X-Ray Diffraction

X-ray diffraction (XRD) is an analytical technique based on the x-ray scattering produced when passing through the matter. The scattering phenomenon occurs when X-ray photons interacts with the electrons present in the studying material. Electrons behave as diffusion centres for X radiation of

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the same wavelength (coherent scattering) or different wavelength (incoherent scattering) of incident radiation. Diffraction is related to the coherent part of the scattering phenomenon: a crystalline solid covered by an X-ray beam produces diffracted X-rays along those directions where all the waves diffused by atoms in the crystal lattice are in phase concordance (constructing interference). To obtain a diffracted beam, certain geometric conditions have to be satisfied and such conditions are well described by Bragg’s law:

2 = (4.14)

where is the interplanar distance between a family of ℎ planes, is the angle between the direction of incident X-ray beam and the lattice planes ℎ , is the incident radiation wavelength and is an integer number.

X-ray diffraction can be performed on both single crystals and powders.

Obviously, the present work needed the analysis of lithologies, not of single phases, so the powder X-ray diffraction was the chosen technique.

Nowadays the applications of Powder Diffraction include, besides the more traditional uses for phase identification work:

- the ab initio crystal structure determination;

- - the Rietveld crystal structure refinement;

- - the accurate phase quantitative analysis (QPA) by line intensity or the Rietveld methods;

- - the quantitative determination of microstructural properties such as lattice strain, domain size and disorder;

- - the kinetic and structural analyses of materials and reaction processes in situ at non ambient conditions also performed in real time, etc.

The choice of instrument components as well as of data collection strategy should be driven by the specific goal (e.g. phase identification, quantitative phase analysis, structure determination, structure refinement, microstructural analysis, time-resolved analysis, etc.). Furthermore, the choice of components and strategy is typically a trade-off between, at least, one of the following factors: sample features, data collection duration, required resolution, costs (purchase, maintenance, accessibility, etc.), and other constrains (e.g. non-ambient environments, etc.). The quality of collected data (hence the reliability of final results) largely depends on (Cruciani, 2006):

- - brilliance and quality (i.e. less divergence) of the primary beam;

57 - - choice of the optics;

- - quantity and shape of the sample under the beam;

- - type and efficiency of detector;

- - scan strategy (i.e. the choice of the angular range, step-size and counting times).

The instrumental set-up used for our analysis is reported in Table 4.2. The instrument is a PanAnalytical X’Pert Pro; it was insert a Co anode.

Table 4.2 Instrumental parameter used for XRD acquisitions

As regard the sample preparation, the powder was finely micronized on a McCrone Micronizing Mill. The grinding vessel (Figure 4.13) consisting of a 125 capacity polypropylene jar fitted with a screw capped gasketless polyethylene closure. The jar is filled with an ordered array of 48 identical cylindrical grinding elements in zirconium oxide (they are also available in agata or corundum). These cylindrical elements grind the samples gently via friction.

The grinding time for optimum micronization is between 3 and 30 . The dry sample volume is 2.5 to which 10 of water had to be added obtaining a moisture (Figure 4.13 c). There are several advantages to use this kind of method: first, crystal lattice is almost entirely preserved during grinding

Instrument Panalytical X’Pert Pro

(Bragg Brentano geometry, theta-theta)

Tube type and settings Long Fine Focus tube with Co anode, 40kV e 40mA

Detector X’Celerator

Sample support Circolar sample holder (32 mm internal diameter)

Sample stage Spinner

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operation (a premise for meaningful X-ray diffraction), secondly very narrow and reproducible particle size distribution is obtained, there is minimal cross contamination, it is a compact, bench-top sized model, and so on.

The grinding operation is a delicate step because the crystallite size of a polycrystalline sample should be the best controlled as possible (i.e. in the 1-3 range, < 10 ) (Cruciani, 2006). An ideal polycrystalline sample is the one consisting of i) a very large number (ideally, infinite) of crystallites, ii) with the same number of crystallites in all possible orientation (random orientation), and iii) with a controlled grain size (usually in the 1-10 range). This are the fundamental requirements of a crystalline sample suited for powder diffraction.

Every significant deviations from one of these requirements would introduce uncertainties in the final results.

Figure 4.13 a) McCrone Micronizing Mill, the grinding vessels consisting in a jar and 48 zirconium oxide grinding elements; the spacer. b) Once the powder is within the jar, 10ml of demineralized water is

poured into the vessel. c) The mixture sample+water after 5 minutes of grinding.

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The very first step for a correct interpretation of a powder pattern is the phase identification occurring in a sample (qualitative analysis). Each crystalline phase shows a peculiar combination of position and intensity of diffraction peaks. In this way, every diffractogram of crystalline compounds is a sort of “fingerprint”

for the compounds, which allow its identification.

The identification process consists firstly in a database search (PDF – Powder Diffraction File) that collects about 300.000 files of inorganic and organic crystalline phases. PDF database is maintained and updated by the International Centre for Diffraction Data (ICDD). PD Files report information related to the crystalline compound, references and list of interplanar distances with relative intensities and characteristic Miller indices of each phase.

The correct phase identification procedure applied to a polyphaser sample assume different difficult degrees depending on the sample preparation, data collection strategy and the complexity of the phase blend.

The diffraction patterns acquired during this study were all qualitative interpreted with X'PertHighScore Plus 3.0 software by PANalytical. The mineral profiles of the compounds were reconstructed by comparing the reflection positions of the detected diffraction patterns with entries of ICDD (International Center for Diffraction Data) and ICSD (Inorganic Crystal Structure Database) databases.

One of the possible methods to determine the relative quantity of each phase in a blend is the Rietveld method. The basic of the Rietveld method lays on the complete exploitation of the whole powder profile without extracting the integrated intensities, all the structure and instrumental parameters are refined during the fitting procedure between the calculated and measured data. The refinement procedure implements the least square regression and it requires a reasonable scheme of starting values that approximate the real datum. Such parameters include:

- A function that describe the peak shape

- A function that describe the instrumental effects (on the shape, position and intensity of diffracted peaks)

- Structural parameters as cell dimension, space group and unit cell atomic coordinates.

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The reliability of the Rietveld method is strictly connected to the quality of powder diffractograms (a well-prepared polycrystalline sample, high counting statistics and limited instrumental problems).

Structural refinements of the compounds were performed through full-profile fitting according to Rietveld method, using DIFFRACplus TOPAS software by Bruker AXS.