Ekhard Salje, FRS (1946–2025)

In March 2025, the annual meeting of the German Crystallographic Society took place at the Gottfried Wilhelm Leibniz University Hannover (LHU). I intended to give an entertaining laudatio for my PhD supervisor and dear friend Professor Ekhard Salje, whom Leibniz University had invited to receive an Honorary Doctorate. Sadly, this honour remained unfulfilled because Ekhard had passed away only two weeks earlier on 24th February 2025.

There are historical connections between Great Britain and Hannover (George I was King of Great Britain), and there are also important scientific links, for instance, between Leibniz and Newton. With a focus on Hannover and Cambridge, the most recent connection was established by the personal union of Professor Ekhard, who emerged from Leibniz University as a truly brilliant scientist.

Ekhard Salje was born in Hannover where he grew up near the royal gardens. He didn't particularly like his school days, but he blossomed when he began his studies at the University of Hannover. His academic career started in 1966 when Ekhard was initially interested in philosophy and maths. However, he finally earned a doctorate in physics. He completed his PhD thesis in 1972 under the supervision of Horst Welling in quantum optics. At that time, Heinrich Heesch was an outstanding group theoretician in mathematics in Hannover. Ekhard attended Heesch's lectures and began working on finite groups, which brought him continuously closer to crystallography. During his PhD thesis, Ekhard went to Zurich for some time in order to work in Alex Mueller's IBM laboratory. There, he met Ortwin Schirmer, with whom he published on polaronic systems. After his habilitation, Ekhard became a professor in 1975, and at the age of 29 years, he was the youngest professor at the LUH.

I was Ekhard's first student and, in 1976 (I was still a graduate student at the time), we published our first joint paper on the electro-optical effect of NH₄IO₃ and, later, on its structural phase transition.

When I first met Ekhard, towards the end of my mineralogy studies, he had just moved from the Institute of Quantum Physics to the Institute of Mineralogy and Crystallography; a little more trendy today, it is called the 'Inst. für Erdsystemwissenschaften, Abt. Mineralogie' (Institute for Earth System Sciences, section Mineralogy). Ekhard began to grow crystals from aqueous solution and worked on different physical effects of perovskites, iodates and tungstates. Each time, Ekhard tackled completely new scientific questions and needed practically no time to familiarise himself with new topics in geosciences. He had all the tensorial material properties for each symmetry immediately to hand and quickly recognised the relevance of the structure-property relationships even for less popular crystallographic space groups. Sometimes, physicists and chemists like to avoid triclinic crystal systems in order to not have to deal with their sort of nasty anisotropy, but Ekhard enjoyed playing with monoclinic and triclinic systems. Nobody in his working group could grasp the symmetry aspects and related properties anywhere near as quickly as Ekhard in order to strategically plan experimental investigations of samples in such a way that something fascinating emerged. In this way, he discovered high electro-optical coefficients of iodates, the electrochromatic blue colouring of pale green tungsten oxide crystals when electric fields were applied, or later, superconductivity in proton-doped domain-walls.

I enthusiastically took up Ekhard's suggestion to grow phosphate-arsenate crystals and to investigate their ferroelastic properties in my doctoral thesis, which was thematically not so far from relaxor materials that later became highly successful and which are encountered today in USB sticks or SSD memories. However, the crystals resulting from the syntheses were so fantastic that we published many years together on their ferroelasticity. Ekhard's ability to formulate texts was also impressive. When writing a manuscript, he had all the results in his memory and usually formulated the text without a break in just a few hours. It was practically never necessary to modify anything afterward. While I was tweaking my texts, Ekhard's were extremely accurate and ready for print right from the start.

In 1980, Ekhard went to Paris to work in Jacques Friedel's group. Another outstanding professor there was Pierre-Gilles de Gennes working in the field of polymers and soft-matter physics. Both met up again later in England. This was the time when Ekhard again focussed on elastic interactions. Theoretically, these are so-called non-local effects, and experimentally it was Raman spectroscopy and X-ray diffraction that interested him in this context. Ekhard learned as much as possible about Raman spectroscopy in a crash course from Gerhard Schaak in Darmstadt, who later worked at the Faculty of Physics and Astronomy in Würzburg.

In Hannover, there was no mineral physics outside of crystallography, and X-ray diffraction only existed in a rudimentary form in the institute. Hence, Ekhard had to do all his diffraction experiments in Göttingen (the crystallographer there was Vladimir Kupcik). This meant that Ekhard drove his car to Göttingen overnight, did the measurements in the morning and drove back the following night. Ekhard was also a member of the university senate at the time, and reforms were almost constantly on the agenda. Often, we met after midnight in the laboratory after his senate meetings, Ekhard exhausted from the never-ending discussions in the senate from over-committed reformers and me exhausted from growing ferroelastic crystals using the Czochralski technique. It was an intensive period of night-time activities, as evidenced by the rims around our eyes.

