Читать книгу Crystallography and Crystal Defects - Anthony Kelly - Страница 45

The conventional space group symbol shows that the space groups have been built up by placing a point group at each of the lattice points of the appropriate Bravais lattice. Thus, for example, Fmm (in full, F 4/m 2/m) means the cubic face‐centred lattice with the point group mm associated with each lattice point; P6₃/mmc (in full, P 6₃/m 2/m 2/c) is a hexagonal primitive lattice derived from P6/mmm by replacing the sixfold rotation axis by 6₃ and one of the mirror planes by a c‐axis glide plane. P6₃/mmc and P6/mmm share the same point group symmetry. The point group symmetry of any crystal is derived immediately from the space group symbol by replacing screw axes by the appropriate rotational axes and glide planes by mirror planes in the space group symbol.