Mapping high-pressure crystallography in a structural chemistry landscape

In this issue of Acta Crystallographica Section B, Michal Kaźmierczak and Ewa PatykKaźmierczak (2021) provide a comprehensive survey of high-pressure crystal structural depositions in the Cambridge Structural Database (Groom et al., 2016) and offer a valuable perspective on the status of high-pressure research within the field of crystal chemistry. For structural chemists, X-ray crystallographic techniques have been recognized as providing an extremely powerful method for the determination of the structure of matter and, specifically, the arrangement of atoms of a crystalline solid in three-dimensional space. The origin of the field comes from the theoretical work of Paul Ewald who, in collaboration with Max von Laue examined the propagation of X-rays through crystals. Encouraged by Laue, Walter Friedrich and Paul Knipping carried out an experiment where they shone a beam of X-rays at a crystal of zinc blende (ZnS) with a photographic film placed behind it to record the diffraction spots. The interpretation of the diffraction images was determined fully by Lawrence Bragg, who inferred that the diffraction events could be understood in terms of mirror-like reflections from planes within the crystal, which he formulated as the now very familiar ‘Bragg’s law’. For their work on translating the information recorded on diffraction images to crystal structure determination at atomic resolution Max von Laue and Lawrence Bragg (with his father William Bragg) won the Nobel Prize for Physics on consecutive years, 1914 and 1915 respectively (Woolfson, 2018). This brief period unlocked the use of X-ray crystal structure analysis for scientists working in disparate fields and by 1929 the output of the fledgling X-ray crystallography community was of sufficient volume for the founding of Strukturberichte, to provide a regularly published source of recent crystal structure determinations. Strukturberichte eventually became Structure Reports as an official publication of the International Union of Crystallography until the 1990s. The period also marked the evolution of X-ray crystallography and crystal structure analysis away from its origins in inorganic chemistry. Long-standing questions on the nature of chemical bonding and interactions in organic chemistry were addressed and the structures of a wide range of natural and synthesized molecules were determined. The biological and life sciences also embraced X-ray crystallography and went on to address several dauntingly complex challenges which, in turn, have revolutionized our understanding of life at the molecular level (Groom & Allen, 2014). From 1929 to the early 1960s, crystal structure compilations and references were print based, with both Strukturberichte and Structure Reports joined by several other publications, and by the late 1940s there were growing concerns about the plethora of sources for primary scientific material, which was dubbed ‘the information explosion’. In 1964, when computer-based systems became feasible, Olga Kennard was invited to create a ‘Crystallographic Data Centre’ with funding from the new UK Office for Scientific and Technical Information (OSTI) and by the following year she had established the Cambridge Crystallographic Data Centre (CCDC) at the University of Cambridge. The remit of CCDC was to establish a comprehensive and retrospective database of organic and metal–organic structures determined by both X-ray and neutron diffraction methods. As well as including the bibliographic, chemical and overall crystallographic information, it was, crucially, to include the three-dimensional atomic coordinate data. The resulting Cambridge Structural Database (CSD), the first fully electronic numerical data depository, was later joined by other databases in the early 1970s, such as the Inorganic Crystal ISSN 2052-5206