Crystal structures of the silver iodide sulfates Ag₃ISO₄ and Ag₄I₂SO₄

The crystal structures of two phases in the AgI-Ag₂SO₄ system, Ag₃ISO₄(AgI:Ag₂SO₄ = 1:1; systematic name: tris-ilver iodide sulfate), and Ag₄I₂SO₄ (AgI:Ag₂SO₄ = 2:1; systematic name: tetra-silver diiodide sulfate), were determined by single-crystal X-ray diffraction. The crystal structure model of Ag₃ISO₄ contains triangularly arranged Ag atoms in zigzag ladder chains extending parallel to the a axis. The Ag zigzag ladder chains alternate with rows of SO₄ groups, while iodine atoms are accommodated within the concavities of the zigzag ladders. The crystal structure of Ag₄I₂SO₄ was refined using a split-atom model for two of the four Ag sites. The disordered Ag sites are situated between the layers containing SO₄ groups parallel to the ac plane, whereas the Ag sites within the SO₄ layers are fully occupied and exhibit no disorder. The positional disorder of some of the Ag sites is considered to be associated with the moderate ionic conductivity of Ag₄I₂SO₄, approximately 10^-3 S cm^-1 at room temperature. This is the first report describing the crystal structures of compounds containing Ag⁺, SO₄ ^2- and I^-.