Can Z, the number of formula units per unit cell, be a fraction? A report of the IUCr Commission on Crystallographic Nomenclature

The IUCr Commission on Crystallographic Nomenclature (the CCN) has considered the question of whether fractional values of Z , the number of formula units in the unit cell, can be permitted. The consideration was initiated by a crystallographer who had determined a very unusual molecular structure, but the discussion was later broadened to cover all non-polymeric structures. Reliable structures having a fractional Z were located in the structural databases and in the literature, and were studied carefully. The CCN concluded that in some rare cases the Z value reported for a structure of a stoichiometric compound may be a fraction, but that that choice would have to be justified very carefully and convincingly. For solid solutions Z should be an integer.