Non-Stoichiometry in Fluorite Structures

  • L. E. J. Roberts


The ideal fluorite structure of CaF2 may be thought of as a simple cubic array of anions with a cation in the centre of each alternate cube, with a coordination number of eight. It has long been recognized that the resulting face-centred cubic structure of cations is essentially preserved over wide variations of the overall composition from MX2. The evidence is of two types. Oxides of transition metals having this structure—the actinide and rare earth oxides—exist with compositions of MO1 · 6 to MO2 · 5, and single phase rare-earth fluorides from MO2 · 0 to MO2 · 25 at least; secondly, both oxides and fluorides readily form ‘anomalous’ solid solutions with compounds of cations of different valency, thus necessarily disturbing the cation: anion ratio of the crystal as a whole, while preserving statistically the fluorite structure, or a structure formed from fluorite with a small distortion.


Uranium Dioxide Fluorite Structure Interstitial Position Oxidation Number Neutron Diffraction Pattern 
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© A. J. Downs, D. A. Long, L. A. K. Staveley 1971

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  • L. E. J. Roberts

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