Solid State Chemistry and its Applications. Anthony R. West

set of X‐ray reflections. For an a glide perpendicular to b, the condition limiting the h0l reflections is that h = 2n (i.e. only even h values may be observed). In the space group C2/m, this condition is part of the more general condition for C‐centring, namely: for hkl, h + k = 2n. Independent evidence for the existence of the glide plane is therefore not immediately available from the X‐ray patterns.

      Two further examples: in space group P222₁, the only symmetry element which causes systematic absences is the 2₁ axis parallel to z, i.e. for 00l reflections, only those for which l = 2n may be observed; there are no general conditions on hkl reflections since the space group is primitive. For space group I4₁, two conditions are imposed on the possible reflections: for the body centring, only the reflections hkl: h + k + l = 2n may be observed; the 4₁ screw axis places the additional condition that: for 00l reflections, l = 4n.