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Paul Niggli was a Swiss crystallographer who was a leader in the field of X-ray crystallography.
Niggli was born in Zofingen and studied at the Swiss Federal Institute of Technology in Zurich and the University of Zurich, where he obtained a doctorate. His 1919 book, Geometrishe Kristillographie des Diskontinuums, played a seminal role in the refinement of space group theory. In this book, Niggli demonstrated that although X-ray reflection conditions do not always uniquely determine the space group to which a crystal belongs, they do reveal a small number of possible space groups to which it could belong. Niggli used morphological methods to account for internal structure and, in his 1928 Kristallographische und Strukturtheoretische Grundbegriffe, he took up what is essentially the reverse process, the task of establishing the connection between space lattices and external crystal morphology. The great aim of his life was to integrate the whole field of Earth sciences.
In 1920, Niggli became the lead scientist at the Institut für Mineralogie und Petrographie, where he brought his systematic approach to the study of crystal morphologies using X-ray diffraction. In 1935, Niggli and his doctoral student Werner Nowacki determined the 73 three-dimensional arithmetic crystal classes. Niggli retired from the Institute in 1949. He was also professor of mineralogy at the Eidgenössische Technische Hochschule and at the University of Zurich.
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