TERNARY HALIDES OF THE A(3)MX(6) TYPE .3. SYNTHESIS, STRUCTURES, AND IONIC-CONDUCTIVITY OF THE HALIDES NA(3)MX(6) (X=CL, BR)

The bromides Na(3)MBr(6) crystallize with the stuffed LiSbF6-type structure (type I; M = Sm-Gd) or with the structure of the mineral cryolite (type II; M = cd-Lu). The structure types were refined from single-crystal X-ray data (Na3SmBr6: trigonal, space group R $($) over bar$$ 3, a = 740.8(2)pm, c =1998.9(8)pm, Z = 3; Na3YBr6: monoclinic, space group P2(1)/n, a = 721.3(4) pm, b = 769.9(2)pm c = 1074.8(4) pm, beta = 90.60(4)degrees, Z = 2). Reversible phase transitions from one structure to the other occur. The phase transition temperatures were determined for the bromides as well as for the chlorides Na(3)MCl(6) (M = Eu-Lu). The refinement of both structures for one compound was possible for Na3GdBr6 (I: trigonal, space group R $($) over bar$$ 3, a = 737.1(5) pm, c = 1887(2) pm, Z = 3; II: monoclinic, space group P2(1)/n, a = 725.2(1)pm, b = 774.1(3)pm, c = 1080.1(3)pm, beta = 90.76(3)degrees, Z = 2). All compounds exhibit ionic conductivity of the sodium ions which decreases with the change from type I to type II. The conductivity of the bromides is always higher when compared with the respective chlorides.