Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 82

Of the 32 point groups, 21 do not possess a centre of symmetry. The absence of a centre of symmetry is an essential but not sufficient requirement for the presence in crystals of optical activity, pyroelectricity and piezoelectricity (Chapters 8 and 10). Optical activity is confined to 15 of the 21 non‐centrosymmetric point groups and piezoelectricity to 20 of these. This is of use in, for example, the search for new materials with piezoelectric activity; it is a waste of time trying to detect piezoelectricity in crystals whose point group is not among the 20 active groups! Crystallographers also make some use of the piezoelectric effect in structure determination. It is a considerable help in solving an unknown structure to know the space group at the outset. If a test for piezoelectricity is carried out with positive results, this limits the choice of space group to the non‐centrosymmetric ones. The absence of piezoelectricity does not necessarily mean, however, that the point group and space group are centrosymmetric.