Rietveld on Rietveld

Thirty-five years ago in the search for mechanically stable fuel element for nuclear reactors, the structure of some uranates had to be determined. Because of the small crystal size only powder diffraction data were available. This restriction was an immense obstacle, because in those days only single crystal diffraction data could lead to a successful refinement. Powder diffraction was regarded to be obsolete for refining structures, because of the problem of overlap with low symmetry compounds and at high diffraction angles. Step scanning yielded a powder diagram that showed definite detail in the profile of these overlapping peaks. The information contained in this detail could however not be extracted by conventional means and one had to resort to taking the sum of these overlapping peaks as data for the least squares refinement. It was therefore clear that full use was not made of the information contained in the powder diagram.

My experience with computers, built up over the years doing single crystal diffraction work, gave me a head start in tackling this problem of extracting the maximum amount of information from a powder diagram. My efforts ultimately led to what is now known as the Rietveld Method. When the method was first reported in Moscow in 1966 at the IUCr Congress, there was hardly any response. A few years later, when I wrote a more comprehensive computer program, first in Algol 60 and later in Fortran IV, the demand for the program and its subsequent use increased, and the popularity of the Rietveld Method grew. I already had foreseen that it could be used for X-ray data, but due to the lack of an actual problem, I did not pursue it any further.

I am totally amazed looking at the ever-increasing use that is being made of the method. Yearly, hundreds of publications use or refer to the method. What began as a solution for a particular problem, turned out to be a tool of much broader value. Now it is not only applied to structure refinement, but also to such diverse fields as quantitive phase analysis, measurement of strain and size and the analysis of time-resolved data. I am deeply gratified that I could have been instrumental in reviving the powder diffraction method. This in itself would have given me enough satisfaction. Receiving the Gregori Aminoff Prize in 1995 from The Royal Swedish Academy of Sciences for my work gave an added luster.