Moore, Paul B. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.
Last reviewed:November 2019
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A mineral of composition CaF2 and an important structure type with space group Fm3m, cell edge = 0.546 nm. The fluorite structure type is written RX2, where the R cations are in cubic (eightfold) coordination by the X anions. Alternatively, the X atoms are tetrahedrally (fourfold) coordinated by the R atoms. The fluorite arrangement may occur when the ratio of r(R), the ionic radius of the R atoms, to r(X), the ionic radius of the X atoms, is equal to or greater than 0.73. The arrangement XR2 is termed the antifluorite arrangement. Here the anions are eight-coordinated and the cations are four-coordinated. Halides of larger divalent cations, oxides of larger quadrivalent (4+) cations, some intermetallic compounds, and XR2 alkali univalent oxides frequently belong to the fluorite structure type. Approximately 100 synthetic compounds are known to have this arrangement; distortions may result in similar arrangements but with lower symmetry, such as in ZrO2, which is monoclinic. A vast number of complex structures can be derived from the fluorite arrangement by ordering vacancies and different cations or anions over the fluorite atomic positions.
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