Structure of Mechanochemical Reaction Products in the Systems PbO–Sb2O5and PbO–Sb2O3

Mechanochemical reactions in 2PbO + Sb2O5and 2PbO + Sb2O3oxide mixtures treated in a high-energy planetary mill were studied by x-ray diffraction. The reactions were found to lead to rapid formation of crystalline products with the pyrochlore and fluorite structures, in line with the reaction-zone model. The temperature ranges of the structural and chemical transformations involved were established, and the compositions of the resulting phases were accurately determined. Heating in air leads to the reduction of the sample containing the pyrochlore phase, with the formation of one crystalline product having a variable lattice parameter because of the chemical inhomogeneity of the material. Under the same conditions, the sample containing the fluorite phase oxidizes in air, yielding four phases with different Pb : Sb ratios: PbSb2O6 , pyrochlore, a new compound Pb1/3Sb2/3O2with the fluorite structure, and Pb3Sb2O8 . The Pb : Sb ratio is found to vary over a wide range, from 0.5 to 1.5. The morphology of the powders produced by mechanical processing at above-threshold rates is considered in terms of nonequilibrium thermodynamics: Since the powder is a dissipative structure, its inhomogeneity cannot be fully eliminated, which correlates with the formation of an intermediate, dynamic state during mechanochemical reactions.