Synthesis and characterization of a metastable LnTh(2)F(11) series (Ln=La-Lu, Y) with cationic and anionic disorder: Crystal structure of SmTh2F11

A series of apparently stoichiometric and metastable phases of composition LnTh(2)F(11) (Ln = La-Lu) was prepared for the first time. The crystal structure of SmTh2F11 was determined from single-crystal data with direct methods and refined to R-1 = 0.043 and wR(2) = 0.107 [space group: Pnma; a = 861.0(2) pm, b = 413.7(1) pm, c = 722.5(2) pm; Z = 1.33]. This new structure type is characterized by a (Th + Sm) disorder on the 4c cationic site and by the presence of anionic vacancies on three of the four 4c anionic sites. The ideal MF4 structure is a three-dimensional network of interconnected MF11 polyhedra but the coordination number of the cations is locally reduced to eight for Sm3+. The structural relationship with the LaF3 (tysonite) type is discussed. (C) 1997 Academic Press.