BONDING TRENDS IN PYRITES AND A REINVESTIGATION OF THE STRUCTURES OF PDAS2, PDSB2, PTSB2 AND PTBI2

PdAs2, PdSb2, PtSb2 and PtBi2 have been formed from reactions of 1-gram samples of the elements in quartz tubes at 923 K, and their crystal structures have been reinvestigated. These materials crystallize with the pyrite structure. The lattice constant, a, and the positional parameter, x, of the nonmetal atoms are the following: PdAs, 5.9790(3) angstrom, 0.38317(3); PdSb2 6.464(1) angstrom, 0.3733(1); PtSb2 6.4423(9) angstrom, 0.3753(2); PtBi2 6.7014(8) angstrom, 0.3710(1). The metal-nonmetal and the closest nonmetal-nonmetal bond lengths are the following: PdAs2, 2.4949(2), 2.4198(3) angstrom; PdSb2, 2.6765(9), 2.838(2) angstrom; PtSb2, 2.671(1), 2.782(2) angstrom; PtBi2, 2.7706(8), 2.995(2) angstrom. The platinum-group dipnictides show a strengthening of the nonmetal-nonmetal bond with heavier nonmetal atoms, as evidenced by a bond-valence analysis. These dumbbell bonds are compared to other observed dumbbells in crystals. Comparisons are made to the transition-metal pyrite disulfides, where electronic effects are correlated with the nonmetal-nonmetal bond strength. The valence-matching principle is shown to correlate well with the observed structures of the alkali metal superoxides and alkaline earth metal peroxides. Bond valence analysis for a hypothetical ''cubic'' MnO2 is given along a fluorite to pyrite pathway.