ATOMIC-STRUCTURE AND CRYSTAL-CHEMISTRY OF SILLENITES - BI12(BI-3+(0.50)FE-3+(0.50))O19.50 AND BI12(BI-3+(0.67)ZN-2+(0.33))O19.33

The atomic structures of single crystals of the sillenites Bi12(Bi0.50Fe0.50)O19.50 and Bi12(Bi0.67Zn0.33)-O19.33 have been determined at 293 K by neutron diffraction. Noticeable deviations from the ideal chemical formula Bi12MO20 have been found. [Fe3+O4] or [Zn2+O4] tetrahedra are replaced by umbrella-like [Bi3+O3] groups statistically oriented with respect to the threefold axes of a cubic cell. A previous supposition concerning the presence of Bi5+ cations in M tetrahedra is not confirmed. Bi12(Bi0.50(3+)Fe0.50(3+))O19.50, M(r) = 2952.2, cubic I23, a = 10.184 (9) angstrom, V = 1056 (3) angstrom 3, Z = 2, D(x) = 9.28 g cm-3, lambda = 1.167 angstrom, mu = 0.002 cm-1, F(000) = 449.08 fm, final R = 0.017, wR = 0.019 for 375 unique reflections. Bi12(Bi0.67(3+)Zn0.33(2+))O19.33, M(r) = 2978.2, cubic, I23, a = 10.207 (3) angstrom, V = 1063.4 (9) angstrom 3, Z = 2, D(x) = 9.30 g cm-3, lambda = 1.167 angstrom, mu = 0.0007 cm-1, F(000) = 444.23 fm, final R = 0.016, wR = 0.017 for 354 unique reflections.