Ternary halides of the A(3)MX(6) type .5. Synthesis, crystal structures, and sodium ionic conductivity of the ternary iodides Na3MI6 (M=Sm, Gd-Dy) and of the solid solution halides Na3GdBr6-xIx

The system Na3GdBrb-xIx exhibits a complete solid solution. The influence of the successive substitution of iodide for bromide on structure and ionic conductivity has been investigated systematically. X-ray structure determination from single crystal and powder data shows that the cryolite-type of structure is observed for x < 1 [Na3GdBr5.1I0.9, monoclinic, P2(1)/n, Z = 2, a = 734.6(10), b = 779.5(9), c = 1101(2) pm, beta = 90.8(2)degrees]. For x > 3 the new Na3GdI6 type is observed [monoclinic, C2/m, Z = 2, a = 787.5(1), b = 1353.4(2), c = 778.3(1) pm, beta = 110.14(1)degrees for Na3GdBr2.4I3.6: a = 772.7(4), b = 1323.3(7), c = 765.8(4) pm, beta = 110.14(3)degrees]. In between, an ordered variant of the Na3GdI6 structure type is found with a doubled c axis [for Na3GdBr4.8I1.2: monoclinic, C2/c, Z = 4, a = 751.5(4), b = 1285.7(6), c = 1413.7(7) pm, beta = 99,65(2)degrees]. Ionic conductivity measurements by impedance spectroscopy exhibt a decline of the activation energies for Na+ conductivity for halides with unchanged crystal structures with increasing substitution by iodide.