Revealing the Beautifulness of Silver Oxide Nitrate
Delving among some books about crystal structures I came upon the remarkable and beautiful crystal structure of silver oxide nitrate: Ag(Ag6O8)NO3.
Space Group: Fm-3m (No. 225)
Crystal system: cubic
Crystal class: m-3m
Lattice parameters: a = b = c = 9.8893 Å, α = β = γ = 90°
Let’s have a first look how its crystal structure looks like:
Fig. 1: Crystal structure of silver oxide nitrate (Blue: nitrogen, red: oxygen, grey: silver, purple: silver).
Uhh, interesting, but partly a mess! But we can turn this into something more beautiful.
Step 1: There are these heavily disordered nitrate anions. Let’s represent them simply with one nitrogen atom in blue.
Fig. 2: The disordered nitrate anions are represented by the blue spheres.
Step 2: One sort of the silver cations are surrounded by 4 oxygen anions in a square-planar fashion. Let’s highlight this feature and slightly change the viewing direction:
Fig. 3: Highlighting the square-planar AgO4 coordination polygons with blue squares.
Ah, doesn’t it then look like an arrangement of rhombicuboctahedra, one of the Archemedian solids?
Step 3: There are additional squares built by 4 oxide anions, but in which no silver cations are located in the center. And then there are these triangular faces built by 3 oxygen atoms, which will complete this rhombicuboctahedron (a rhombicuboctahedron has 24 vertices (here realized by 24 oxygen anions), 26 faces and 48 edges). Let’s highlight these squares in orange and the triangular faces in grey :
Fig. 4: Completing the rhombicuboctahedra…
Now, we see in which way the face-centered cubic arrangement of the rhombicuboctahedra are connected to each other – by orange cubes!
This means, we have two cavities, both forming a fcc-like arrangement. In the center of the rhombicuboctahedra ( = at the corners and face centers of the unit cell) the disordered nitrate anions are located and in the center of the cubes ( = at the middle of the edges of the unit cell) there are silver ions. In this sense, silver oxide nitrate is only a kind of a ‘decorated’ rock salt structure 🙂
Fig. 5: Visualization of the silver and nitrate ions, being located in the center of the rhombicuboctahedra and cubes, respectively.
Isn’t this beautiful?
Further fun facts
1. While mixed-valence compounds are very common, here we have a mixed-valence compound in which the silver ions that are covalently bonded to oxide anions have two different oxidation states, i.e. Ag(I) and Ag(III). So the actual formula can be written as
2. Silver oxide nitrate has a relatively high electrical conductivity of 2.1 x 102 S/cm.
 C.H. Wong, T.-H. Lu, C.N. Chen, T.-J. Lee, Journal of Inorganic and Nuclear Chemistry (1972), 34, 3253-3257
 I. Náray-Szabó, G. Argay, P. Szabó, Acta Crystallogr. (1965), 19, 180-184 DOI: 10.1107/S0365110X65003043
 W. Levason, M.D. Spicer, Coord. Chem. Rev. (1987), 76, 45-120