ON CHEMISTRY AND CRYSTAL-CHEMISTRY OF ACLNX6 COMPOUNDS (AC = TH, U, LN = RARE-EARTHS, X = BR, I)

Reactions of AcX4 (Ac: Th, U; X: Br, 1) with Rare Earth Dihalides LnX2 lead to AcLnX6 compounds with Ln = Sm, Eu, Dy, Tm, Yb. In the case of the other Rare Earth compounds redox reactions and formation of the trihalides of Ac and Ln occur. Thermodynamic calculations help to understand the reaction paths. Different structure types (beta- and gamma-ThSnI6-type) occur depending on the radius ratio of the ions. They are statistical or ordered substitutions variants of AB3 structures (BiI3- or PuBr3-type). The compounds UBI6 (B: Sr, Eu, Ba) crystallize in their own structure type. All these compounds and those having a beta-ThSnI6 structure can be transformed to a gamma-ThSnI6-type modification under pressure.