Such a CN would seem to be larger than was determined in previous Figure 1

Such a cn would seem to be larger than was determined

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Such a CN would seem to be larger than was determined in previous Figure 1 | Reverse Monte Carlo modelling to reproduce the experimental X-ray diffraction and absorption data. a , Solid lines are the experimental XRD and Fourier-transformed EXAFS spectra for the Ni 80 P 20 amorphous alloy. The circles are for the XRD pattern calculated for the eventual RMC configuration. b , c , The solid lines present the inverse Fourier transforms of the first peaks in the Fourier-transformed EXAFS data in a . x ( k ) is the EXAFS, where k is the photoelectron wavevector. R is the distance between atoms. The EXAFS spectra calculated for the RMC configuration (circles) and for the ab initio molecular dynamics configuration (green line) are in good agreement with the experimental data. d , A view of the final RMC configuration. The blue and pink balls represent Ni and P atoms, respectively. Figure 2 | The evolution of partial RRDFs of the Ni 80 P 20 liquid observed in ab initio molecular dynamics simulations. The partial RRDFs, G ij ( R ), change as a function of temperature during the cooling and glass formation of the Ni 80 P 20 liquid. G ij ð R Þ ¼ 4 p R ½ r ij ð R Þ = c j 2 r 0 , where r ij ( R ) is the atomic density of the j atom at distance R from an i atom, and r 0 is the mean atomic density. The experimental data for this glass are also shown for comparison (orange circles, redrawn from ref. 13). ARTICLES NATURE | Vol 439 | 26 January 2006 420
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© 2006 Nature Publishing Group RDF work 7,13 by counting the number of atoms belonging to the ‘first shell’, which is very sensitive to the predefined cut-off distance. However, we confirmed that the results are consistent, as the CN turned out to be 9.4 when we used a cut-off distance of 2.8 A ˚ in the RDF (Fig. 2), in good agreement with previous analyses of TM–metalloid MGs 7,13 . Dependence of local packing on atomic size ratio To observe general features and trends, we analysed a number of model MGs using ab initio molecular dynamics simulations. These structural configurations are validated by comparing the calculated RRDFs with the experimentally measured ones (see below). Owing to the different atomic size ratios, the CN distribution of these MGs shifts consistently, as shown in Fig. 3. The solvent atoms surrounding the centre solute atom apparently form different types of coordination polyhedra in these various systems. As opposed to Pauling’s rule of parsimony 27 , which states that the number of the chemically different environments of an (ionic) compound tends to be small (for example, even the most complex of silicates minerals typically has only three to four different kinds of coordination polyhedra 28 ), the solute-centred polyhedra in each of the MGs studied involve a large number (as many as a few hundred) of coordination polyhedron types. Such diversity has previously been reported in classical molecular dynamics simulations of model systems 23,29 , and has recently been enumerated 25,26 . This finding suggests that the local arrangements cannot be modelled by a uniquely prescribed stereo- chemical structure. This conclusion was also corroborated by our
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