RE-EVALUATION OF THE CRYSTAL STRUCTURE OF SELECTED OXIDE COMPOUNDS.

The interatomic distances in selected oxide compounds were re-evaluated using the most recent crystal chemical data available. The relationships between the crystal structure, lattice parameters, ionic radii, interionic distances, coordination number, and bond angles were determined for selected oxides. The primary effort was to determine the best value for the ionic radius of O2- at six-fold coordination. The three major factors considered were the effects of covalency as a function of the electronegativity differences, the coordination number, and the structural defects. Discrepancies in the data and the formation of solid solutions were also considered. The literature was studied for a total of eighteen oxide compounds having the general formula types AX, A2X, AX2, A2X3, and ABX3. From the data compiled, it was concluded that the most consistent O2- radius value for the compounds studied was 1.46 A at six-fold coordination. In all of the compounds studied, the nature of the bond was never purely ionic. However, the higher the degree of ionic bonding, the closer the computed value of the O2- radius came to 1.46 A. Spinel and the inverse spinel crystal structures and various polymorphic phases of SiO2 were also studied. A translation of Some Fundamental Problems in Crystal Chemistry and Their Use in Mineralogy, presented in Zapaski Vsesoyuznoga Mineralogicheskogo Obshchestva, Vol. 84, No. 4, 1955, pp. 470-492, is included in this report as an appendix because of its relevance to this study.