2 National Institute of Standards and Technology Gaithersburg Maryland 20899

2 national institute of standards and technology

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2 National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA. 3 High-Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Vol 439 | 26 January 2006 | doi:10.1038/nature04421 419
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© 2006 Nature Publishing Group Figure 2 illustrates the evolution of the reduced radial distribution functions (RRDFs) as a function of temperature during the cooling of the ab initio Ni–P system. The development of SRO with decreas- ing temperature is clearly observed from the structure developed in the second peak. The partial RRDFs of the final 300 K configuration (purple curve) agree remarkably well with those directly measured from experiments 13 (orange dots). Furthermore, the EXAFS spectra in Fig. 1 (as well as the XANES spectra in Supplementary Fig. S1) obtained using first-principles calculations 22 for the 300 K ab initio configurations also exhibit an excellent match with the experimental data. These validations lend strong support to the use of the configuration obtained from the ‘virtual’ quenching experiments for structural analysis of the 3D atomic packing. An additional check for both the RMC and molecular dynamics configurations is that their mass (or atomic) densities are all within 1 to 2% of the experimentally measured value for the Ni–P (and 0.7% for Ni–B discussed below) MG. Structural analysis based on Voronoi tessellation The atomic positions in both the RMC and ab initio molecular dynamics configurations can then be used, and compared, to unravel the short-to-medium-range details. Our first objective was to identi- fy the possible presence of building blocks, or ‘clusters’. The first step was to resolve the local nearest-neighbour coordination. The coordi- nation number (CN) can be determined unambiguously by using the Voronoi tessellation method 23–26 , which also characterizes the local atomic environment. We use the Voronoi index k n 3 ; n 4 ; n 5 ; n 6 ; l , where n i denotes the number of i -edged faces of the Voronoi polyhedron and i P n i is the total CN, to designate and differentiate the type of the coordination polyhedron surrounding a centre solute atom. For example, tri-capped trigonal prism packing (TTP) corre- sponds to a Voronoi index of k 0,3,6,0 l , while a mono-capped square archimedean antiprism (slightly distorted from the TTP) has a Voronoi index of k 0,5,4,0 l (see Supplementary Fig. S3). In the Ni–P glass the solute P was found to be coordinated in the first neighbouring shell by Ni atoms only, and the statistics of the Voronoi analysis indicated that the average first-neighbour CN of the P solute is 10.5 for the ab initio configuration, and 10.6 for the RMC model, with very similar distribution for the two 3D configur- ations (see the distribution in Fig. 3 for the ab initio configuration).
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