Jozef Bicerano

X̃ 1A1, ã 3B1, and à 1B1 electronic state of silylenes. Structures and vibrational frequencies of SiH2, and SiHF, and SiF2

Abstract The spectroscopic characteristics of the three lowest-lying electronic states of SiH 2 , SiHF, and SiF 2 have been predicted via a priori quantum-mechanical methods. Where experimental results are available, the theoretical predictions usually agree well. Specifically Raos identification of the 3 B 1 state of SiF 2 is confirmed here. Also Milligan and Jacoxs very tentative assignment of an absorption at 2032 cm −1 to ν 1 (SiH 2 ) is shown to be reasonable. Many spectroscopic features have been predicted for which there are no experimental observations at present.

Chemical bond approach to glass structure

Abstract The importance of fundamental chemical bonding considerations, based on an examination of the chemical bonds expected to be present, in the determination of the structures and properties of a wide variety of glasses, is discussed. The average number of near neighbors of each type expected to surround a given central atom in an alloy glass, can often be estimated by using simple considerations based on chemical ordering, coordination numbers and bond energies. The bonding patterns thus obtained provide insight into the average local order present in these glasses, as well as enabling the explanation and prediction of various physical, chemical and electrical properties. Several examples are given to illustrate these ideas.

The infrared spectrum of silaethylene

The harmonic vibrational frequencies and infrared intensities of the H2Si=CH2 molecule have been predicted by ab initio molecular quantum mechanics. Self‐consistent‐field (SCF) and configuration interaction (CI) methods have been used in conjunction with a double zeta plus polarization (DZ+P) basis. Comparison with pertinent experimental data is made.

Ab initio calculations on (SiH3)2F+: stability in the gas phase and model for bridging fluorine atom in ion-implanted amorphous silicon

Abstract Ab initio calculations have been performed on model molecular clusters simulating bridging fluorine configurations in fluorinated amorphous silicon. Optimized geometries, total energies and vibrational frequencies have been computed for (SiH 3 ) 2 F + clusters with the terminal SiH 3 groups eclipsed or staggered. The stable minimum on the potential energy surface corresponds to a linear, but very flexible, Si-F-Si bridging configuration. (SiH 3 ) 2 F + appears to be stable with respect to unimolecular decomposition. The calculated vibrational frequencies include a strongly infrared-active antisymmetric stretch mode at 740 cm −1 , similar to the metastable “ B band” experimentally observed at 750 cm −1 in the ion-implanted samples. These results are compared with calculated geometries and vibrational frequencies of SiH 3 F, SiH 3 F + , SiH 2 F + and Si 2 H 5 F.

SiLiF: The competition between electronic effects favoring singlet and triplet ground states. A case study

Electronegative substituents such as F favor closed‐shell singlet, while electropositive substituents such as Li favor triplet ground electronic states in carbenes and silylenes. Therefore, SiLiF presents a very instructive case study of the competing electronic effects involved. We have examined the minima on the lowest‐lying 1A’, 1A‘, and 3A‘ potential surfaces of SiLiF, using self‐consistent‐field and configuration interaction methods with a basis set at the double zeta plus polarization level. Two minima, one with a wide and the other with a narrow bond angle θ (LiSiF), have been found for each of the three electronic states, in analogy with SiHLi. The most stable minimum at all levels of approximation is the narrow‐angle triplet (3A‘ I). It lies below the second most stable minimum, the narrow‐angle closed‐shell singlet (1A’ I), by 10.5 kcal/mol at the CISD level of theory. This result is similar to the singlet–triplet separation in SiHLi at the same level of theory (10.7 kcal/mol), demonstrating tha...

Chemical Bonding and the Nature of Glass Structure

The use of fundamental chemical bonding considerations in the determination of the structures and properties of a wide variety of glasses with predominantly covalent bonding is reviewed. Simple considerations based on chemical ordering, coordination numbers and bond energies, can often be used to obtain information concerning the nature of the average local order around any given type of central atom in such an alloy glass. This information enables the explanation and prediction of various physical, chemical and electrical properties. Examples of these procedures are given by examining some chalcogenide glasses of interest due to their reversible threshold switching (Si18Ge7As35Te40) and memory switching (Ge15Te81Sb2S2 and Ge24Te72Sb2S2) properties.

Theoretical studies of hydrogen storage in binary Ti-Ni, Ti-Cu, and Ti-Fe alloys

Theoretical studies have been carried out to examine hydrogen storage in some binary transition metal alloys which include titanium as one of the alloying elements. Quantum mechanical calculations at the Extended Hückel level of approximation have been performed on numerous clusters of compositions Ti18Ni18, Ti18Ni18H, Ti18Ni18H12, Ti24Ni12, Ti24Ni12H, Ti24Ni12Hi12, Ti16Cu16, Ti16Cu16H, Ti24Cu2, Ti16Fe16, Ti16Fe16H9, and Ti16Fe16H32, to yield information on energetics, densities of states, charge distributions, and the effects of hydrogenation on these properties. In addition, ab initio calculations at the split valence level of approximation have been performed on several smaller clusters. The hydrogens have been shown to acquire a partially anionic character in all cases. Another conclusion is that the preference of H for certain types of sites (for example the tetrahedral Ti4 sites in crystalline TiCu) is more likely to be related not to the intrinsically greater stability of a hydrogen atom located in such a site, but to more general topological and electronic considerations. Qualitative concepts related to the classification, spatial distribution, and sizes and shapes of “hole” sites which could become occupied by hydrogen atoms, have been shown to correlate with hydrogen storage capacity for crystalline materials. These qualitative concepts have been extended to amorphous materials and corroborate the observations that under optimized conditions amorphous alloys can be found with better reversible hydrogen storage properties than the crystalline or microcrystalline systems. Distorted tetrahedral and octahedral holes have been examined in detail, and parameters (volume, area, “tetrahedrality”, and “octahedrality”) have been introduced to describe their sizes and shapes. An algorithmic surveying technique has been introduced, and shown to provide useful information about the limiting amounts of hydrogen uptake.

119Sn electric field gradients in model clusters of chalcogenide glasses

Calculations of 119Sn electric field gradients (EFG) have been performed using the Extended Hückel approximation on characteristic molecular clusters simulating possible types of sites in chalcogenide glasses. The motivation for these calculations derives from theoretical concepts on varying near neighbor relationships in these types of glasses, and from recent 119Sn Mössbauer experiments on Sn-doped Gex(Se or S)1−x bulk glasses which reveal three types (A, B and C) of chemically inequivalent sites, with distinct values and composition dependences for their isomer shifts and quadrupole splittings. The model clusters chosen for the calculations were the ethane-like (Ge2Se3)n quasi-one-dimensional chains of varying lengths which have been proposed as possible sources of the B site. In addition, calculations were also carried out on several additional types of clusters, in order to help in interpreting the results for the chains. We find that the magnitude of the quadrupole splitting in isolated linear ethane-like chains is very small, and almost independent of the particular site along the chain at which Sn replaces Ge. It therefore seems unlikely that such isolated linear clusters would be the source of the B sites. These sites are more likely to be related to distortions of the ethane-like clusters into non-linear configurations, as well as interactions with neighboring clusters, as forced by the constraints of the packing in the structure of the glass.