As-As Bonds Research Articles

Spontaneous generation of stable pnictinyl radicals from "jack-in-the-box" dipnictines: a solid-state, gas-phase, and theoretical investigation of the origins of steric stabilization.

The molecular structures of the stable phosphinyl and arsinyl radicals, .PnR(2) [Pn = P (2); As (4); R = CH(SiMe(3))(2)], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the "dimers", R(2)PnPnR(2) [Pn = P (1), As (3)], and the chloro derivatives R(2)PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe(3))(2) ligands that permit the formation of the P-P and As-As bonds in 1and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state by melting, dissolution, or evaporation. The insights gleaned from these unusual systems have permitted a deeper understanding of the functioning of sterically demanding substituents.

Gilluyite Tl2(As,Sb)8S13; reinterpretation of the crystal structure and order-disorder phenomena

The crystal structure of gillulyite Tl 2 (As, Sb) 8 S 13 was reinterpreted based on the average structure determined by Foit et al. (1995). It consists of alternating PbS-like slabs comprised of As-S polyhedra and Tl-bearing slabs with partly zeolitic properties. In the latter, TlS 5 and As 2 S 5 groups alternate regularly along the b direction and across the width of the slab, thus eliminating the need for S-S and As-As bonds postulated by Foit et al. (1995). These two types of slabs are also unit order-disorder (OD) layers, the ambiguities in stacking of which lead to several potential gillulyite polytypes. The actually observed OD phenomena are caused by ambiguity in the position of [010] TlS 5 -As 2 S 5 sequences (or of entire such layers), which can with equal probability assume two positions 1b apart.

Effect of Small Amounts of Silver on the Electrical Properties of As2S3 Glasses

The dc conductivity, σ, of As 2 S 3 glasses, pure and doped with Ag up to 1.0 at%, and As 2 S 2.82 glass have been measured. Conduction behavior of As 2 S 2.82 glass that contains supplementary As-As bonds was nearly the same as that of As2S3 glass. By introducing 0.1 at% of Ag into As2S3 glass, the dc conductivity markedly increased, the activation energy E decreased from 1.14 to 0.81 eV, and the pre-exponential factor slightly decreased, while further addition of Ag up to 1.0 at% resulted in little changes in both σ and E. A structural model for the doped glasses is presented, and the relationship between the electrical properties and structures is discussed. It is suggested that Coulomb interactions between the Ag + ions and S atoms in As 2 S 3 layers play an important role in the above remarkable change in the conduction behavior.

TOF-neutron diffraction study of liquid Tl-As 2X 3 systems (X: Se, Te)

Time-of-flight neutron diffraction has been measured on both liquid Tl-As 2Te 3 and Tl-As 2Se 3 systems, which exhibit the metal-non-metal transition at the Tl-rich composition. Liquid As 2Te 3 contains direct As-As bonds in the first coordination shell as well as As-Te and perhaps Te-Te bonds, while liquid As 2Se 3 contains only As-Se bonds in the first coordination shell. The coordination number of component atoms in both liquids was determined to be three for As, and two for Te and Se, through a least-squares fit to the observed intensity function. The first sharp diffraction peak in S(Q) in liquid As 2X 3 (X: Te or Se), relating to -As-X-As- linkages, rapidly decreases upon the addition of Tl and disappears completely in the transition composition range in both liquid Tl-As 2X 3 systems. In the liquid Tl-As 2Te 3 system, Ass-Te bonds are gradually replaced by partially ionic Tl-Te bonds with increasing Tl content, leading to the sharp metal-non-metal transition at Tl 6As 2Te 3. On the other hand, As-Se bonds in the liquid selenide system remain even in the vicinity of the transition composition, strongly influencing the electronic properties over a wide composition range.

