. The redistribution in energy of free-atom valence levels is studied by means of local-densities-of-states diagrams.

Dynamic Jahn-Teller effect forCr2+in MgO: Acoustic paramagnetic resonance

An empirical evaluation of the parameters defining the dynamic Jahn-Teller effect for ${\mathrm{Cr}}^{2+}$ in MgO is made using the temperature dependence of the acoustic-paramagnetic-resonance (APR) spectra. The molecular-cluster model for the tightly bound ${\mathrm{Cr}}^{2+}$ and its nearest neighbours is applied to the experimental results to determine the tunneling splitting and spin-orbit splitting. The magnitude of the tunneling splitting ($\ensuremath{\delta}\ensuremath{\sim}19$ ${\mathrm{cm}}^{\ensuremath{-}1}$) and spin-orbit splitting ($D\ensuremath{\sim}2.3$ ${\mathrm{cm}}^{\ensuremath{-}1}$) indicates the ${\mathrm{Cr}}^{2+}$ impurity is in a more extreme dynamic limit of the Jahn-Teller effect than previously assumed ($\frac{\ensuremath{\delta}}{D}\ensuremath{\sim}8$). The gyromagnetic ratio for the paramagnetic triplet levels of ${\mathrm{Cr}}^{2+}$ ions exhibits a significant anisotropy resulting from local distortions of the cubic site due to internal strains. The anisotropy depends on the acoustic mode used to observe the spectra. For longitudinal waves the observed anisotropy of the $g$ factor can be explained as due to induced transitions in ions in a primarily tetragonal environment. The magnitude of the internal strain is found to be approximately ${10}^{\ensuremath{-}5}$. Only doublet transitions are observed for the triplet even though the strain splitting (singlet-doublet) is found to be approximately 0.2 ${\mathrm{cm}}^{\ensuremath{-}1}$ which would allow $\ensuremath{\Delta}m=1$ transitions for the acoustic frequencies used in this investigation. The slow transverse mode couples more strongly to the chromous ion and exhibits an APR spectrum which is more complex than the longitudinal wave under similar conditions. The anisotropy of the $g$ factor is due to the same sets of ions as the longitudinal mode but the magnitude of the APR peaks depends on the direction of the magnetic field in the crystal.

Molecular Cluster Approach to Magnesium and Calcium Oxide Crystals III. Charge Distribution Analysis of Some Hole Centres

AbstractThe molecular cluster model is applied to the analysis of charge distribution and of symmetry of some hole centres (V−, V0, Li0, Na0), and also to impurity atoms of aluminium and to nitrogen ions in MgO. The calculations yield theoretical evidence of the models of hole centres derived from experimental data. Besides the essential effect of charge redistribution in the vicinity of impurity it is shown, e.g., the aluminium ion may be considered as triply charged only accounting for the charge of its nearest environment.

Molecular cluster theory for chemisorption of first row atoms on nickel (100) surfaces

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of first row atoms on Ni(100) surfaces are presented. Energy levels and ground state charge distributions for XNi S clusters with the adatom X = H, C, N, O located in C 4 V symmetry at a fixed height of h = 2.0 au above the surface are given. The variation of properties with h was studied in detail for the case of oxygen. Theoretical results compare rather well with experimental photoelectron and energy loss data. Local-densities-of-states diagrams are used to clarify the interaction between adsorbate levels and metal conduction bands.

Comparison of the molecular cluster model with the phonon model for Jahn—Teller active impurities in crystals

For comparing the molecular cluster model with the phonon model in describing the Jahn-Teller interaction for magnetic impurities in crystals, the Jahn-Teller energy and the Ham reduction factors are expressed in terms of the phonon Green's function of the host crystal for an orbital triplet coupled to E g modes of vibrations. Numerical results for the case of MgO lattice have been obtained using the phonon Green's function derived from the breathing shell model.

Theory of Electron Paramagnetic Resonance of Trivalent Iron Ions in II-VI Tetrahedral Crystals

Theory of g-shift and D tensor of Fe3+ ions incorporated into II-VI tetrahedral crystals is developed and compared with experiment. Electron-transfer mechanism based on molecular cluster model is shown to be adequate for the interpretation of positive g-shift and D tensor.

Anisotropy of the g Tensor of Fe3+ in Zinc Oxide

ESR measurements of Fe 3+ in ZnO were used to determine the g shift and to compare the data with a theory proposed by Watanabe. The results agree very well with the assumed molecular cluster model.