Bonding and doping of simple icosahedral-boride semiconductors

A simple model of the bonding and doping of a series of icosahedral-boride insulators is presented. Icosahedral borides contain clusters of boron atoms that occupy the 12 vertices of icosahedra. This particular series of icosahedral borides share both the stoichiometry B₁₂X₂, where X denotes a group V element (P or As), and a common lattice structure. The inter-icosahedral bonding of these icosahedral borides is contrasted with that of B₁₂O₂ and with that of α-rhombohedral boron. Knowledge of the various types of inter-icosahedral bonding is used as a basis to address effects of inter-icosahedral atomic substitutions. The inter-icosahedral bonding is maintained when an atom of a group V element is replaced with an atom of a group IV element, thereby producing a p-type dopant. However, changes of inter-icosahedral bonding occur upon replacing an atom of a group V element with an atom of a group VI element or with a vacancy. As a result, these substitutions do not produce effective n-type dopants. Moreover, partial substitution of boron atoms for atoms of group V elements generally renders these materials p-type semiconductors.     

Influence of Ti, V, and Ni Dopants on the Electronic Structure and Chemical Bonding in Cubic Silicon Carbide

The electronic structure and chemical bond parameters were studied for 3C–SiC systems with Ti, V, and Ni dopants by the ab initio discrete variation method. Possible dopant (M) sites in crystal are taken into account. As dopant sites we consider the interstitial (M_i) and substitution (M_s) positions. Pair type defects are modeled: M_i–Si vacancy and M_s–Si_i defect.     

Exploring Electronic Properties of Si20-n H20-n P n Heterofullerenes (N = 1, 2, 5, and 10) Based on NMR and NBO Analysis: A DFT Study

Abstract

Density functional theory (DFT) calculations are performed to characterize Si_20-nH_20-nP_nheterofullerenes (n = 1, 2, 5, and 10), and to examine the stability of encapsulated X@Si_20-nH_20-nP_nwhere X = Li, Na, and K. To this aim, ²⁹Si, ³¹P, and ¹H chemical shielding (CS) tensors as well as natural charges are calculated for the optimized structures. The local structures around silicon as well as phosphorus nuclei are found to show a good correlation with CSs. However, the similar values of ¹H calculated CSs (26–28 ppm) obtained for all the heterofullerenes mean hydrogen atoms do not detect the local structure around the adjacent silicon and also do not distinguish between isomers of heterofullerenes and the number of P dopants. According to calculated endo-hedral inclusion energies (E_inc), formation of the Li@Si_20-nH_20-nP_ncomplexes, unlike the K@Si_20-nH_20-nP_nones, are exothermic while E_incof the Na@Si_20-nH_20-nP_ncomplexes strongly depends on the position and number of P dopants. Moreover, binding energies for the considered models are found to be in the order of Si_20-nH_20-nP_n> Li@Si_20-nH_20-nP_n> Na@Si_20-nH_20-nP_n> K@Si_20-nH_20-nP_nwhile strongly depending on the pattern of P dopants on the surface of the cage.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the following free supplemental files: Additional figures and tables]     

Transition metal-doped titanium(IV) dioxide: Characterisation and influence on photodegradation of poly(vinyl chloride)

The effects of transition metal dopants (V(IV), V(V), Mn(II), Cr(III), Mo(V), and W(V)), introduced into TiO₂, upon the rate of photodegradation of poly(vinyl chloride) (PVC) films containing TiO₂ have been measured. The rates were determined mainly by monitoring carbonyl group formation. In another set of experiments, the rates of chloride ion release from irradiated PVC particles suspended in water undergoing agitation with air or O₂ in the presence of particles of doped TiO₂ were measured electrochemically. The doping of TiO₂ (rutile) with Cr(III), V(V) or Mn(II) reduces the photoactivity of the pigment, while doping by Mo(V) or W(V) enhances its photoactivity; the results obtained from carbonyl index measurements are paralleled closely by those from chloride ion release. Even the most aggressive doped pigments were less reactive than Degussa P25 pigment, while the greatest protection to PVC film was offered by TiO₂ particles coated with Al₂O₃ or SiO₂. Overall, the photoactivity of doped TiO₂ is a complex function of dopant concentration, the energy levels of the dopants in the TiO₂ lattice, their d electronic configuration and their local distribution. Photoactivity is also linked to other factors such as crystal type, particle size distribution and surface area. There is a clear relationship between the tendency of the dopant to induce the rutile-to-anatase transition and its effect in enhancing the photoactivity of the pigment. The characterisation of the doped pigments was achieved using X-ray powder diffraction, EPR spectroscopy, diffuse reflectance UV-vis spectrophotometry, scanning optical and electron microscopy and particle size analysis using LALLS.     

