Milestone 15: Powder Struggle

MILESTONES

MILESTONE 15 more comprehensive description of the procedure now known as the Rietveld refinement method. The 1969 paper is specifically about neutron diffraction Powder struggle but it speculates about extending the method to X-ray powder diffraction, “The powder method has gained a new which would eventually be achieved importance in neutron diffraction owing almost a decade later, and is still widely to the general lack of large specimens for AAAS used today. single-crystal methods”. With this brief The secret of the method’s longevity opening sentence of his landmark paper and widespread use is its strong in 1969, Hugo Rietveld had explained computational nature. With time, very clearly the scenario of solid-state ever-growing computational power crystallography at the time. has allowed increasing amounts of In a powder diffraction experiment, information to be obtained and analysed, the sample is composed of a large well beyond the simple structure of a number of randomly oriented crystalline material. The method is widely used in grains, so that the peaks from scattering metallurgy, mineralogy, forensic science, by diffraction of all lattice planes can archaeology, condensed-matter physics, in principle be detected at the same and the biological and pharmaceutical time. The technique originated several Powder diffraction pattern of a sample of the soil sciences, and it can provide information decades before Rietveld’s work, of Mars, collected by the Curiosity rover. Figure on quantitative phase analysis, strain when Peter Debye and Paul Scherrer reprinted with permission from D. L. Bish et al. and defect distribution. The technique Science 341, 6153 (2013). performed X-ray diffraction experiments also played a central role in the recent that led them to discover the structure of only powder samples existed. He geochemical analyses carried out by the graphite in 1916 (Milestone 4). Around had experience in using computer Curiosity rover on the surface of Mars. 30 years later, the first neutron powder software to model diffraction patterns As Rietveld himself put it in 2002, “I diffraction experiments appeared. from single-crystalline samples, and am totally amazed looking at the ever Although it had proved useful to solve he set off to extend his previous work increasing use that is being made of the relatively simple structures, by the to powder diffraction patterns. The method. […] What began as a solution 1960s it was realized that powder software he generated used some of for a particular problem, turned out to be diffraction was unwieldy to use with the parameters that could be extracted a tool of much broader value.” structures of increasing complexity. The from the actual data, like the peaks’ Fabio Pulizzi, Chief Editor, presence of multiple phases, different positions, intensities and widths, to Nature Nanotechnology

grain sizes, and reflections due to the calculate a theoretical diffraction ORIGINAL RESEARCH PAPERS Rietveld, H. M. experimental conditions, for example profile. This was used as a starting Line profiles of neutron powder-diffraction the sample or detector geometry, point for an iterative least-squares fit to peaks for structure refinement. Acta Crystallogr. were generating a large number of 22, 151–152 (1967) | Rietveld, H. M. A profile the measured spectrum. In Rietveld’s refinement method for nuclear and magnetic overlapping and intermixing peaks that method, a curve-fitting procedure structures. J. Appl. Crystallogr. 2, 65–71 (1969) were difficult to separate. for the whole diffraction pattern FURTHER READING Cheetham, A. K. et al. Hugo Rietveld was a crystallographer replaces a comparison of individual Crystal structure determination by powder neutron diffraction at the spallation neutron at the Netherlands Energy Research peak intensities. source, ISIS. Nature 320, 46–48 (1986) | Foundation, and one of his tasks was The first concept and results were Dinnebier, R. (ed.) International Union of to unveil the structures of possible published in 1967, and a second Crystallography Commission on Powder Diffraction Newsletter 26 (IUCR, 2001) fuels for nuclear reactors, for which publication two years later provided a

NATURE MILESTONES | CRYSTALLOGRAPHY AUGUST 2014