Solid State Chemistry and its Applications. Anthony R. WestЧитать онлайн книгу.
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.
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