Vacancy patterns, high temperature

TCP, Ca3(PC>4)2, exists in two crystal forms, viz. a-TCP and p-TCP. Both forms can only be prepared by calcination at high temperature (>800 °C).70 a-TCP is meta stable at room temperature and is more soluble than p-TCP in water. The crystal structure of a-TCP shows that there are 16 inequivalent phosphorus sites.110 Its 31P MAS spectrum contains only 14 peaks, where the chemical shifts range from —3 to 5 ppm.111 Although there are only three crystallographically inequivalent phosphorus sites for (3-TCP,112,113 there are as many as 16 resolved peaks in the 31P MAS spectrum because of the presence of some calcium vacancies.114 The 31P chemical shifts of (S-TCP are in the same range as reported for a-TCP, but the overall patterns of their 31P MAS spectra are quite different. In addition, it has also been found that even a minor substitution of Ca2+ by Na+, Mg21 or Zn2+ ions will perturb the 31P MAS spectrum, revealing the substantial effect of the counter ions near the phosphorus sites.114,115... 

The defects which disrupt the regular patterns of crystals, can be classified into point defects (zero-dimensional), line defects (1-dimensional), planar (2-dimensional) and bulk defects (3-dimensional). Point defects are imperfections of the crystal lattice having dimensions of the order of the atomic size. The formation of point defects in solids was predicted by Frenkel [40], At high temperatures, the thermal motion of atoms becomes more intensive and some of atoms obtain energies sufficient to leave their lattice sites and occupy interstitial positions. In this case, a vacancy and an interstitial atom, the so-called Frenkel pair, appear simultaneously. A way to create only vacancies has been shown later by Wagner and Schottky [41] atoms leave their lattice sites and occupy free positions on the surface or at internal imperfections of the crystal (voids, grain boundaries, dislocations). Such vacancies are often called Schottky defects (Fig. 6.3). This mechanism dominates in solids with close-packed lattices where the formation of vacancies requires considerably smaller energies than that of interstitials. In ionic compounds also there are defects of two types, Frenkel and Schottky disorder. In the first case there are equal numbers of cation vacancies... 

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