TL2NB2O6+X (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) - A CONTINUOUS CUBIC PYROCHLORE TYPE SOLID-SOLUTION

Thallium metaniobate, Tl2Nb2O6, belongs to the cubic pyrochlore structural type (space group Fd3m). It is demonstrated by TGA, chemical analysis, 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 progressively incorporated into the network while x Tl3+ ions substitute to x Tl+ ones. Rietveld refinements of X-ray powder diffraction data reveal that, in the classical model of the pyrochlore structure, Tl+ and Tl3+ are 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 splitting of Tl cations from 32e to 16d positions when x increases, provided that a correlation exists between oxygen insertion and the Tl3+ distribution: each extra oxygen O' oxidizes one of its closest Tl neighbors to +3 while the three remaining monovalent Tl cations are repelled. (C) 1995 Academic Press, Inc.