Structural-Properties Of Amorphous Hydrogenated Carbon .1. A High-Resolution Neutron-Diffraction Study

The structure of samples of amorphous hydrogenated carbon, prepared from acetylene and propane precursors, containing 35 and 32 at.% hydrogen, respectively, was investigated by time-of-flight neutron diffraction in the range 0.2-50 angstrom-1 using the ISIS spallation source. The large dynamic range of the data ensures a real-space resolution sufficient to reveal directly the proportions of sp2 and sp3 hybridized carbon. The results show that, in these hard carbon materials, the carbon-atom sites are predominantly sp2 bonded, and the carbon-carbon single bond:carbon-carbon double bond ratio is about 2.5:1. The detailed information on atomic correlations thus provided is used to discuss current structural models, and in particular the data are used to show that these models require significant modification

Splitting ‘intervocalic’: Expanding the typology of lenition environments

The basic types of lenition environments (‘initial’, ‘intervocalic’, ‘final’) need to be separately evaluated as they differ along parameters like word position (e.g., pre-consonantal vs. final codas) or stress relations. This paper argues that we need to recognise an additional such parameter: the length of the vowel preceding an intervocalic consonant. We show that a number of phenomena from varieties of English and German show lenition patterns which draw a distinction between reflexes found in post-short (vc) and post-long (vvc) environments. The theoretical consequence of our observations is that phonological theory needs to be able to account for the post-short vs. post-long distinction in the form of a parametrically-determined representational difference

A spectroscopic study of the structure of amorphous hydrogenated carbon

A range of amorphous hydrogenated carbon (a-C:H) samples have been studied using inelastic neutron spectroscopy (INS) and Fourier transform infrared (FTIR) spectroscopy. Using these complementary techniques, the bonding environments of both carbon and hydrogen can be probed in some detail, with the INS data providing not only qualitative but also quantitative information. By comparing the data from each of the samples we have been able to examine the effects of different deposition conditions, i.e. precursor gas, deposition energy and deposition method, on the atomic-scale structure of a-C:H

Structural properties of amorphous hydrogenated carbon. IV. A molecular-dynamics investigation and comparison to experiments

Hydrogenated amorphous carbon structures, a-C:H, with densities of 1.8 and 2.0 g/cm3, have been generated by semiempirical density-functions (DF) molecular-dynamics (MD) rapid cooling of a liquid phase of 128 carbon and 64 hydrogen atoms within periodically arranged cubic supercells. The electronic bonding properties of the model structures are analyzed within a local-orbital description. The structural properties are compared to relevant statistical and diffraction data obtained by neutron scattering and NMR in order to achieve a fundamental understanding of structure-related properties on the molecular level of chemical bonding

The effect of hydrogen dilution on the structure of a-C : H

Two a-C:H samples were prepared using a fast-atom deposition system from acetylene and an acetylene/hydrogen gas mixture. Their structure was investigated using neutron and x-ny diffraction and infrared spectroscopy measurements. Compositional analysis shows that a 1:1 C2H2:H-2 mixture results in a change from a-C-77:H-23 to a-C-79:H-21, i.e. has a very small effect on the composition. The diffraction data also show that the addition of hydrogen to the precursor gas has no significant effect on the average bond distances and angles but shows a small change in the H-C-H and C-C-H correlations between the two samples. However, the infrared data show that there are significant changes in the bonding of hydrogen within the sample-changes which do not affect the average network structure. We observe a decrease in the amount of sp(3) CH2 and CH3 groups, and an increase in the fraction of sp(2) and sp(3) CH groups, with the formation of a second sp(2) CH bonding environment in the hydrogen-diluted sample. Therefore, in addition to providing useful structural information on these a-C:H samples, this set of experiments illustrates very well the complementary nature of the data from diffraction and spectroscopic techniques

Inelastic neutron scattering of amorphous hydrogenated carbon

The hydrogen bonding environment of three samples of a-C:H have been extensively studied by means of inelastic neutron scattering. The observed inelastic neutron scattering spectra can be modelled by approximately equal quantities of sp3 CH and CH2 groups,i.e. approximately two-thirds of the hydrogen incorporated in a-C:H is in the form of CH2 groups

The structure of amorphous hydrogenated carbon by neutron and x-ray scattering

Whilst the short-range order in a-C:H is of central importance in explaining its mech. properties, it is order at intermediate distances that is likely to account more fully for the electronic and optical properties. The role of H would appear to be significant in promoting tetrahedral bonding-but no direct study of the H sites has been made. It is in this context that the preliminary results of neutron diffraction measurements, and of some novel synchrotron x-ray expts., are presented and discussed. The x-ray diffraction studies were carried out using a technique which may prove useful for as-deposited films in which the angle of incidence is fixed at, or near, the crit. angle. The results of a "reverse Monte Carlo" computer modeling study of the neutron data are presented. [on SciFinder(R)