Siberian Federal University

The concentration stability region of magnesium- containing pyrochlores based on bismuth tantalate (Bi2+xMg1+yTa2-yO9-δ, –0.11≤x≤0.325, –0.01≤y≤0.20) was experimentally determined. On the basis of experimental data the conditions for the formation of mixed pyrochlores containing ions of divalent elements (Mg(II) type) in Ta(V) octahedral positions were defined. It was shown that phase- pure pyrochlores are formed in the case of a vacant, by 37–22.5 at.%, bismuth sublattice, and the degree of substitution by magnesium ions of cations of the Ta(V) octahedral sublattice is limited to the interval 33–40 %. It was demonstrated that magnesium(II) ions within 4–6 at.% are distributed in the bismuth position. It was found that the degree of substitution of the octahedral sublattice ions is not affected by the degree of the bismuth sublattice vacancy. It was shown that the condition of observing the permissible substitution of Ta(V) ions is equal to the requirement of achieving the average charge of B cations of the B2O6 octahedral sublattice to the value +4. Apparently, this is the second factor of pyrochlore structure preservation, equal to the tolerance factor, suitable for predicting the homogeneity region of mixed pyrochlores