When he was in Paris, Ekhard had met Neville Mott. Neville Mott and Jacques Friedel had a family connection, so Neville often came to Paris and asked Ekhard if he would like to come to Cambridge. At that time, Ekhard actually had other plans. However, in 1984 he met Michael Carpenter and Volker Heine from Cambridge who visited him in Hannover. I was present when Heine entered the room. It was a freezing cold winter day. Heine was impressive with his Hawaii shirt, his clogs and his thick winter coat torn open at one side. Heine and Carpenter had a similar tempting plan: if Ekhard came to Cambridge, he could choose the institutes in which he wanted to work. He would also have very few lectures and almost no administration. The latter aspect was highly convincing so he chose the Earth Sciences and Physics departments in Cambridge. In Physics at the Cavendish Laboratory, Ekhard continued with Raman spectroscopy and mainly the theory of 'pattern formation'. In Earth Sciences, he used electron microscopy and diffraction and focused on his main topic 'structural phase transitions'. He worked on theory with Volker Heine and experimentally with Michael Carpenter. Together they published around 300 papers and they clarified the kinetics of feldspars, which previous generations of geoscientists had cut their teeth on. Ekhard also published several books. One of his most widely used textbooks is entitled Phase Transitions in Ferroelastic and Co-elastic Crystals, first published in 1990.

Ekhard writing in his garden in Cambridge (ca. 1986).

Ekhard was not only an outstanding scientist but also an exceptional teacher. His lectures and talks were at a demanding level, but at the same time, he was able to convey the content in an incredibly clear and enthusiastic way, and he attracted a huge number of PhD students, PostDocs and guest scientists to join his group.

Ekhard received offers from Princeton, ETH Zurich and various German universities. He became a Fellow of the Royal Society and received several prizes, including the AvH Research Prize. He also became Fellow of the Royal Society of Arts, was appointed to the Leopoldina, and received the Schlumberger Medal and the DMG Agricola Medal. In France, Ekhard became 'Chevallier dans ordre des Palmes Academiques' (the order was created by Napoleon Bonaparte in 1808). In China, he became an Honorary Professor of the Jiao-Tong University in 2012. In Germany, Ekhard was awarded the Federal Cross of Merit First Class, the Werner Heisenberg medal in 2017 and an honorary PhD at the University of Wuerzburg in 2019. The number of his memberships of international committees and on review panels is too long to be listed here.

However, even at universities such as Cambridge, top academics are often favoured with self-administration and Ekhard's innocent academic life came to an abrupt end when he became Head of the Earth Science Department at the University of Cambridge in 1987. He was then responsible for the administration and, in particular, the research and teaching strategy of the department. In 2001, Ekhard was appointed Master of the College 'Clare Hall' in Cambridge. Incidentally, Ekhard was the first German to hold the position of a Master (i.e. President) of a College in Cambridge. The appointment period at the College was 7 years. He initiated numerous new international co-operations at Clare Hall. For example, ca. 50 visiting professorships for various disciplines can be traced back to his initiative. He and his wife Lisa also started music and art events, which are still very popular in Clare Hall today. In fact, Ekhard was also passionate about painting, and at their home in Cambridge there are plenty of his own oil paintings on the walls. He also initiated intensive collaborations with Asian research centres in Japan, China and Korea. By 2008, Ekhard had fulfilled all his administrative duties and returned to research.

Over the last 20 years, Ekhard not only worked on the mathematically correct and physical description of microstructures, as well as on phase transformations and the thermodynamic stability of minerals, but also on the mechanical stability of minerals, metals and biominerals. Ekhard set up laboratories for this purpose, some of them in China with colleagues in Xi'an and Chengdu. In addition, he developed an acoustic examination method with colleagues in Cambridge. Samples were slightly compressed to create microstructures, which then emitted acoustic signals. This 'crackling noise' spectroscopy was further developed with Trudy Krieven on porous minerals. Today, data-intensive measurement protocols are used that make it possible to recognise mechanical instabilities 'in situ', for instance, in mechanical structures such as bridges or bicycle frames at an early stage and before potential damage occurs. Ekhard published over 700 papers, and his PhD students remember his motto: 'One can steal everything from you but nobody can steal your knowledge'.

Ekhard was a true bridge-builder of international cooperation who was extraordinarily successful in the search for laws in mineral physics and their geoscientific contexts. His sound judgement and the curiosity that guided his outstanding research will be missed by the mineral physics and crystallography community, and his friends all over the world.

Editor’s note: Ekhard was a personal friend of mine for over 45 years, ever since I discovered that he and I were working on the same topic while he was in Hannover. He was not only a superb scientist, as Ulli Bismayer explains, but he also had a great sense of humour. I recall visiting him at his home in Cambridge once for lunch to see his back garden full of guinea-pigs running freely everywhere. He explained that it saved him from having to mow the lawn!