Oligomerization in As (III) sulfide solutions: Theoretical constraints and spectroscopic evidence

Bond distances, vibrational frequencies, gas-phase energetics, and proton affinities for various thioarsenite molecules and ions are predicted from molecular orbital theory and used to interpret EXAFS and Raman spectra of dissolved thioarsenites in undersaturated, alkaline 1 M NaHS solutions. From MO predictions, Raman peaks at 325 and 412 cm − are assigned to AsS(SH) 2 − and a peak at 382 cm − to AsS 2(SH) 2− At alkaline pH, As-S distances in dissolved thioarsenites are 2.21–2.23 Å and no statistically significant As-As interactions are recorded, consistent with predominance of the monomers, AsS(SH) 2 − and AsS 2(SH ) 2−. Estimated proton affinities suggest that thioarsenites with a negative charge greater than 2 are unstable in water. In seeming contradiction to this spectroscopic evidence, a new analysis of published solubility studies reinforces previous inferences that the trimer, As 3S 4(SH) 2 −, is the predominant thioarsenite in systems saturated with As 2S 3. Previously proposed dimeric species of the form, H x As 2S 4 x− , are rejected based on predicted thermodynamic properties. Dimer plus tetramer combinations also are rejected. Estimated free energies for AsS (OH)(SH) − and AsS(SH) 2 − are presented. We reconcile the spectroscopic and solubility evidence by showing that in undersaturated solutions monomers can become thermodynamically favored over oligomers. This pattern should be looked for in other sulfide systems as well. Sulfidic natural waters are in many cases undersaturated with respect to AS 2S 3 phases, so monomeric thioarsenites could be more important in nature than the trimers that have been characterized in saturated solutions. EXAFS spectra show that amorphous AS 2S 3 resembles orpiment in the first shell around As, but that higher shells are disordered. Disorder may be caused by occasional realgar-like, As-As bonds, consistent with the observation that amorphous AS 2S 3 is slightly S deficient.

Structure and some physical properties in relation to average co-ordination number, 〈r〉, in TeX and TeXAs glasses

Vibrational spectra and differential scanning calorimeter analysis of glass structure, transition temperature, Tg, and heat capacity jump, ΔCp, at Tg indicate that the new glasses in the TeSeAs system can be divided into two groups. The first, ‘TeX’-type glass, with the average coordination number, 〈r〉 < 2.2, has a quasi one-dimensional structure consisting of SeTe chains principally, and the fraction of chains containing As atoms is negligible. Some Physical properties of this glass, such as Tg, ΔCp and thermal expansion coefficient, α, etc., deviate from the universal linear function of 〈r〉 advanced for covalent bonding inorganic glasses with networks consisting of long polymer chains, because of the existence of Se8 rings and TeITe groups in the glass network. The second As-rich ‘TeXAs’-type glass, with the average coordination number, 〈r〉 > 2.2, has a two-dimensional glass network by the cross-linkages between SeTeAs chains through AsAs bonds. No Se8 rings and two-coordinated I atoms exist in this glass because of the formations of [AsSe3] pyramids and AsI bonds. The physical properties of this glass have an excellent agreement with the topological linear function of 〈r〉 as in the GeAsSe system, and they are optimized at the Phillips-Thorpe rigidity percolation threshold value, 〈r〉 = 2.4.

Nuclear-quadrupole-resonance studies of As2Se3 fibers.

Pulsed nuclear-quadrupole-resonance (NQR) experiments have been performed at 77 K on rapidly drawn fibers of ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The NQR line shape at 77 K was compared to the line shape of bulk glassy ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ and to thin-film ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. The fiber was found to have a line shape that is intermediate between the glassy and thin-film line shapes. The line shape of the fiber was also compared to the line shapes of glassy ${\mathrm{As}}_{0.5}$${\mathrm{Se}}_{0.5}$ at 77 K. The absence of a peak near 78 MHz in the fiber spectrum indicates that there are no As-As bonds in this sample.

