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International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. B. ch. 4.2, pp. 435-436
Section 4.2.4.4.9. Comparison between X-ray and neutron methods |
Apart from experimental arguments in favour of either method, there are some specific points which should be mentioned in this context. The diffuse scattering in question must be separated from Bragg scattering and from other diffuse-scattering contributions. Generally both methods are complementary: neutrons are preferable in cases where X-rays show only a small scattering contrast: (heavy) metal hydrides, oxides, carbides, Al–Mg distribution etc. In favourable cases it is possible to suppress (nuclear) Bragg scattering of neutrons when isotopes are used so that ![[\sum_{\nu} c_{\nu} f_{\nu} = 0]](/teximages/bach4o2/bach4o2fi527.svg)
for all equivalent positions. Another way to separate Bragg peaks is to record the diffuse intensity, if possible, at low ![[|{\bf H}|]](/teximages/bach3o4/bach3o4fi45.svg)
values. This can be achieved either by measurement at low θ angles or by using long wavelengths. For reasons of absorption the latter point is the domain of neutron scattering. Exceeding the Bragg cut-off, Bragg scattering is ruled out. In this way `diffuse' background owing to multiple Bragg scattering is avoided. Other diffuse-scattering contributions which increase with the value are thus also minimized: thermal diffuse scattering (TDS) and scattering due to long-range static displacements. On the other hand, lattice distortions, Huang scattering, … should be measured at large values of . TDS can be separated by purely elastic neutron methods within the limits given by the energy resolution of an instrument. This technique is of particular importance at higher temperatures where TDS becomes remarkably strong. Neutron scattering is a good tool only in cases where (isotope/spin-)incoherent scattering is not too strong. In the case of magnetic materials confusion with paramagnetic diffuse scattering could occur. This is also important when electrons are trapped by defects which themselves act as paramagnetic centres.
As mentioned in Section 4.2.4.4.4![[link]](/graphics/greenarr.gif)
the evaluations of the γ, δ, ɛ depend on the assumption that the f's do not depend on strongly within the range of measurement. Owing to the atomic form factor, this is not always well approximated in the X-ray case and is one of the main sources of error in the determination of the short-range-order parameters.
