XRD and SAXS

X-ray diffraction (XRD) is a perfect tool to define the structure of materials and, in particular, of nanoparticles. The cores of the particles will provide the diffraction peaks, which may appear modified in their position due to the frequent lattice parameter modification in nanoparticle state (respect to bulk). In addition, the relative intensity of the peaks may reflect modifications in the atomic occupancy within the core.

Another aspect inherent to the nanometric scale is the increase of peak width, which can also stem from microstrain effects. Occasionally the surface shell is present as a disordered crystallographic phase; this will appear as a broad contribution in the spectra. In consequence, we will be able to give quantitative parameters of core structure, particle size and microstrain. This will be evaluated by Rietveld refinement, which is routinely used in our analysis at UNICAN.

If fluorescence problems are important, we eventually will combine the Cu-Ka radiation with Mo-Ka results and combine them in Multipattern Rietveld refinements. Studies using different combination of slits and different radiations are then mandatory. Calibration of intensity variations and of resolution functions is important to provide adequate Cagliotti coefficients. Counting times to enable sufficient statistics will be also taken into account. The sample environment also includes a heating chamber if temperature variations are needed (up to 450 ºC). In consequence, the samples will then be continuously analyzed using Si sample holders including protection to eventually avoid air contamination. The use of a monochromator in the detection arm is mandatory. Typically, the spectra will be collected over 6 hours, although a time-tuning is expected. For some specific samples, which may be conceptually regarded as case examples, it may be possible to obtain XRD spectra at Large Facilities such as, for example, ESRF, or in Spain ALBA (apart from other Installations all over Europe).

The results will then be compared to those obtained with TEM and neutron diffraction described in the project and the magnetic granulometry derived from DC-magnetization and AC-susceptibility measurements. On the other hand, small-angle x-ray scattering (SAXS) enables the study of particle correlations at the mesoscopic scale. This can determine the existence of nanoparticles in single and multicore crystals. SAXS is sensible to electronic density inhomogeneities usually arising from scattering objects embedded in a medium of another density. In some limiting cases when the media sample presents a sharp interface and large Q-values a Porod´s law is described. By contrast in particles isolated and without interaction a Guinier exponential dependence LnI vs.Q² is found. In general, there could be convolutions of the particle form factor coupled with the structure factor of the assembly.

In some cases the shape of correlated nanoparticles can also be ascertained. Naturally this can be performed either in-house or in Large Facilities. There is also the possibility of combining SAXS with wide-angle X-ray diffraction WAXS. Frequently, Large Facilities employ easy-to-operate data reduction packages for analysis and strong scientific support. The downside is that beamtime should be requested at certain periods along the year. SAXS should be recorded in the projects in single o multicore systems with a relatively low degree of polydispersity. In this sense it’s possible to analyze collection of magnetic nanoparticles if, especially different volume fractions of the magnetic materials (core) is achieved. It should be assured a good contrast between the particles and the eventual matrix. The fact that the particle systems may present a single or a multi-core should produce drastic difference in the spectra of SAXS. Experiments can take 2 days. A collection of Fe-based particles with different volume fraction should be achieved to define properly the methodology and metrology.

Project Partners

Project partners


	This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 604448