α-Silver iodide (superionic) AgI

Unit cell

Lattice constants

a = 5.040 Å, b = 5.040 Å, c = 5.040 Å; α = 90°, β = 90°, γ = 90°

Cell volume

128.0 ų

Atoms in the cell

2 (I 2)

Formula units Z

2

Calculated density

3.29 g cm⁻³

Shortest contact

4.365 Å

Coordination

I

CN 8 at 4.36 Å (8 I)

About the structure

• Above 147 °C silver iodide becomes α-AgI: the I⁻ ions form a rigid bcc array while the two Ag⁺ per cell are spread over 12 tetrahedral sites (the markers), hopping between them like a liquid.
• Only the I⁻ frame is drawn as atoms; switch the tetrahedral markers on to see the Ag⁺ sites — 12 sites, 2 occupied at any instant.
• Ionic conductivity ≈ 1 S cm⁻¹, comparable to a molten salt; the prototype superionic conductor.
• Below 147 °C AgI is wurtzite-type (β) or zinc-blende-type (γ).

Atom positions (fractional, one cell)

Z, coordination numbers, distances, density and packing on this page are computed from these coordinates.