Die Darstellung der Oligosilane Si8 X 18 (X-Cl,OCH3) durch Photolyse von Silylquecksilberverbindungen

The formation of SiSi-bonds by a photochemical reaction of silylmercury compounds is described. The silylmercury compounds [(X ₃Si)₃Si]₂Hg (X=Cl, OCH₃) were synthesized via theVyazankin Hydrid method with (X ₃Si)₃SiH and Bis(t-butyl)mercury. By UV-irradiation of these products in hexane as a solvent, the oligosilanes [(X ₃Si)₃Si]₂ are formed in good yields. All these compounds are charactericed by spectroscopical methods.

     

A Computational NICS and 13C NMR Characterization of C60−n Si n Heterofullerenes (n = 1, 2, 6, 12, 20, 24, 30)

Density functional theory (DFT) calculations are performed for a representative set of low-energy structures of C_60-n Si _n heterofullerenes (n = 1, 2, 6, 12, 20, 24, 30) to investigate the effect of silicon doping on the electron structure of fullerene. The results show that chemical shielding (CS) parameters are so sensitive to the structural distortion made by outwardly relaxing silicon doped atoms from the fullerene surface which results in puckered Si-doped rings. As a result, the chemical shifts of the nearest carbon sites of silicon atoms considerably shift to downfield. Our survey shows that those first neighbors of silicon atoms which have minor ¹³C chemical shift belong to normal (un-puckered) rings. Meanwhile, the chemical shielding anisotropy (Δσ) parameter detects the effects of dopant so that Δσ values of the carbon atoms which are contributed to the Si–C bond are mainly larger than the others. Compensation between diatropic and paratropic ring currents lead to less negative NICS values at cage centers of Si-doped fullerenes than that of C₆₀ except C₅₈Si₂-b and C₅₄Si₆-b in which more negative NICS values may be attributed to more spherical geometries of their carbon cages.     

Molecular and electronic structure of some binary polyhedral clusters

The molecular and electronic structures of some polyhedral alternate molecules X_nY_n, where n=12, 16; X=B, Al, Si; Y=N,P,C, and of homoatomic clusters C₂₄, Si₂₄, C₃₂, and Si₃₂ are calculated in a valence approximation by the MNDO method. It is suggested that the σ-frameworks of these molecules are formed of four- and six-membered rings, with each X atom having only Y atoms as neighbors. The singlet states of all these systems have local minima on the corresponding potential energy surfaces with T_h symmetry for n=12 and T_d symmetry for n=16. The main structural parameters, heats of formation, ionization potentials, and effective charge distributions are given. It is concluded that the X_nY_n heteroatomic clusters can exist when X and Y are atoms of Group III and V elements, respectively, or both are atoms of Group IV elements. A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp.976–982, November–December, 1995. Translated by L. Smolina     

Über die Stärke der Si—Si-bindung in Abhängigkeit vom Substituenten

The strength of the Si?Si-bond in disilanes varies with the substituents of silicon. It is shown, that the increase of the Si?Si stretching force constant inX ₃SiSiX ₃ parallels an increase of the SiH-stretching force constant in the correspondingX ₃SiH-compound. The coupling constantJ(²⁹SiH) alters in a similar manner. Exceptions are found in same disilanes with strong d π p π-interactions from the substituents.     

Site engineering in chemical-solution-deposited Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films using Ce,Zr, Mn and Si atoms

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were fabricated by depositing sol-gel solutions on Pt(111)/Ti/SiO₂/Si (100) substrates. Crystallographic orientations of the BLT films were random, but the preferred orientations along (00l) and (117) axes were found. All films showed a single-phase bismuth-layered structure but the orientation in the films could be engineered by optimizing the growth condition, as well as by introducing dopant atoms such as Ce and Zr in the films, which in turn influenced the ferroelectric properties of the films significantly. The shape of c-axis-oriented grains was more plate-like, while that of (117)-oriented grains was rod-like. Small % substitution of Ce, Mn, and Zr atoms at Ti site enhanced the remanent polarization by approximately 20%, while substitution of Si atoms reduced the remanent polarization in BLT films but improved insulating properties. It was also demonstrated that fatigue endurance could be controlled by the concentration of dopant atoms, which was thought to be due to the decrease in oxygen vacancy concentration.     

