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What is it about?
Since structural crystallography began in 1914, the sizes of atoms and ions have played a major role in solving crystal structures and systematizing many metric aspects of crystal structures and their properties, to the extent that Pauling’s rules, first published in 1929, are still taught in many introductory Chemistry and Mineralogy courses. There is a major contradiction between the sizes of ions/atoms in crystals (1) derived from mean experimental interatomic distances (ionic radii), and (2) derived from quantum-mechanical calculations of the electron density (bonded radii) that has persisted for nearly 50 years. Users of ionic radii persist as such radii have proved very useful, whereas (some) producers of bonded radii dismiss the results of using ionic radii as nonsense.
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Photo by Ayush Kumar on Unsplash
Why is it important?
These different views of the sizes of atoms/ions in crystals represent a major hiatus between theory and experiment, and obviously need to be resolved. There are two types of use of ion radii: (1) methods which use the relative sizes (radii) of cations and anions; (2) those methods which compare the radii of different cations (or the radii of different anions). Type (1) methods do not work (See Featured Image) and should not be used. Type (2) methods can be very precise and lead to useful and accurate predictions. This behaviour can be understood by recognizing that ionic radii (derived from experimental interatomic distances) are proxy variables for mean interatomic distance and do not represent real sizes of ions in crystals.
Perspectives
Recognition of the proxy nature of ionic/ion radii encouraged us to derive new radii for all ions in all their coordination numbers from ~180,000 observed bond lengths. Moreover, realization of the proxy nature of ionic radius will hopefully (1) defuse the arguments between the experimental and theoretical communities on who is correct; (2) stop work on tinkering with the values of ionic radii to get better agreement with Pauling’s first rule; (3) promote the realization that radii accurately order ions according to their size without knowing those sizes. Future work could involve (1) comparison of calculated bonded radii with radii measured from electron-density measurements on the same crystals; (2) attempts to systematize/understand physical properties of crystals using calculated bonded radii.
Frank Hawthorne
University of Manitoba
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Read the Original
This page is a summary of: New ion radii for oxides and oxysalts, fluorides, chlorides and nitrides, Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, July 2024, International Union of Crystallography,
DOI: 10.1107/s2052520624005080.
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