Calcium carbonate, crystal spectra

The broadening of the spectrum upon treatment at 790 K is similar to observations made when calcium carbonate single crystals are subject to argon ion bombardment. The defects induced in the calcium oxide component of the catalyst cannot be caused by an initial reduction of the oxide, because calcium oxide is stable to hydrogen at 790 K. The defects may be indicative of the formation of a ternary calcium iron oxide since there is no observed increase in the dispersion of calcium, which would be associated with a disintegration of the CaO crystals. It should be pointed out that the other structural promoter element, aluminum, exhibits the same spectral changes in its A12 emission. The dispersion of the alumina increases, however, with reduction of the catalyst, particularly when the wet reduction method is applied (see Table 2.1). 

Figure 7 shows the relative increase in sideband intensities for CP over BD as HPO4" and CO3" ions are introduced into the lattice and the considerable broadening of the CHA-B peaks [42]. The effect of HPO " ions is obvious, since they give pronounced sideband patterns and cross-polarize very efficiently. This has already been explained during the discussion of BRU. Figure 8 proves that the appearance of sideband patterns is very sensitive to the structure of the material [20]. It was found [42] that decrease in crystallinity results in considerable increase of linewidths and sideband intensities, and in dramatic reduction or disappearance of DD peaks (Fig. 9) [42]. The centreband in the CP spectrum of poorly crystalline HA looked like a superposition of a sharp line on a broader background. The CP and BD spectra of amorphous calcium phosphate were virtually identical within the experimental error. Aue et al. [42] admitted that they found no satisfactory explanation for the latter observation. Likewise, the discussion of the CO3" effect (Fig. 7) did not go very far. It was stated that the CHA-B lines were 5-10 times broader than those from various HA samples and that this broadening showed rather small dependence on carbonate concentration. It was inferred that the introduction of CO3" increased the CP efficiency of some PO " ions in the crystal lattice and imparted some changes in the PO " shift anisotropies. The former is questionable, because no protons were introduced into intracrystalline sites proximate to PO ions and the latter acceptable, because the CHA lattice must be less ordered, at least in the...

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