From RefleXions Spring 2014
T. W. J. M. (Ted) Janssen.
Aloysio Janner.
Aloysio Janner and T. W. J. M. (Ted) Janssen, Emeriti Professors at U. of Nijmegen, The Netherlands, have been awarded the tenth Ewald Prize, 'for the development of superspace crystallography and its application to the analysis of aperiodic crystals'. The award will be presented on August 5, 2014, at the opening ceremony of the IUCr congress in Montreal, Canada.
Aperiodic crystals are well ordered materials that lack translational lattice periodicity. One of the most famous examples of an aperiodic crystal is calaverite, a gold telluride mineral discovered in the gold mines of California. At the beginning of the 1900's, mineralogists had noticed that the morphology of calaverite's crystal faces did not follow the accepted empirical rule of crystal growth (Haüy's law of rational indices). The diffraction patterns of calaverite also departed from those of a perfect crystal, then defined as an ordered and periodic arrangement of atoms in space. De Wolff linked these spots to the absence of lattice periodicity in Na2CO3 crystals. He described these crystals as 'incommensurately modulated structures' - they are superimpositions of basic periodic structures, with a symmetry that can be described by a three-dimensional space group. Janner and Janssen, who were investigating the relationship between symmetry and physical properties in condensed matter, began collaboration with de Wolff. Together they conceived and developed the 'super space' theory, publishing the first complete list of (3+1)-dimensional super space groups in 1981. De Wolff, Janner and Janssen shared the 1998 Gregori Aminoff Prize awarded by the Royal Swedish Academy of Sciences.
Janner and Janssen have dedicated more than thirty years to expanding the theoretical treatment of aperiodic crystals. They laid the groundwork for the development of methods to solve and refine the structure of aperiodic crystals, with applications from condensed matter physics to structural biology. Their superspace formalism also applies to the analysis of quasicrystals, a specific class of aperiodic crystals, described in 1982 by Schechtman who won the 2011 Nobel Prize in Chemistry for the discovery. The award of the Ewald Prize to Janner and Janssen highlights the tremendous impact that their work has had on the development of crystallography.