TERNARY HALIDES OF THE A(3)MX(6) TYPE .4. TERNARY HALIDES OF SCANDIUMWITH 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 Na 3ScF6 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)degre es) and Na3ScBr6, as the only ternary bromide of the rare earth elemen ts 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 powd er samples of Na3ScF6, Na3ScEr6 and of Na3ScCl6 was studied by impedan ce spectroscopy. Activation energies were determined as 1.22 eV, 0.80 eV and 0.71 eV for the fluoride, chloride and bromide, respectively. T he differences are explained from the crystal structures and the sizes and polarizabilities of the anions.