The structure factor S͑Q͒ of high purity amorphous Si membranes prepared by ion implantation was measured over an extended Q range (0.03 55 Å 21). Calculation of the first neighbor shell coordination (C 1) as a function of maximum Q indicates that measurement of S͑Q͒ out to at least 40 Å 21 is required to reliably determine the radial distribution function (RDF). A 2% change in C 1 and subtle changes in the rest of the RDF were observed upon annealing, consistent with point defect removal. After annealing at 600 ± C, C 1 3.88, which would explain why amorphous Si is less dense than crystalline Si.

No abstract

Structural modeling of amorphous eumelanin has been carried out by comparing calculated data, S(q) and RDF(r), in reciprocal and real space, respectively, for limited random network models with the experimental X-ray scattering data of tyrosine melanin (Cheng et al., 1994). A basic picture of the atomic arrangements in amorphous eumelanin, which accounts for the short and intermediate range order, has been formulated. This reveals domains of a fundamental "particle" dimension of R approximately 15 A, consisting of a paracrystalline array of disordered planar networks polymerized by 4-8 DHI monomers with a graphite-like stacking spacing of approximately 3.45 A, 4-5 layers thick.

No abstract

The intrinsic local structure characterization of natural sepia melanin and L-dopa and tyrosine synthetic melanin powder has been carried out by X-ray diffraction using synchrotron radiation. The derived structure factor, S(q), shows six significant diffuse peaks within the q-range from 0.3 A-1 to 16 A-1 in the reciprocal space (q = (4 pi sin theta)/lambda, 2 theta is the scattering angle). The Fourier transform of S(q), which yields the radial distribution function (RDF), gives us information in real space of a 1.42 A distance averaged over the C-C, C-O and C-N bond lengths as well as peaks at 2.40-2.41 A, 3.67-3.71 A and 4.67-4.70 A discrete neighbor distances. There is a great similarity in the scattering intensity profiles of the natural and synthetic melanins indicating that the synthetically prepared material may be essentially similar to "real" melanin in its local atomic arrangements. An evidence of a prepeak at q congruent to 0.45 A-1 has been confirmed which indicates a preferred length scale of approximately 13-20 A that corresponds to the initial particle size in colloidal melanin solutions.

No abstract

No abstract

It has been previously established on the basis of an Ising model of alloy ordering (part I of this series) that the observed maxima of short-range-order diffuse scattering in disordered alloys mark the positions of the minima of V(k), the Fourier transform of the pairwise interatomic potential V(r). It was suggested in I that the superlattice spots of the ordered state should occur at these same positions. This would establish a consistency requirement connecting the ordered and disordered configurations. We prove here that this is a sufficient but not a necessary condition, and we derive the full set of necessary conditions. We also show that for most of the diffuse maxima locations observed in bcc and fee alloys, it is possible to establish the ordered configuration required by the disordered phase, or conversely to use a knowledge of the ordered configuration to restrict the possible choices of V(r) in fitting the disordered-phase scattering data. The linear approximation for correlation functions in binary alloys presented by the authors in I is compared quantitatively with the more exact but less general formula of Fisher and Burford. Except at temperatures very close to T c , it is found that the theoretical shape of the short-range order diffuse scattering is in agreement but the temperature dependencies differ. We note also that all the currently known approximate calculations of a(k) for arbitrary V(r) can be represented in the same functional form, a(k)=G2(r)/[l+Gi(r)F(k)], where the temperature-dependent factors G\ and G2 vary according to the method of calculation used, but that V(k), the Fourier transform of V(r), is the same for all. This leads to the conclusion that the ratios V(r n +i)/V(r n ) determined by fitting the theoretical form to experimental data are insensitive to the particular choice of G\ and G2, but that the magnitudes of the V (r n ) so determined vary according to this choice. As a consequence, the values obtained for V(r n +i)/V(r n ) are to be regarded as more accurate than those for V(r n ).