Theo Hahn (1928-2016)

excerpted from Acta Cryst. (2016), 72, 294-295
doi: 10.1107/S2053273316004770

The mineralogist and crystallographer Theo Hahn died in Aachen on 12 February, 2016, six weeks after his 88th birthday. He was born in Duisburg, Germany, in 1928. He experienced the end of World War II at the age of 17 and started studying mineralogy and crystallography at the Universities of Marburg (as a student of Carl Hermann) and Frankfurt/Main in 1946. There he obtained a doctoral degree (Dr. rer. nat.) at the age of 24. The title of his dissertation was Fluoberyllates as Model Compounds of Silicates. This formed the basis for one of his early research fields, the study of 'stuffed' tetrahedral-framework structures, which he considered as models for silicates.

Theo worked as a postdoc for Martin J. Buerger at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. There, and at the neighbouring Harvard U., he came into contact with Sydney C. Abrahams, Norio Kato, Andrew R. Lang and Charles T. Prewitt. In Buerger's laboratory he learned single-crystal structure determination by X-ray diffraction and developed his love for symmetry in crystallography and art, and its group-theoretical treatment.

Returning to Germany (1956), Theo became an assistant to Herbert O'Daniel at the Mineralogical Inst. of the U. Frankfurt/Main. He devoted his attention to the application of computers and the development of programs for single-crystal structure determination. Theo worked on this topic for his Habilitation (Methods and Results of the Exact Crystal-Structure Research, Treated on Specific Examples), obtaining the Venia legendi for Crystallography and Mineralogy in 1960.

At the age of 35, Theo was appointed full Professor and Chair for Crystallography of the Technical U. of Aachen (RWTH). He was director of this 'Institut für Kristallographie' until his retirement in 1993. He was also Dean of the Faculty of Mining, Metallurgy and Geosciences of RWTH Aachen, Chair of the German Mineralogical Society, and consultant and reviewer for the German Research Foundation.

Theo received several honours for his scientific work, including the Abraham Gottlob Werner Medal in Silver of the German Mineralogical Society (1997) and, with Hans Wondratschek, the Carl Hermann Medal of the German Association for Crystallography (2001). He was also an Honorary Member (from 1997) of the German Association for Crystallography.

Theo used X-ray and neutron diffraction methods for crystal powders and single crystals. Under his management a very effective neutron research group (U. Triple-Axes Spectrometer UNIDAS for Inelastic Neutron Scattering), affiliated to the Institut für Kristallographie, RWTH Aachen, and led by Götz Eckold, was established at the Nuclear Research Centre Jülich. Recently, his research concerned polymorphism, phase transformations and domain formation. He and I were invited to write a review article Twinning of Crystals for Volume D (Physical Properties of Crystals, 2004) of International Tables for Crystallography.

Hahn's greatest achievements for the worldwide crystallographic community started during the 1963 IUCr Congress in Rome, when a new Commission on International Tables was established with the aim of preparing and publishing a Pilot Issue in preparation for a future new edition of the first volume of International Tables for X-ray Crystallography. During the 1972 IUCr Congress in Kyoto, Theo was appointed as Chair of the revised Commission on International Tables with the task of editing a revised Volume A (Space-Group Symmetry) of International Tables for Crystallography (hereafter abbreviated as IT A). Theo was the author of several parts of IT A. It was more than ten years until, in 1983, the first edition of the volume went to press. He introduced numerous amendments and supplements in the following four editions. Five years after the appearance of the fifth edition of IT A in 2002 and after more than 35 years in the role, Theo retired as Editor of the volume at the age of 79. However, until the end of 2015 he was engaged with the revision and update of Chapter 3.2 of the Sixth Edition of IT A, which is edited by his successor Mois I. Aroyo and will appear soon.

Theo served the International Union of Crystallography for 40 years (since 1963) as Chair of the Commission on International Tables, as editor of IT A (five editions), as author of various contributions to IT A and IT D, as a member of the IUCr Executive Committee and as President of the IUCr (1984-1987). His long service for the crystallographic community, in particular the editions of the new International Tables Volume A, forms a major part of his lifetime achievements.

Theo organized, usually with his close friend Hans Wondratschek, about 15 international summer schools on crystal symmetries and space groups. A school in Sofia led to the publication of the book Symmetry of Crystals by Th. Hahn and H. Wondratschek (1994).

Theo Hahn was a creative scientist of high international reputation, and his obliging and charming nature made him a fascinating partner for discussion and conversation. The community of crystallographers worldwide will miss him as an inspiring teacher and a wonderful person. Theo is survived by his wife Jane, one son and three grandchildren. Our sincere condolences are with them.