6-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) - A CONTINUOUS CUBIC PYROCHLORE TYPE SOLID-SOLUTION(X (0)

Thallium metaniobate, Tl2Nb2O6, belongs to the cubic pyrochlore struct ural type (space group Fd3m). It is demonstrated by TGA, chemical anal ysis, and X-ray thermodiffractometry that it gives a continuous solid solution Tl2Nb2O6+x (0 less than or equal to x less than or equal to 1 ) under oxidation; the cubic pyrochlore structure is maintained with a continuous decreasing unit cell edge: x oxide ions being progressivel y incorporated into the network while x Tl3+ ions substitute to x Tlones. Rietveld refinements of X-ray powder diffraction data reveal tha t, in the classical model of the pyrochlore structure, Tl+ and Tl3+ ar e disordered on half of the 32e positions for 0 less than or equal to x < 0.5 and on the 16d ones for 0.5 < x less than or equal to 1. Bond valence calculations are in good agreement with the decrease in the sp litting of Tl cations from 32e to 16d positions when x increases, prov ided that a correlation exists between oxygen insertion and the Tl3+ d istribution: each extra oxygen O' oxidizes one of its closest Tl neigh bors to +3 while the three remaining monovalent Tl cations are repelle d. (C) 1995 Academic Press, Inc.