Mechanism of radiation-structural transformations in amorphous As2S3

Abstract The mechanism of reversible radiation-induced structural transformations in vitreous As2S3 is studied using IR Fourier-spectroscopy technique. It is established that observed effects are associated with destruction-polymerization processes (transformations of the products containing homopolar S-S and heteropolar As-S bonds into the products on the heteropolar As-S and homopolar As-As bonds basis respectively) in vitreous matrix which lead to the stabilization of (As4 +, S1) coordination defects.

A theoretical study of stoichiometric and As-rich amorphous GaAs

We have investigated the electronic and atomistic structure of stoichiometric and As-rich GaAs by treating this material as a ternary alloy of fourfold-coordinated As4+, Ga3+ and threefold-coordinated As3+. Using the configurational averaging technique of Verges we have calculated the density of states (DOS) of this material, not only in terms of the stoichiometric index x and proportion of As3+ atoms alpha , but also in terms of the short-range order (SRO) parameters. By correlating with experiment we obtain valuable information about the SRO. In particular, we have shown that compensation occurs at a value of alpha that is a function of x only. Using this value of alpha and assuming that less than 1% of As4+-As4+ bonds occur (all other As-As bonds being allowed) we were able to (i) reproduce the EXAFS data and the variation of the optical gap with x and (ii) account for the observed shoulder in the lowest peak of the valence band and the subsidiary shell of As neighbours of Ga. Finally, at other values of the SRO parameters, we find features in the DOS that could explain some experiments on other amorphous III-V semiconductors.

Segregation of As on GaAs(100) Surface during Abrasion Process Studied by X-Ray Photoelectron Spectroscopy and Auger Electron Spectroscopy

When a GaAs(100) surface was mechanically scribed by a diamond tip in an ultrahigh vacuum (UHV), surface segregation of As was observed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The same phenomenon was also seen when the surface was abraded in air using emery paper. In the latter case, angle-resolved XPS analysis of As3d and Ga3d spectra showed the formation of As-As bonds on the abraded surface. Ga-Ga bonds existed beneath the layer of As-As bonds isolated from Ga-As bonds. The mechanism of the surface segregation of As is discussed.

On the optical properties of wedge-shaped thin films of Ag-photodoped As30S70 glass

Abstract Thin films of a-As 30 S 70 prepared by thermal evaporation were photodoped with Ag. The optical transmission was measured over the 0.3 to 2.0 μm spectral region in order to derive the refractive index and absorption coefficient of these Ag-photodoped chalcogenide films. Furthermore, the analytical expressions proposed by Swanepoel, enabling the calculation of the optical constants of a thin film with non-uniform thickness, have successfully been applied. In addition, thickness measurements made by a surface-profiling stylus were also carried out to cross-check the results corresponding to the envelope method. On the other hand, the dispersion of n was discussed in terms of the single-oscillator Wemple and DiDomencio model. Finally, the value of the optical band gap decreased from 2.47 eV in the case of the undoped films down to 1.91 eV in the almost saturated Ag-photodoped films. It is plausible that the decrease in E opt g by the incorporation of Ag arises from the smaller binding energy of Ag-S and As-As bonds compared to that of As-S bonds.

Compositional trends of the glass-transition temperature, urbach-tail slope and optical gap in As 0.5Se 0.5− xTe x glasses

The dependence of the glass-transition temperature ( T g ), Urbach-tail slope (Δ), and optical gap ( E 03) on the chemical composition in As-Se-Te glasses has been studied. It was found that both T g and Δ are compositionally invariant, while E 03 increases with increasing selenium content. It is supposed that softening of the studied glasses is controlled by the strength of the As-As bonds, a fraction of which is compositionally invariant. Similarly the As-As bonds broken during glass formation are supposed to be responsible for frozen-in potential fluctuations giving rise to the Urbach tail. Hence the As-As bonds and broken As-As bonds control both T g and Δ, which were found interrelated Δ ≈ 1.3 K B T g . On the other hand, the optical gap is controlled by anion substitution, and the E 03( x) dependence can be calculated using the virtual crystal model (VCM). The bowing parameter calculated using VCM has been found to be γ = 0.54 eV, in good agreement with experiment.

