Remembering Hartmut Bärnighausen: Architect of the Group–Subgroup Approach in Crystallography

Hartmut Bärnighausen was born in February 1933 in Chemnitz, Saxony, Germany. He studied chemistry at the University of Leipzig, which was in the German Democratic Republic at that time. In 1958, before the Berlin Wall was built, like many people of his age, he fled from the communist regime and settled in Freiburg in West Germany. He obtained his doctorate in solid-state chemistry in 1959 at the University of Freiburg, working in Georg Brauer’s group. He continued in that group for post-doctoral research. In 1967, he was appointed as professor of Inorganic Chemistry at the University of Karlsruhe, where he remained until his retirement in 1998.

From the outset, his research was oriented towards the synthesis and properties of compounds of the rare earths in uncommon states of oxidation, such as LiEu₃O₄, Sm₃S₄, and the ‘Vernier structures’ Ln_xCl_2x+1 (e.g. Eu₅Cl₁₁, Dy₅Cl₁₁, Tm₆Cl₁₄, Tm₇Cl₁₅). The crystal structures of all of these compounds were determined by X-ray diffraction, even at times when this still was a time-consuming task.

When Bärnighausen began with his X-ray studies, he sought support at the Department of Crystallography at the University of Freiburg, where he came to know Hans Wondratschek, who had begun to tabulate the subgroups of all space groups. That huge, purely theoretical task kept Wondratschek busy for 35 years. Bärnighausen became familiar with this group-theoretical aspect of crystalline matter and realized that this is most useful in crystal chemistry. He developed the concept of relating crystal structures by trees of group–subgroup relations, now called Bärnighausen trees. He published this in his seminal paper in MATCH (Bärnighausen, 1980) (see figure below). 

Bärnighausen family tree for perovskite structures. Reproduced from Bärnighausen (1980).

In addition, the concept was propagated in courses that were taught repeatedly in Germany, Italy, France, Czechia, Bulgaria, Russia and South Africa. In the beginning, Bärnighausen and Wondratschek were the main lecturers for these courses; later, numerous other colleagues followed.

Bärnighausen’s scripts for these courses were the basis for the book Symmetry relationships between crystal structures by Müller & de la Flor (2024).

The group–subgroup relations were also incorporated into the continuing experimental work, often in cooperation with physicists. A central topic concerned phase transitions involving ferroelastic and ferroelectric materials such as CaCl₂, K₂ZnI₄, and CsCd(NO₂)₃.

Bärnighausen’s work was honoured by the German Society of Crystallography in 1997 by the awarding of the Carl-Hermann medal. His publications excel in conspicuous scientific and linguistic accuracy. Colleagues appreciated his expertise and critical attitude.

References

Bärnighausen, H. (1980). MATCH Commun. Math. Comput. Chem. 9, 139–175.

Müller, U. & de la Flor, G. (2024). Symmetry relationships between crystal structures, 2nd ed., IUCr Texts on Crystallography. Oxford University Press.