TERNARY HALIDES OF THE A3MX6 TYPE .2. THE SYSTEM AG3-XNAXYCL6 - SYNTHESIS, STRUCTURES, IONIC-CONDUCTIVITY

The influence of the substitution of Ag+ by Na+ ions on the crystal structure and the ionic conductivity of Ag3YCl6 (stuffed LiSbF6-type structure) has been investigated. The system Ag3-xNaxYCl6 forms a complete solid solution. The stuffed LiSbF6-type structure is stable for all compositions. For compounds with Na+ contents of x > 1.67, the cryolite-type structure is observed as the high-temperature form. The transition temperature decreases steadily with increasing Na+ content. The ''end member'' phase Na3YCl6 transforms at 243 K from the monoclinic cryolite-type structure to the stuffed LiSbF6-type structure (trigonal, R3BAR; a = 697.3(1), c = 1 868.4(14) pm, Z = 3; R = 0.094; R(W) = 0.069). The crystal structures of Ag1.3Na1.7YCl6 (trigonal, R3BAR; a = 691.5(2), c = 1853.7(6) pm, Z = 3; R = 0.099, R(W) = 0.081) and AgNa2YCl6 (trigonal, R3BAR; a = 691.7(1), c = 1853.9(5) pm, Z = 3; R = 0. 099, R(W) = 0.064) have also been determined. Both chlorides crystallize like Ag3YCl6 and Na3YCl6-I in the stuffed LiSbF6-type structure. The monovalent cations, Ag- and Na+, are distributed over the five octahedral voids that are occupied by the Ag+ ions alone in Ag3YCl6. The ionic conductivity for compounds within the solid solution Ag3-xNaxYCl6 decreases with increasing Na+ content. The values for Na3YCl6 (sigma = 1 . 10(-6) OMEGA-1 cm-1 at T = 500 K) are by 2.5 to 3.5 orders of magnitude smaller than those for Ag3YCl6 (sigma = 6 . 10(-4) OMEGA-1 cm-1 at T = 500 K).