As-As Bonds Research Articles

New analytic method for investigation of the distribution of bonds in As-S system

This paper describes a simple analytical method enabling the determination of the concentration of As-As bonds. We have proved that immediately after evaporation the samples contain at least 32% of As-As bonds from the total number of bonds by which arsenic is bound to its neighbourhood. Their content markedly decreases after annealing and illumination.

Extended X-ray absorption fine-structure spectroscopy study of photostructural changes in amorphous arsenic chalcogenides

Abstract Extended X-ray absorption fine-structure spectroscopy (EXAFS) measurements have been performed on amorphous arsenic sulphide, in both bulk-glass and thin-film forms, in order to probe the changes in local order accompanying the photostructural changes which occur in such materials. The irreversible photo-induced changes occurring in the thin films are associated with changes in the first coordination shell; these are ascribed to the conversion of As-As bonds to As-S bonds. In contrast, no significant changes in the first coordination shell, as revealed by EXAFS, appear to accompany the reversible photo-induced changes observed in glassy As2S3. Instead, changes in the region of the second shell are observed, and these are ascribed to either an increase in (bond-angle) disorder, or to an increase in inter-molecular distances, occurring on illumination.

The electronic structure of defects in amorphous GaAs

Abstract The local electronic structure of like-atom bonds and threefold coordinated Ga and As sites (dangling bonds) in amorphous GaAs have been calculated by the tight-binding recursion method. Singly occupied dangling bonds are found to give gap states just inside the band edges, but their relaxed diamagnetic configurations only give band resonances. This behaviour differs from a-Si: H but is similar to that on surfaces of GaAs crystals. Ga-Ga bonds do not give gap states, but isolated As-As bonds give states just below the conduction-band edge.

Ternary arsenides with LaFe 4P 12-type structure

Ten new ternary arsenides with composition LnT4As12 (Ln = rare earth element, T=Fe, Ru, or Os) were synthesized by reaction of the elemental components in silica ampoules. Their X-ray powder patterns show them to crystallize with the LaFe4P12-type structure. Their lattice constants are reported. The crystal structure of LaFe4As12 was refined from single-crystal X-ray counter data to a conventional R value of 0.039 for 312 independent reflections. While the La-As and Fe-As distances are considered normal, the As-As distances of 2.57 and 2.58are found too be unusually large, as compared to distances of other two-electron As-As bonds. It is suggested that this is due to filling of As-As antibonding states.

Resonance Raman scattering in AsS glasses

The Raman spectra of glasses with compositions in the range As 35S 65 yo As 43S 57 have been recorded using near band-gap excitation ( λ i = 5145 A ̊ ) and also weakly absorbed excitation ( λ i = 6328 A ̊ ). With 5145 Å excitation there is a significant enhancement at ∼231 cm −1 for the glasses with ⩾ 40 at. % As. This band is attributed to the presence of As-As bonds in the AsS network. Both As-As and S-S bonds are believed to occur in the stoichiometric glass, a-As 40S 60, though in small numbers. The enhancement of the 231 cm −1 band for 5145 Å excitation is shown to be primarily a resonance effect, suggesting that As-As bonds constitute electronic “defect” states within the mobility gap.

The structure of vapour-deposited arsenic sulphides

High resolution X-ray and neutron correlation functions have been obtained for arsenic sulphide bulk glass and vapour-deposited films by making measurements to high momentum transfers. These data are compared with a vatiety of models and it is concluded that the structure of the films is dominated by the presence of As 4S 4 molecules in the vapour phase, which results in more As-As bonds than the minimum required by stoichoimetry. The extent to which these As 4S 4 molecules polymerise in the as-deposited film is unclear, but depends strongly on preparation conditions.

Thermally induced effects in evaporated chalcogenide films. I. Structure

The structure of evaporated ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$, ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, and Ge${\mathrm{Se}}_{2}$ films, and the influence of annealing at the glasstransition temperature, are studied by extended x-ray-absorption fine structure (EXAFS) and by Raman spectroscopy. In addition, the topology of each film is analyzed by calculating the x-ray diffraction for several models. The films were prepared by evaporation onto substrates held near room temperature. All the as-deposited films exhibited significant homopolar bonding in contrast to the almost totally heteropolar bonding of the corresponding bulk quenched glasses. Upon annealing of the films, the measurements indicate that the density of homopolar bonds decreases, and the films more closely resemble the bulk quenched glasses. The Raman spectra are quantitatively analyzed with two models to characterize the disorder, and then compared to the EXAFS results. The analysis indicates an As-As coordination of \ensuremath{\sim} 0.6 and \ensuremath{\sim} 0.4 and a Ge-Ge coordination of \ensuremath{\sim} 0.3 for the ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$, ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, and Ge${\mathrm{Se}}_{2}$ as-deposited films, respectively. The measurements also indicate that the As-As bonds of the ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ film are incorporated into ${\mathrm{S}}_{2}$As-As${\mathrm{S}}_{2}$ units, suggesting the presence of ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ molecules. Calculations using this model are in good agreement with x-ray diffraction data in the literature. The data from ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ evaporated films also indicate that molecular structures may be present. There is no evidence, however, for molecular structures in Ge${\mathrm{Se}}_{2}$ films.

Infrared Absorption Spectra of Glasses in the System As2S3–Ag and Oxygen Impurity Bands

The infrared absorption spectra of samples of As2S3-Ag composition with varying Ag content, and As2Sx glasses with x≤3, prepared by melting elements As and S of different purities in evacuated sealed tubes were investigated, and in addition X-ray diffraction and electron microscopic observation of As2S3-Ag glasses were carried out. It was found that the effect of oxygen impurity on the absorption bands of As2S3-Ag glasses became more pronounced with the increasing Ag content, and the impurity bands corresponded to the band characteristic of a mixture of the band of cubic form As2O3 and that of glassy form. It may be concluded that Ag-S bonds are born by the addition of Ag, and the >As-S-As< linkages in the As2S3 glass are eliminated, leading to the formation of As-As bonds. It is suggested that the glass structure has changed sharply in the composition As2S3Agx with x between 0.311 and 0.402.