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 influe nce of the successive substitution of iodide for bromide on structure and ionic conductivity has been investigated systematically. X-ray str ucture determination from single crystal and powder data shows that th e cryolite-type of structure is observed for x < 1 [Na3GdBr5.1I0.9, mo noclinic, 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 [m onoclinic, 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 v ariant of the Na3GdI6 structure type is found with a doubled c axis [f or 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 measu rements by impedance spectroscopy exhibt a decline of the activation e nergies for Na+ conductivity for halides with unchanged crystal struct ures with increasing substitution by iodide.