PROMOLECULE RADII FOR NITRIDES, OXIDES, AND SULFIDES - A COMPARISON WITH EFFECTIVE IONIC AND CRYSTAL RADII

Promolecule radii have been calculated for nitride, oxide, and sulfide coordinated polyhedra with bond lengths fixed at the sums of effectiv e ionic and crystal radii. Radii calculated for both nontransition and transition cations from the first four rows of the periodic table are highly correlated with crystal radii derived for oxide and sulfide cr ystals and with ionic radii derived for nitride crystals. Promolecule radii calculated for the coordination polyhedra in the minerals danbur ite, bromellite, and stishovite match experimentally determined bonded radii to within approximately 0.02 angstrom, on average, indicating t hat the position of the minimum in the electron density in the directi on of a bond is governed in large part by the length of the bond and t he electron density of the atoms that form the bond. Radii calculated for nitride, oxide, and sulfide anions correlate with bond type, tendi ng to match ionic radii when bonded to highly electropositive cations and atomic radii when bonded to highly electronegative cations.