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

A reduced phase of niobium pyrophosphate containing Nb4+ has been prep ared from the reaction of Nb6Cl14. 8H(2)O and phosphoric acid. The X-r ay powder diffraction and electron diffraction studies have shown that the compound belongs to the Pa (3) over bar space group and has the Z rP2O7 structure with a cubic superstructure (a' 3a(0)) Magnetic suscep tibility 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 constan ts of these samples are a 8.0830(4) and 8.0705(2) Angstrom, respective ly. As the mean oxidation number of niobium increases, the color of th e compound varies from brown to gray. When the compound is heated in o xygen, it changes into the known white niobium pyrophosphate, in which all niobium is in the +5 oxidation state. Rietveld refinements indica te that niobium pyrophosphates have defects in the phosphorus sites. T he topotactic extraction of phosphorus atoms in the reaction with oxyg en was confirmed by the analysis of phosphorus oxide generated during the reaction. (C) 1995 Academic Press, Inc