Structural Principles of o-Agl and Related Double Salts

The structures of RbAg415 and related superionic onductors are discussed, and quantitative measures of chemical bonding are proposed which explain the formation of double salt compounds. Alternative diffusing cation candi-dates are proposed. The o-phases of AgI, CuI, and CuBr are perhaps the most remarkable binary solids known. Although the anions form a bcc lattice, the cations are distributed among many nearly equivalent sites; the distribution is so nearly uniform that the cations may be said to form a liquid (1-3). The temperatures, T ~ for the (wurtzite)- ~ ~ transition and the melting points, Tin, for these three crystals are.listed in Table I. For CuI and CuBr, T ~ and Tm are close enough together (es-pecially for CuBr) that one might say that the cation sublattice has merely melted (or premelted) just be-fore the anion sublattice. However, no other single phase compounds are known to melt in stages, and general theories of first order phase transitions do not suggest hat such phenomena should occur. Moreover, the s-phase of AgI is stable over a very wide range of temperature (Table I). A second explanation for these extraordinary phases is that they are in some way peculiar to Ag and Cu salts. Apart from its alchemical character, this expla-nation is not satisfactory because AgC1 and AgBr do not form a-phases. However, Tubandt and co-workers showed long ago that Cu2S, Ag2S, ~Ag2Se, and Ag2Te all have ionic conductivities much higher than in ordi-nary salts. A particularly interesting case, studied by Ketelaar (4), is (Ag, Cu)2 HgI4. This compound has es-sentially the zinc blende structure, but only three of the four cation sites in cubic unit cell are occupied. In principle, this may give rise to a tetragonaI struc-ture, and in the Cu case it does (c/a-- 1---- 0.010), but in the Ag case the structure remains cubic (c/a-- 1---- 0.000). The disordering temperature, Td, for Ag2HgI4 is 50 ~ considerably lower than T ~ _-- 146 ~ in AgI