High-resolution (GaAs)1-x(Ge2)x x-ray photoelectron valence-band spectra: Implications for proposed electronic and structural models.

Valence-band density of states from sputter-deposited epitaxial metastable (GaAs${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(${\mathrm{Ge}}_{2}$${)}_{\mathit{x}}$ alloys have been acquired by high-resolution x-ray photoelectron spectroscopy (XPS). Based upon a comparison of XPS results and previously published models of electronic and atomic structure, we conclude that (1) the virtual-crystal approximation is not valid for this system, (2) there are no significant numbers of Ga-Ga and As-As bonds, and (3) Ge is not incorporated randomly. The electronic and atomic structure of these metastable alloys is determined primarily by the large energy-level differences among Ga, As, and Ge atoms.

The influence of the composition of the layers and of the inorganic solvents on photoinduced dissolution of As-S amorphous thin films

Dissolution of As x S 100−x (x=28;40;42;45) thin layers in alkaline solutions was studied. The dissolution rate of as-evaporated and exposed layers increases with temperature, pH of solution and with presence of oxidizing agents in solvent. It decreases with As content in the layer and in presence of reducing agents in etching solution. It is lower in as-evaporated layers. The differences in solubility rates are related with different content of As-As bonds or As 4 S 4 structural units in the layers.

Electron beam-induced current imaging of antiphase domain boundaries in heteroepitaxial GaAs on group IV semiconductors

A viable GaAs heteroepitaxy technology is desirable in order to exploit the high mobility and direct band gap properties of GaAs while avoiding the high cost, defect density, and weight of bulk GaAs. Microstructural defects such as dislocations, microtwins, and stacking faults found in most heteroepitaxial GaAs films have thus far inhibited the achievement of device quality films. In addition, the formation of antiphase domain boundaries (APBs), at which As-As bonds and Ga-Ga bonds exist, is undesirable because they are electrically active. The APBs are believed to form due to the presence of single atomic steps on the underlying group IV substrate. Transmission electron microscopy investigations have previously reported the presence of APBs in GaAs heteroepitaxial films. Since the APBs act as electron-hole recombination sites, the electron beam-induced current. (EBIC) imaging mode of the scanning electron microscope (SEM) is ideally suited for straightforward and statistically significant. GaAs heteroepitaxial film evaluation.

Photodarkening in amorphous arsenic chalcogenides: Microscopic mechanisms and compositional trends

The microscopic origin of photodarkening, the reversible light-induced shift in the optical absorption edge in chalcogenide glasses, is studied by use of X-ray absorption spectroscopy. For the stoichiometric compound As 2S 3, a change in intermediate-range order correlations and a slight (0.5%) increase in the concentration of homopolar As-As bonds with photodarkening are identified. For nonstoichiometric compounds of the type As x S 1− x , the results are presented together with a review of other recent experimental studies on arsenic sulfide glasses and the equivalent selenide system ( As x Se 1− x ). Despite significant differences between the sulfide and the selenide glasses, photodarkening increases for As rich compositions with x >;0.4 in both cases, pointing to the crucial role of homopolar As-As bonds in the process. The photodarkening effect in As-rich selenide glasses reveals additional annealing and bleaching aspects not observed in the stoichiometric compound

Nuclear quadrupole resonance in the glassy Cu-As-S and Cu-As-Se systems.

Nuclear quadrupole resonance (NQR) of $^{75}\mathrm{As}$ has been used to study bonding arrangements in the metal chalcogenide glass systems, Cu-As-S and Cu-As-Se. In the compositions ${\mathrm{Cu}}_{\mathrm{x}}$(${\mathrm{As}}_{2/5}$${\mathrm{Z}}_{3/5}$${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$, where Z=S or Se, the $^{75}\mathrm{As}$ NQR spin-echo measurements are consistent with the appearance of As-As bonds whose density increases with x. For the compositions (Cu-As-Se) the NQR measurements yield no evidence for the presence of As-As bonds. Both of these results are consistent with a recent structural model proposed to explain the local structural order in metal chalcogenide glasses.

