The advantage of neutron powder diffraction is that the scattering power of light atoms like oxygen is as great as for heavy atoms; for X-rays, the heavy atoms dominate, so errors for the light atoms will be greater. A second advantage is that with neutrons you have a larger sample, so a better powder average is obtained and systematic errors will be generally smaller. You can also see magnetic structure, and low/high temperature measurements may be easier. The advantage of X-rays is that X-ray diffractometers are more common, you can use smaller samples, high pressures etc. Generally, if you can actually do the experiment using first class diffractometers with either X-rays and neutrons, the neutron structure will be better with few exceptions. Rietveld himself considered that his method was not so reliable for X-rays because of potential systematic error, though of course some progress has been made since then. Alan.
Davide Levy said: > Dear All, > > I have a philosophic question for you: the structure obtained by NPD are > better than the ones found by XRPD? > > I explain a little bit more my question, I studied a MgAlFeO4 spinel both > by > NDP and XRPD from RT to 1000°C. The O coordinate it is different for two > data set (0.2581 vs.0.25953). The found error on the oxygen coordinate is > about 10-5 and 10-4 for NPD and XRPD data respectively. The coordinate > calculated from cation partitioning and cation radii is similar to the > XRPD result. > > At the moment I did not found a justification of this and I do not know > which result is the right one. > > Do you have any suggestion? > > Thank you > > Davide ______________________________________________ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE <[EMAIL PROTECTED]> +33.476.98.41.68 http://www.NeutronOptics.com/ ______________________________________________