> when the superconductor craze hit in the mid-1980s... it was recognized the Rietveld Method was useful only for REFINING structures which were generally accepted from single crystal x-ray analysis.
That is an interesting observation, and it's a pity you didn't send it to the whole list because people will have different opinions. Certainly Rietveld designed his program to refine a known model structure, but today people use Pawley (1980) and LeBail (2005) methods to determine unknown structures. These later methods use profile refinement, but cannot really be called the "Rietveld method" because no previous structure model is refined. But even with Rietveld's program, it is possible to choose between different structure models, so "structure determination" is correct. Your superconductor example is a good one. From single crystal x-ray analysis, it was concluded that the "100K superconductor" YBa2Cu3O9-x was a defect perovskite structure, similar to Muller's original La2CuO4. This was important because Muller believed that CuO6 co-ordination in the perovskite structure was essential for superconductivity. But neutron powder profile refinement immediately claimed that there were no CuO6 octahedra. For a while there was conflict, with Muller himself publishing his "alternative structure" <https://link.springer.com/article/10.1007/BF01307250> containing CuO6 octahedra. Muller's paper is open access and well worth reading, along with the citations therein, because it gives some idea of the impact of powder diffraction on one of the most important topics of the day... and how even Nobel laureates can be wrong :-) 1987 was the year when neutron powder profile refinement became almost the only crystallographic technique of interest in Physics, and when it started to be taken seriously by the crystallography community. And don't worry if the Rietveld list discussion is sometimes conflictual. “*If necessity is the mother of invention, conflict is its father*.” Alan ______________________________________________ * Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE * <alan.he...@neutronoptics.com> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________
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