Crystallographic Environment Research Articles

NMR study of the influence of aluminum alloying on the titanium-hydrogen system

The addition of aluminum to titanium is known to reduce the embrittling effect of hydrogen on the metal. NMR measurements of hydrogen have been performed on Ti-Al-H alloys in order to ascertain how aluminum affects the behavior of hydrogen in titanium. NMR absorption curves and the temperature dependence of T 1 were obtained for various concentrations of hydrogen in titanium-aluminum alloys representative of the random and ordered phases of the Ti-Al system. The results are compared to those of hydrogen in aluminum free titanium and show that the hydrogen is located in three different crystallographic environments having different diffusion characteristics. Heat treatment seems to shift the hydrogen sites and a tentative crystallographic model is presented to explain our results. The role of aluminum on the inhibition of hydrogen embrittlement of titanium is discussed and our data indicates that heat treatment may be beneficial in alleviating this problem. It is also shown how the techniques applied in this study can be used to find quantitatively the optimum heat treatment and alloy concentration for reducing hydrogen embrittlement.

Probability-plot comparison of crystallographically independent molecules

Abstract : Molecules may be described in terms of chemical coordinates (bond lengths, bond angles and torsion angles) or bonded and non-bonded interatomic distances, as well as by crystallographic coordinates. For studies of molecular geometry, the former is more generally useful. Since random errors in interatomic distances should be normally distributed, they may be used in probability-plot analyses of differences in molecular geometry. Several examples of such analyses are discussed. The method is shown to be applicable to entire molecules or molecular fragments, even when the chemical or crystallographic environment is different. The technique is sensitive to minor changes in conformation which might otherwise be overlooked. (Author)

Relationship among structure factors due to identical molecules in different crystallographic environments

Relationship among structure factors due to identical molecules in different crystallographic environments

Lattice spacings and colour in the system alumina-chromic oxide

Precision lattice spacing determinations have been made on well crystallized specimens throughout the system, and the diffuse reflectivity of the samples measured. There is no anomaly at 8 mol per cent Cr 2O 3 as found by earlier workers, the change in both the colour and the crystal field surrounding the chromium ions being continuous and smooth, though changes in the crystallographic environment of these ions cannot be followed in detail. Atomic parameters have been obtained for pure chromic oxide, revealing that the structure is rather less distorted than the similar structures of α-alumina and α-Fe 2O 3.