Theoretical study of isomerism of carbonand silicon-substituted aluminum clusters M<Subscript>6</Subscript>Al<Subscript>38</Subscript> and M<Subscript>12</Subscript>Al<Subscript>32</Subscript>

More than twenty M₆Al₃₈ isomers and several M₁₂Al₃₂ isomers of carbon- and silicon-substituted aluminum clusters with six and twelve dopant atoms of general formula M_nA_l44–n(M = C and Si, n = 6 and 12) have been studied by the density functional theory method. Calculations predict that, in the lowest-lying M₆Al₃₈, isomer, all substitutions of C atoms for Al are localized in one outer surface layer of the aluminum cage. In the course of optimization, the C atoms with a negative charge of about 1e are incorporated into positions of the intermediate layer to transform it into a 12-atom face composed of three adjacent vertex-sharing six-membered rings with short C–Al bonds. In the favorable isomer of M₆Al₃₈, the dopants are scattered as individual Si atoms located in both outer layers or in the subsurface space between the outer layers and the inner core of the cluster. Optimization of low-lying isomers with twelve starting substitutions of C and Si for Al in both outer layers has localized two preferable C₁₂Al₃₂ isomers. One of them contains three covalently bonded diatomic C₂ anions, which are combined through bridging aluminum atoms in the three-dimensional [C₆Al₇] cluster inside the severely distorted outer cage. In the second, most favorable, isomer, the dopants are distributed as isolated C anions; together with the bridging Al atoms, they form the [M₁₂Al₃₂] inner cage with an unusual dumbbell-like structure. For M₁₂Al₃₂, the aluminum cage undergoes moderate distortions. The silicon atoms remain in the outer layers and form five-membered ring subclusters [Si₅] and [Si₂Al₃] bound to the neighboring intermediate layers through elongated and weakened Si–Al bonds. Evaluation of the energies of the model exchange reactions Al₄₄ + M₆ → M₆Al₃₈ + Al₆ and Al₄₄ + 2M₆ → M₁₂Al₃₂ + 2A_l6 shows that for M= C both reaction are exothermic, whereas for M = Si the former reaction is nearly isothermal and the second reaction is endothermic and requires significant energy inputs. The differences between the equilibrium structures and the relative positions on the energy scale of the isomers of the C₆Al₃₈–Si₆Al₃₈ and C₁₂Al₃₈–Si₁₂Al₃₈ clusters are examined.     

A study on the stability,chirality, and theoretical spectra of the heterofullerenes C69X (X = N,P, As,B, Si,Ge)

There are two chiral and three achiral C₆₉X isomers possible. The achiral structures belong to the C_S group of symmetry while the chiral ones possess no symmetry elements. The stability of all five C₆₉X heterofullerenes for each of the X heteroatoms (X = N, P, As, B, Si, Ge) was determined at the (U)B3LYP/6-31G¹ level. The isomer population in an equilibrium mixture varies with the heteroatom type: the highest content of the chiral isomers was predicted for boron (92%) and nitrogen (47% plus 48%), while for the other heteroatoms studied the achiral isomers significantly dominate. For the chiral structures, four different sinister-rectus chirality measures, ^SRCM: pure geometrical, labeled, mass, and charge, were calculated by our CHIMEA program. We found that the labeled and mass chirality measures calculated for the two types of chiral isomers did correlate with each other. For each chiral C₆₉X molecule, spectroscopic VCD, IR, Raman, and NMR characteristics are discussed and demonstrated for their possible use for future identification and distinction of the isomers.     

Coordination Polyhedra OsXn(X = F, Cl, Br, I) in Crystal Structures

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to perform crystal-chemical analysis of compounds containing complexes [Os _a X _b ] ^z–(X = F, Cl, Br, I). Atoms of Os(V) at X = F and Cl, of Os(IV) at X = Cl, Br, and of Os(III) at X = Br were found to exhibit a coordination number of 6 with respect to the halogen atoms and to form OsX₆octahedra. The coordination polyhedra of Os(III) for X = Cl, I are square pyramids OsX₄. Each Os(III) atom forms one Os–Os bond; as a consequence, the OsBr₆octahedra share a face in forming Os₂Br^3– ₉complexes, while the OsX₄pyramids (X = Cl, I) dimerize to produce [X₄Os–OsX₄]^2–ions. The influence of the valence state of the Os atoms and of the nature of the halogen atoms on the composition and structure of the complexes formed and some characteristics of the coordination sphere of Os were considered.     

