Distortions of Diffraction Spectra

A diffraction spectrum is considered a fingerprint for identifying examined materials. An acquired spectrum should be compared with the standard from an X-ray diffraction database. The standard spectrum has been collected from a pure powder sample with fine powder size, usually several micrometers in diameter. We would expect to match both peak locations and relative peak intensities between the acquired spectrum and the standard if the sample is the same substance as the standard. Matching relative intensities, however, is not always possible even when the examined specimen has the same crystalline stmcture as the standard. One reason for this discrepancy might be the existence of preferential crystal orientation in the sample. This often happens when we examine coarse powder or non-powder samples. Preferential orientation of crystals (grains in solid) may even make certain peaks invisible. 

Any factor that changes the lattice parameters of crystalline specimens can also distort their X-ray diffraction spectra. For example, residual stress in solid specimens may shift the diffraction peak position in a spectrum. Residual stress generates strain in crystalline materials by stretching or compressing bonds between atoms. Thus, the spacing of crystallographic planes 

Identification of crystalline substance and crystalline phases in a specimen is achieved by comparing the specimen diffraction spectrum with spectra of known crystalline substances. X-ray diffraction data from a known substance are recorded as a powder diffractionfile (PDF). Most PDFs are obtained with CuKa radiation. Standard diffraction data have been published by the International Centre for Diffraction Data (ICDD), and they are updated and expanded from time to time. For one crystalline substance, there may be more than one file. The most recently updated file is recommended for phase identification. The early PDFs may contain errors in data obtained experimentally. More recently published PDFs are either obtained by more accurate experimental measurements or by theoretical calculation. A specimen to be identified should be in a powder form for most accurate matching. When we need to identify the crystal structure of a specimen that cannot be prepared as powder, matches of peak positions and relative intensities might be less than perfect. In this case, other information about the specimen such as chemical composition should be used to make a judgment. 

In a modern diffractometer, computer software performs the search-match task. All PDFs of the ICDD can be stored in computer. A search-match program can find all the possible matches for a specimen. The software algorithms also can identify more than one crystalline phase in a specimen. It searches the recorded pattern with its background, and adds candidate crystalline phases together to compose, rather than decompose, an observed multiphase pattern. 

Section 2.2.3 described factors that can affect relative intensities of peaks. These include the preferential orientation of crystal grains, residual stress and other crystal defects. Eventually, we must rely on human judgment to make the final identification. In this case, the matches of relative intensities are very good in general and no other calcium phosphate PDFs can provide matches as good as hydroxyapatite. Thus, hydroxyapatite can be positively identified. 

---