Crystallite perfection

In situ analysis of mineral content and crystallinity in bone. Bone, a functionally gradient material, is composed of protein and mineral components which give rise to spectral absorptions in the mid and far-infrared spectral range. Recently, Miller et al. (2001) have initiated an investigation of cross sections of human iliac crest bones, collecting the IR absorption spectra around a human osteon. The focus of this investigation was to measure the acid phosphate content and determine mineral crystallite perfection from the . spectra. The crystallite perfection was determined from a concurrent study of the correlation of IR absorption spectra with X-ray powder diffraction results from a series of synthetic hydroxyapatite crystals and natural bone powders of various species and ages. 

Crystallite size, crystallite distribution, and crystallite perfection... 

In Section 1.3 it was noted that the energy of adsorption even for a perfect crystal differs from one face to another. An actual specimen of solid will tend to be microcrystalline, and the proportion of the various faces exposed will depend not only on the lattice itself but also on the crystal habit this may well vary amongst the crystallites, since it is highly sensitive to the conditions prevailing during the preparation of the specimen. Thus the overall behaviour of the solid as an adsorbent will be determined not only by its chemical nature but also by the way in which it was prepared. 

The stoichiometry of decomposition of [Ni(NH3)4](NCS)2 was dependent on the method of salt preparation [1126]. Ammonia was lost in three successive steps (—NH3, —NH3, —2 NH3) from the solution-prepared salt, but the first intermediate could not be isolated from the similar reaction of material prepared by heterogenous reaction. The difference in behaviour was ascribed to differences in perfection of the crystallites resulting from the alternative preparative methods. 

X-Ray diffraction has an important limitation Clear diffraction peaks are only observed when the sample possesses sufficient long-range order. The advantage of this limitation is that the width (or rather the shape) of diffraction peaks carries information on the dimensions of the reflecting planes. Diffraction lines from perfect crystals are very narrow, see for example the (111) and (200) reflections of large palladium particles in Fig. 4.5. For crystallite sizes below 100 nm, however, line broadening occurs due to incomplete destructive interference in scattering directions where the X-rays are out of phase. The two XRD patterns of supported Pd catalysts in Fig. 4.5 show that the reflections of palladium are much broader than those of the reference. The Scherrer formula relates crystal size to line width ... 

This analysis gives satisfactory results concerning the average crystallite sizes even in unfavorable experimental conditions such as overlapped or very weak and noisy peaks, and it allows an easy treatment of non-perfect monochromaticity of the radiation. But, it is important to emphasize that it is almost impossible to obtain the promised detailed description of the crystallite size and strain distributions. This is a fundamental problem related to the adopted procedure that is based on the a priori choice of the peak shape that inevitably imposes the general shape of such distributions [40]. For these reasons, the average dimension and strains remain the only reliable information. 

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