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The crystal structure of lead niobate Pb2.8Nb2O7.8 [a = 7.520(1) Å, b = 32.513(8) Å, c = 7.542(1) Å; Cmc21; Z = 10; ϱ = 7.96(12)g/cm3]1 is closely related to the pyrochlore type of Pb1.5Nb2O6.5. Sheets with the pyrochlore type structure parallel (010), displaced by (1/2,1/2,0) are interconnected by layers, which only contain lead atoms, sixfold coordinated by oxygen atoms. These lead atoms occupy the sites of niobium and lead atoms as they are present in the undistorted pyrochlore structure. But they have changed the oxygen coordination because of their position at a regularly spaced antiphase boundary of the two adjacent pyrochlore type sheets with an antiphase-vector c/2. The cation sites in this structure are fully occupied, but similar to Pb1.5Nb2O6.5 at one of the oxygen positions, which do not belong to the framework of NbO6-octahedra, half of the sites are vacant.
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