Diffraction analysis of metallic, semiconducting and inorganic glasses

Abstract

Recent advances in both diffraction theory and experiments have led to the determination of the topological and chemical short-range order in metallic, semiconducting and inorganic glasses. The introduction of the variable wavelength experiments in x-ray diffraction (energy-dispersive x-ray diffraction) and in neutron diffraction (time-of-flight measurements with polychromatic neutrons) has extended the reciprocal space to the values of K = ( 4π γ ) sinϑ as high as 40 Å −1. These extended diffraction data have yielded high resolution radial distribution functions for such glasses. The structure of two-component systems, consisting of elements 1 and 2 (binary alloys), is characterized by three partial structure factors S NN (K), S CC (K), and S NC (K) which describe interference functions I 11 (K), I 22 (K) and I 12 (K), or the topological short-range order, the chemical short-range order, and the correlation between number density and concentration, respectively. These partial functions can be evaluated from three total interference functions obtained by (1) isomorphous substitution of one or both elements, (2) isotopic substitution (for neutrons), (3) three different radiations (x-rays, neutrons, and electrons), (4) anomalous dispersion, and/or (5) polarized neutrons for magnetic materials. Three special cases facilitate the evaluation of the partial interference functions or structure factors: (1) x-ray or neutron experiments on two-component systems with elements i, whose coherent scattering amplitudes f i are similar, will yield the topological shortrange order S NN(K) = c 1 2I 11(K) + c 2 2I 22(K) + 2c 1c 2I 12(K) where c i is the atomic concentration of element i. (2) If we choose a system for which c 1f 1 + c 2f 2 = 0, (zero-alloy), we can directly determine the chemical short-range order S CC(K) = c 1c 2{1 + c 1c 2[I 11(K) +I 22(K) − 2I 12(K)]}. (3) If the coherent scattering amplitude f 2 of element 2 is zero (null-element), then we obtain directly the partial interference function I 11(K). Examples of recent attempts to determine the partial interference functions I ij(K) and their corresponding atomic distribution functions ϱ ij(r) will be given for metallic, semiconducting and inorganic glasses.