A fitting model based on the use of two independent blocks resulting from distributions of a hyperfine field and of one sextet of lorentzian lines is discussed for Mössbauer spectra recorded for Fe(Cu)MB nanocrystalline alloys. One distributed subspectrum is ascribed to the amorphous residual matrix, while the other independent block, from the hyperfine-field distribution, is attributed to Fe atoms located in the so-called interface zone. This region comprises atoms of nanocrystalline-grain surfaces and also atoms originating from the amorphous precursor, in close contact with the nanocrystalline grains. A sextet of lorentzian lines is attributed to the crystalline grains that have emerged from the amorphous alloy, which are unambiguously identified as α-Fe phase. The distribution with low hyperfine fields can be eventually analysed in terms of two components accounting for the coexistence of electric and magnetic hyperfine interactions. In such an analysis, distributions of both quadrupolar splittings and hyperfine magnetic fields are employed. Examples of the present fitting model are provided for Mössbauer spectra of FeCuMB (M = Zr, Ti, and NbCr) nanocrystalline alloys in the first stage of crystallization. The spectra have been recorded under various experimental conditions comprising low (77 K) and high (373 K) temperatures as well as an external magnetic field. More detailed discussion about the consequences of this novel fitting procedure with respect to the topography of hyperfine interactions within Fe-based nanocrystalline alloys is reported in part II, the following paper.
The atomic structure of Ni-Pd nanoparticles has been studied using atomic pair distribution function (PDF) analysis of X-ray total scattering data and with transmission electron microscopy (TEM). Larger nanoparticles have PDFs corresponding to the bulk face-centered cubic packing. However, the smallest nanoparticles have PDFs that strongly resemble those obtained from bulk metallic glasses (BMGs). In fact, by simply scaling the distance axis by the mean metallic radius, the curves may be collapsed onto each other and onto the PDF from a metallic glass sample. In common with a wide range of BMG materials, the intermediate range order may be fit with a damped single-frequency sine wave. When viewed in high-resolution TEM, these nanoparticles exhibit atomic fringes typical of those seen in small metallic clusters with icosahedral or decahedral order. These two seemingly contradictory results are reconciled by calculating the PDFs of models of icosahedra that would be consistent with the fringes seen in TEM. These model PDFs resemble the measured ones when significant atom-position disorder is introduced, drawing together the two diverse fields of metallic nanoparticles and BMGs and supporting the view that BMGs may contain significant icosahedral or decahedral order.
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