Bildung siliciumorganischer Verbindungen. 77. Zur Bildung von Carbosilanen aus Methylsilanen

Formation of Organosilicon Compounds. 77. Formation of Carbosilanes from Methylsilanes The products formed by pyrolysis of me₃SiH, me₂SiH₂, and meSiH₃ are reported. Sime₄ and the mentioned methylsilanes were reacted in a plasma, and the products are compared to those of the pyrolysis. The pyrolysis of me₃SiH and me₂SiH₂ essentially yields the same groups of carbosilanes which are accessible by thermal decomposition of Sime₄, if the range is restricted to compounds with 4 Si atoms at most. Cylic carbosilanes are the main products of the pyrolysis of me₃SiH, and amoung these, 1,3,5-trisilacyclohexanes and 1,3,5,7-tetrasilaadamantanes are preferrently formed. From me₂SiH₂ above all linear compounds as 1,3-disilapropanes are obtained. This is attributed to the chosen experimental procedure in which they not subject to further reaction. In the pyrolysis of meSiH₃ a yellow solid is formed besides little amounts of meH₂Si? SiH₂me. Compared to the compounds formed by pyrolysis of Sime₄, the carbosilasen obtained from me₃SiH and me₂SiH₂ possess more SiH substituents. Also the decomposition of Sime₄ in a plasma preferrently yields carbosilanes, mainly linear compounds with 2 or 3 Si atoms.     

The IrX6 Coordination Polyhedra (X = F,Cl, Br) in Crystal Structures

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to perform crystal-chemical analysis of compounds containing [Ir _a X _b ] ^z– complexes (X = F, Cl, or Br). The coordination number of Ir atoms with respect to halogen atoms was found to be 6, irrespective of the oxidation state (III, IV, or V), and the coordination polyhedra formed by Ir were found to be always octahedra. The influence of the site symmetry and the valence state of the Ir atoms on the distortion of the IrX₆ octahedra is considered. It is shown that characteristics of the VDP of Ir atoms can be used for quantitative estimation of the crystal-chemical role of Ir atoms in the halide structures.     

Theoretical Study of Isomers of Doped Clusters M<Subscript>2</Subscript>Al<Subscript>42</Subscript> with Light Element Dopants M inside and on the Surface of the Aluminum Cage

Structures, energies, and spectroscopic characteristics of the isomers of the family of doubly doped М₂Al₄₂ clusters with dopants M from the first two periods and H, Cu, and Zn atoms located in different positions at the surface and in the inner cavity of the aluminum cage have been calculated by the density functional theory method. The effect of the dopant nature on the relative energies of isomers and on the energies of their dissociation along the channels М₂Al₄₂ → 2М + Al₄₂ and М₂Al₄₂ + 2Al → 2М+ Al₄₄. The results are compared with the results of previous DFT calculations of endohedral (M@Al₁₂) and exohedral (Al@)MAl₁₁) isomers of the simpler doped clusters MAl₁₂ with the same dopants M. The influence of the aluminum cage size on the relative energy stability of the surface and and internal sopant positions is considered.     

Theoretical Study of Pd11Si6 Nanosheet Compounds Including Seven‐Coordinated Si Species and Its Ge Analogues

Nanosheet compounds Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNtBu)₁₀ ( 1 ) and Pd₁₁(SiiPr)₂(SiiPr₂)₄(CNMes)₁₀ ( 2 ), containing two Pd₇(SiiPr)(SiiPr₂)₂(CNR)₄ plates (R=tBu or Mes) connected with three common Pd atoms, were investigated with DFT method. All Pd atoms are somewhat positively charged and the electron density is accumulated between the Pd and Si atoms, indicating that a charge transfer (CT) occurs from the Pd to the Si atoms of the SiMe₂ and SiMe groups. Negative regions of the Laplacian of the electron density were found between the Pd and Si atoms. A model of a seven‐coordinated Si species, that is, Pd₅(Pd?SiMe), is predicted to be a stable pentagonal bipyramidal molecule. Five Pd atoms in the equatorial plane form bonding overlaps with two 3p orbitals of the Si atom. This is a new type of hypervalency. The Ge analogues have geometry and an electronic structure similar to those of the Si compounds. But their formation energies are smaller than those of the Si analogues. The use of the element Si is crucial to synthesize these nanoplate compounds.     

