Study of a mixed-polytype crystal of silver iodide containing a new 12H polytype

A mixed-polytype crystal of AgI is studied by X-ray diffraction. The constituent polytypes are found to be the common 2H and 4H varieties plus a new 12H polytype, and their proportions are determined. The stacking sequence of the 12H polytype is 1211111121 (Zhdanov notation). Disorder-diffuse scattering observed in the diffraction pattern is attributed to intrinsic-2h stacking faults in 2H regions of the crystal. The degree of disorder corresponds to a fault probability α = 0.15.