ISOMORPHOUS REPLACEMENTS IN FLUORITES AND SILLENITES V.I.Simonov, Institute of Crystallography, Moscow 117333, Russia

On the basis of the data obtained from accurate X-ray and neutron diffraction structure studies we analyzed the character of isomorphous replacements in fluorite-related single crystals in the MF₂-R₂F₃ system, where M=Ca, Sr, Ba; R=Y, La, Pr, Lu and sillenites Bi₁₂M_1+/-xO_20+/-y, where M=Ge, Ti, (Fe, P), (Bi, Ga), (Bi, Fe), (Bi, Zn), (Bi, V). In single crystals of solid solutions with the fluorite-related structure the known clusters Ln₄F₂₆ or Ln₆F₃₆ are formed, depending on the relation of ionic radii of M²⁺ and R³⁺ cations. If a large number of M²⁺ cations are replaced by R³⁺ cations, more complex clusters are formed with a simultaneous probable accommodation of complementary F^1- anions on the two and three-fold symmetry axes. The mechanism of isomorphous replacements in sillenite structures is totally different. In contrast to the fact that the valence of a M cation is always +4, while the number of oxygen atoms is strictly 20, as was always reported in the literature, in the above compounds the effective valences of M cations are different. For instance, in (Bi,Zn)-sillenite it is less than +4, a Ge-sillenite it equals +4, while in (Bi,V) it exceeds +4. The amount of oxygen is also different. When Bi cations in the isomorphic mixture with other cations occupy the M site in the structure, their valence in all the cases was found Bi³⁺. In fact, in this case MO₄ tetrahedron are replaced by umbrella-like BiO₃ groups, the statistics of these groups being in accordance with their orientation. The average cubic symmetry is retained over the entire crystal structure. The site of the fourth missing oxygen atom in the BiO₃ group is occupied by electron lone pair of a Bi atom. Pentavalent V⁵⁺ at the M site results in the occupation of structure voids by additional O atoms. The lability of sillenite framework is so great that Fe²⁺ and P⁵⁺ cations with totally different ionic radii are allowed to occupy statistically the M site. The unusually remarkable isomorphism of cations in fluorites and sillenites permits controlled changes of physical properties of these compounds.