IR- und NMR-spektroskopische Untersuchungen zu Substituenteneffekten in Siloxy- und Alkoxysilanen

I.R. and N.M.R. Spectroscopic Investigations on Substituent Effects in Siloxy and Alkoxysilanes Siloxy and alkoxysilanes of the types (RMe₂SiO)₃SiH, (RMe₂CO)₃SiH, (Me_3–nPh_nSiO)_m(Me₃SiO)_3–mSiH (m = 1—3, n = 1—3), and Me_3–n(RMe₂SiO)_nSiH (n = 1—3) have been prepared. The influence of the substituent effects through the (Me₂)Si? O- a (Me₂)C? O- group on the Si? H band is approximately equal.     

Vibrational spectra in the ν(SiH) and ν(SiD) regions of chloro and bromodisilanes and ab initio geometry studies of C2H5Cl,Si2H5Cl and 1,1-Si2H4Cl2

Infrared measurements in the gas phase are reported for the ν(SiH) and ν(SiD) regions of Si₂H₅X, Si₂D₅X, 1,1-Si₂H₄X₂ and 1,1-Si₂D₄X₂ species where X = Cl, Br. Incomplete Raman data have also been obtained. All three possible isolated SiH stretching frequencies are observed in the spectra of the Si₂D₄X₂ samples, but only two from the Si₂D₅X ones. The missing ν^is(SiH) values are obtained by use of the frequency sum rule, and by harmonic local mode force field treatments of all the available ν(SiH) and ν(SiD) data, using a procedure previously tested on disilane.Ab initio calculations of the geometries of C₂H₅Cl, Si₂H₅Cl and 1,1-Si₂H₄Cl₂ using the 6-31G* basis set are reported. Trends in r_e(CH) or r_e(SiH) values reflect trends in ν^is(CH) or ν^is(SiH) ones. The alpha, trans and gauche effects of halogen are similar in CH and SiH compounds, although smaller in the latter. In both cases, ab initio calculations predict larger effects than are observed in the spectra, especially for the α effect of halogen.A kinetic isotope effect in the halogenation of disilane may occur. Reassignment of earlier spectra of disilyl iodide species is proposed.     

Organocobalt cluster complexes XXIX. Novel silicon compounds containing the nonacarbonyl tricobaltcarbon substituent

The reaction of bromomethylidynetricobalt nonacarbonyl or, more effectively, of methylidynetricobalt nonacarbonyl with diverse silicon hydrides (R₃SiH, Ph₃SiH, Me₂(EtO)SiH, R_nCl_3-nSiH (n = 02), etc. results in formation of silylmethylidynetricobalt nonacarbonyl complexes. Silicon-functional interconversions such as SiCl → SiOH, SiCl → SiOMe, SiOH → SiF, and SiOH → SiOSiMe₃, have provided still other substituted silylmethylidynetricobalt nonacarbonyl complexes, generally in high yield. The compounds Me(HO)₂SiCCO₃(CO)₉ and (HO)₃SiCCo₃(CO)₉ have been incorporated into methylsilicone polymers by H₂SO₄-induced reactions with cyclo-(Me₂SiO)₃.     

Induced Fitting and Polarization of a Bromine Molecule in an Electrophilic Inorganic Molecular Cavity and Its Bromination Reactivity

Dodecavanadate, [V₁₂O₃₂]^4? (V12), possesses a 4.4 Å cavity entrance, and the cavity shows unique electrophilicity. Owing to the high polarizability, Br₂ was inserted into V12, inducing the inversion of one of the VO₅ square pyramids to form [V₁₂O₃₂(Br₂)]^4? (V12(Br2)). The inserted Br₂ molecule was polarized and showed a peak at 185 cm^?1 in the IR spectrum. The reaction of V12(Br2) and toluene yielded bromination of toluene at the ring, showing the electrophilicity of the inserted Br₂ molecule. Compound V12(Br2) also reacted with propane, n‐butane, and n‐pentane to give brominated alkanes. Bromination with V12(Br2) showed high selectivity for 3‐bromopentane (64 %) among the monobromopentane products and preferred threo isomer among 2‐,3‐dibromobutane and 2,3‐dibromopenane. The unique inorganic cavity traps Br₂ leading the polarization of the diatomic molecule. Owing to its new reaction field, the trapped Br₂ shows selective functionalization of alkanes.