Na//2Ca//3Al//2F//1//4: A NEW EXAMPLE OF A STRUCTURE WITH 'INDEPENDENT F** minus ' - A NEW METHOD OF COMPARISON BETWEEN FLUORIDES AND OXIDES OF DIFFERENT FORMULA.

Article abstract of DOI:10.1016/0022-4596(88)90239-3

Na//2Ca//3Al//2F//1//4is cubic. The absolute configuration was determined from 1412 X-ray independent reflections. The structure contains isolated left bracket AlF//6 left bracket **3** minus octahedra and an independent fluoride ion, F//3, linked to three Ca**2** plus and one Na** plus ions. The tetrahedra left bracket FCa//3Na right bracket **6** plus linked by Ca**2** plus ions, build up a three-dimensional network in which the octahedral entities left bracket AlF//6 right bracket **3** minus are inserted. A structural analysis leading to a comparison with oxide structures is given for compounds containing 'independent' fluoride ions and isolated left bracket MF//6 right bracket octahedra, despite different chemical formulations for the fluorides and the corresponding oxides.