Effect of H/D Substitution in Powder Samples

The first figure shows the powder neutron diffraction data on HBr phase Ib (collected in 1976 on D1A at the ILL, Grenoble). The majority of the very high background is due to the incoherent scattering of the hydrogen in HBr. The remainder of the background is attributable to the quartz-glass sample holder and dynamic disorder in the sample. The data would probably be unusable but for the fact that crystal structure at this temperature is cubic: the relatively weak Bragg scattering is therefore concentrated into just a few peaks. You might wonder why data was collected on a protonated sample in this instance given the problems of the incoherent scattering of HBr. The answer is that the equivalent phase does not exist for DBr due to the isotopic effect on the phase transition temperature.


For comparison, the figure below shows the powder diffraction data of deuterium iodide, DI, phase II. The peak to background ratio is very much improved in comparison with the top figure, but is not zero. The background will have appreciable components as above from the quartz-glass sample holder, and dynamic disorder in the sample. In addition, there will be a noticeable contribution even from the deuterium in the sample.

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© Copyright 1997-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft