PREPARATION OF CUBIC NIOBIUM PYROPHOSPHATE CONTAINING NB(IV) AND TOPOTACTIC EXTRACTION OF PHOSPHORUS ATOMS

A reduced phase of niobium pyrophosphate containing Nb4+ has been prepared from the reaction of Nb6Cl14. 8H(2)O and phosphoric acid. The X-ray powder diffraction and electron diffraction studies have shown that the compound belongs to the Pa (3) over bar space group and has the ZrP2O7 structure with a cubic superstructure (a' 3a(0)) Magnetic susceptibility was measured for two samples, and the mean oxidation numbers of niobium in them are deduced to be +4.66 and +4.88. The cell constants of these samples are a 8.0830(4) and 8.0705(2) Angstrom, respectively. As the mean oxidation number of niobium increases, the color of the compound varies from brown to gray. When the compound is heated in oxygen, it changes into the known white niobium pyrophosphate, in which all niobium is in the +5 oxidation state. Rietveld refinements indicate that niobium pyrophosphates have defects in the phosphorus sites. The topotactic extraction of phosphorus atoms in the reaction with oxygen was confirmed by the analysis of phosphorus oxide generated during the reaction. (C) 1995 Academic Press, Inc