Chemical order in the glassy AsxS1-x system: An x-ray-absorption spectroscopy study.

We have examined chemical ordering in the glassy ${\mathrm{As}}_{\mathrm{x}}$${\mathrm{S}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ system by determining the effect of composition on the local structure of these chalcogenide glasses using x-ray-absorption spectroscopy. Structural changes associated with composition indicate that with increasing S content, the S-rich glasses on the As site have a similar local structure to crystalline ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ (orpiment), but the As-S-As linkages are replaced by As-S-S linkages at higher S concentration. In As-rich glasses a breakdown of the local ${\mathrm{AsS}}_{3}$ configuration is evident and the formation of As-As bonds is observed. Further comparison between As-rich alloys and crystalline ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ (realgar) suggests that a significant fraction of disordered ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ molecular fragments is contained in the As-rich region.

Antiphase domains in GaAs grown by metalorganic chemical vapor deposition on silicon-on-insulator

Distinct antiphase domain structures in GaAs epitaxial layers grown on a Si/SiO2/Si-substrate structure by metalorganic chemical vapor deposition have been revealed by using a silicon etchant (HF/HNO3). The antiphase is characterized by the [011]-oriented etching textures which rotate 90° between adjacent domains. The corresponding lattice rotation is further confirmed by a convergent beam electron diffraction technique. The size of the antiphase domains is found to increase with increasing film thickness and to grow upon annealing at temperatures above 700 °C. The maximum size of the domain, however, is found to be limited by the film thickness. The majority of the domain boundary lines revealed by chemical etching on the (100) surface do not correspond to any crystalline orientation. Only small segments are found to orient along [011], [010], [021], and, occasionally, [031] and [041] directions. Cross-sectional transmission electron microscopy studies confirmed that the boundaries are generally in curved configurations or zigzag configurations constituted of (011), (010), and (121) planes. All the boundaries are initiated at the interface and propagate through the film in the growth direction. Diffraction contrast experiments show a stacking-faultlike contrast of intrinsic type, indicating an inward relaxation of the lattice planes at the boundary. The rapid chemical reaction of the boundary with the silicon etchant, the intrinsic nature of the lattice distortion at the boundary, and the curved configuration of the boundary indicate that the boundary atoms are replaced by Si atoms. The higher concentration of Si atoms at the antiphase boundary has further been verified by energy dispersive x-ray analysis. In view of the reduction of bond distortion energy, the segregation of Si atoms at the antiphase boundary eliminates the highly distorted GaGa and AsAs bonds and is, therefore, an energetically favorable process. The possible antiphase boundary structure and a mechanism for its migration are discussed. Spatially resolved photoluminescence and cathodoluminescence studies reveal that both the antiphase boundaries and the defective interfacial regions contain nonradiative recombination centers. The luminescence efficiency of the domains increases strongly after annealing.

Validity of the recursion method for calculating the energy gap of (GaAs)1-xGe2x: Comments on a paper by Gu, Newman, and Fedders.

We have shown previously that the energy gap of (GaAs${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{2\mathrm{x}}$, as calculated by the recursion method, is sensitive to the presence of As-As nearest-neighbor pairs. When significant concentrations of such As-As bonds are incorporated into alloys with near-midrange compositions the gap decreases from about 0.5 eV, in agreement with optical-absorption data, to zero, in clear disagreement with experiment. This provides a strong reason for rejecting the Newman-Dow (ND) thermodynamic model of the alloy, which predicts large concentrations of As-As bonds. In a recent paper Gu, Newman, and Fedders claimed that the recursion calculations, upon which our rejection of the ND model is based, are invalidated by an elementary error. We demonstrate that this assertion is unfounded.