# PETER DEBYE (1884 - 1966) and PAUL SCHERRER (1890 - 1969). *Interferenzen an Regellos Orientierten Teilchen im Röntgenlicht*. (*Nachrichten von der Königl. Gesellschaft der Wissenschaften zu Göttingen. Mathematisch-Physikalische Klasse* **1916**, *29*, 1-15.)

This discovery of the technique of powder X-ray diffraction grew out of work conducted between 1915 and 1917 by Scherrer and his Ph.D. supervisor, Debye, at the University of Göttingen. After Scherrer received his doctorate in 1916, he continued to work on powder X-ray diffraction, especially an investigation of the effect of limited particle size on X-ray diffraction patterns. Scherrer showed that smaller particles gave broader diffraction beams, and he embodied this relationship in an equation now known as the Scherrer equation. His paper on the effect of crystallite size on the width of X-ray diffraction peaks appeared in 1918.

Scherrer derived his equation for the ideal condition of a perfectly parallel, infinitely narrow and monochromatic X-ray beam incident on a powder of cube-shaped crystallites all of the same size. He then considered the case in which the crystals adopted shapes other than cubes, and modified his equation to include numerical factor K that depended on the crystallite shape.

All subsequent improvements to Scherrer's original equation have essentially been refinements to the numerical factor K arising from more detailed analysis of the instrumental and non-instrumental factors that lead to broadening of the diffracted X-ray beams.

When the Swiss Institute for Nuclear Research and the Federal Institute for Reactor Research were merged in 1988, the new institute was named the Paul Scherrer Institute, in recognition of his outstanding contributions to science. Debye is remembered, for example, through the Debye equations that describe frequency-dependent effects in dielectric materials, and also for the Debye theory of the specific heat capacity of solids.

Holzwarth, Uwe and Gibson, Neil. &qThe Scherrer Equation versus the 'Debye-Scherrer Equation'," *Nature Nanotechnology* **2011**, *6*, 534.