Ternary halides of the A(3)MX(6) type .4. Ternary halides of scandium with sodium, Na(3)ScX(6) (X=F,Cl,Br): Synthesis, structures, ionic conductivity

X-ray studies on single crystals of Na3ScF6 and Na3ScBr6 show, that Na3ScF6 crystallizes with the cryolite type (monoclinic, P2(1)/n, Z = 2, a = 560.16(9), b = 580.31(8), c = 812.1(2) pm, beta = 90.720(14)degrees) and Na3ScBr6, as the only ternary bromide of the rare earth elements with sodium, in the Na3CrCl6 type (trigonal, <P(3)over bar 1c>, Z = 2, a = 728.95(7), c = 1309.29(17) pm). The ionic conductivity of powder samples of Na3ScF6, Na3ScEr6 and of Na3ScCl6 was studied by impedance spectroscopy. Activation energies were determined as 1.22 eV, 0.80 eV and 0.71 eV for the fluoride, chloride and bromide, respectively. The differences are explained from the crystal structures and the sizes and polarizabilities of the anions.