Hydroxyapatite-poly-L-aspartic acid (HA-PASP) and hydroxyapatite-polyacrylic acid (HA-PAA) composite crystals have been prepared by direct synthesis in aqueous solution. The polyelectrolytes are quantitatively incorporated into the crystals up to about 8-9 wt%, as a function of their concentration in solution. The structural and morphological properties of the crystals vary as a function of polyelectrolyte content. TEM images show that the composite crystals display a greater length/width ratio with respect to the control HA crystals. The broadening of the X-ray diffraction reflections on increasing polyelectrolyte content was investigated using three different methods: (i) the Scherrer method, (ii) the Warren-Averbach approach, and (iii) a whole pattern analysis approach. Both polyelectrolytes induce a greater reduction of the mean crystallite size along a direction perpendicular to the c-axis direction, suggesting a preferential interaction of the polymers with the crystal faces parallel to the c-axis. PASP interaction with HA structure provokes a greater increase of strain in comparison to PAA. The data indicate that anionic polyelectrolytes can be usefully applied to modulate the structural and morphological properties of hydroxyapatite crystals.

The crystalline microstructure of ground sepiolite has been investigated. A reference sample of sepiolite and products of its comminution by dry grinding were studied through X-ray diffraction pattern analysis, specific surface measurements by nitrogen adsorption and complementary analysis of field emission scanning electron microscope images. A statistical model of polycrystals was applied to describe and determine the crystalline microstructure of the studied specimens. The model parameters characterizing the microstructure were prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice strain distribution, and they were determined for each sample by modelling a selected part of the X-ray diffraction pattern and fitting the simulated pattern to a measured one. A strict correlation of microstructure parameters with grinding time and with specific surface magnitudes was observed. A parallelepiped with edge-length ratios almost independent of grinding time (for longer times) was found to be the predominant crystallite shape. The crystallite size distributions were found to be close to logarithmic normal ones, with the mean values decreasing with increasing grinding time and the standard-deviation-to-mean-value ratios approximately constant. The second-order crystalline lattice strain distributions were found to be close to some simple function with the mean value equal to zero, the mean deviation increasing with increasing grinding time and the standard-to-mean-deviation ratios approximately constant. It was demonstrated that the specific surface can be calculated on the basis of the microstructure characteristics. Some details of the relation between crystallites and crystalline grains were explained by comparing the results of analyses via X-ray diffraction and scanning electron microscopy.

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A polycrystalline material may be considered as a set of crystallites. Since the crystallites have rather regular shapes, the assumption about the same shape is not far from physical reality for most polycrystals, especially powders. Such a system may be characterised in a statistical manner by two functions, the crystallite size distribution and the crystalline lattice strain distribution (for some materials other lattice distortions inside the crystallites, like stacking faults or dislocations, are to be considered additionally). The crystalline microstructure can be determined by investigating an X-ray diffraction pattern, what should be based on comparing an experimental pattern with a simulated one, derived from an appropriate physical model. Pure X-ray diffraction line profiles, containing information about crystalline microstructure, can be extracted from experimental data. An important step in analysing them is the separation of contributions from crystallite shapes and sizes and from strains, enabling the proper determination of both distributions together with the estimation of prevalent crystallite shape. A model of polycrystalline material combined with a description of X-ray diffraction on it, making such an analysis possible, is presented in this article. An approximate formula for separating both effects is based on results of computer simulation of pure X-ray diffraction line profiles from different crystalline powders, done under simplifying assumptions that the crystallites are prismatic or spherical, the size distribution is logarithmic-normal and the second-order strain distribution is normal.