Site Symmetry in Crystals. Wyckoff Positions

The different Wyckoff positions are labeled by smaU Roman letters. The maximum number of different Wyckoff positions of a space group is 27 (in the group 2/, — Pmmm). The various possible sets of Wyckoff positions for all the space 

Multipl. Wyck. letter Site symm. Coordinates 

For this group there are 11 different Wyckoff positions denoted by letters from a to k. The number of crystallographic orbit points in the primitive unit cell (multiplicity) equals np/Uq where np = 16 is the order of the point group and Uq is the order of the site symmetry group Gg. The number of points in a Wyckoff position and their coordinates are given in the International Tables with respect to the conventional unit cell of the lattice (for the space group D j with a simple Bravais lattice, the 

The difference between oriented site-symmetry groups of different Wyckoff positions is due to different orientations of the elements of the site-symmetry group G, with respect to the lattice. The difference arises when similar symmetry elements (reflections in planes and rotations about twofold axes of symmetry) occur in more than one class of elements of the point group F. Only eleven site groups [C2(2), Cs m), C2h 2./m), C 2 (2mm), CsyiZmm), 2(222), Ds(322), D2d(42m), D3d 32m), D hijnrnm), and 3 (62m)] can have different orientations with respect to the Bra-vais lattice. Oriented site- symmetry symbols show how the symmetry elements at a site are related to the symmetry elements of a space group. The site-symmetry 

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