Trapping phosphate anions inside the [Ag4I]3+ framework: Structure, bonding, and properties of Ag4I(PO4)

Orange-red Ag4I(PO4) crystallizes in the monoclinic system, space group P21/m (No. 11), with the unit cell dimensions a=9.0874(6) Å, b=6.8809(5) Å, c=11.1260(7) Å, β=109.450(1)°, and Z=4. The crystal structure is fully ordered; it comprises the silver–iodine three-dimensional positively charged framework hosting the tetrahedral PO43− guest anions. The framework features high coordination numbers for iodine and manifold Ag–Ag bonds ranging from 3.01 to 3.46 Å. The Ag–Ag interaction is bonding, it involves silver 4d and 5s orbitals lying, together with the orbitals of iodine, just below the Fermi level. Though the orbitals of silver and iodine define the conducting properties of the title compound, the interaction between the framework and the guest anions is also important and is responsive to the number of the silver atoms surrounding the PO43− tetrahedra. Ag4I(PO4) melts incongruently at 591 K and produces a mixture of the silver phosphate and an amorphous phase upon cooling. Pure Ag4I(PO4) is a poor conductor with a room temperature conductivity of 3×10−6 S m−1. The discrepancies between the properties observed here and those reported previously in the literature are discussed.

Regular [PO4] tetrahedra fill large voids in the Ag–I framework to form a host–guest compound, Ag4I(PO4). It has a perfectly ordered crystal structure, atypical for this kind of compounds, rendering the study of the manifold Ag–Ag bonds and the host–guest interaction. However, this ordering leads to low ionic conductivity.Figure optionsDownload as PowerPoint slide