Glide plane definition

Up to this point, our description has been general it applies to crystals of rhenium chloride as accurately as to crystals of the ribosome. In addition to the translational symmetry (periodicity) that is inherent in the definition of a crystal, other symmetry can occur, but the kinds that can occur are restricted to crystals of biological macromolecules. Because the molecules are chiral, the symmetry operations in crystals must not change the handedness of the molecule, and so mirror planes, inversions, and glide planes (sliding mirror planes) do not occur. This leaves only rotations and screws (helical-type symmetry, sliding rotations). 

Two further elements of symmetry enter into the definition of the extended space lattice, namely glide planes and screw axes. 

The crystallographic data will indicate the proper index k, i.e., into which site thejth is sent by the operator 0. The (+1) is a definition of phase. For example, 0 may be chosen as a two-fold screw axis, or a glide plane, and so on. Having chosen the nature of 03, the remaining operators in the factor group, the y3, behave as follows ... 

A particular crystal system has some definite number of point groups and for this monoclinic system it has symmetry operations like 2, m, and 2/m, that is, twofold rotation, a mirror plane, and twofold with mirror plane of symmetries. Now, for three-dimensional crystal the possible symmetry elements will include also screw axes and glide planes, and when screw axes and glide planes are added to the point group of symmetries for this system, we can say that different possibilities that may exist are 2, 2i, m, c, 2/m, 2i/m, 2/c, and 2j/c. Now each of these symmetry groups are repeated by lattice translation of the Bravais lattices of that system. As monoclinic system has only primitive P and C, all the symmetry possibilities may be associated with both P and C. Therefore, if they are worked out, they come out to be 13 in number and they are Pm, Pc, Cm, Cc, P2, P2i, C2, P2/m, P2i/m, C2/m, P2/c, P2i/c, C2/c, etc. 

Thus it is possible to define with respect to a (111) surface a polyhedron described by the 111 family. It is, in effect, a tetrahedron. However, closer inspection requires the definition of two tetrahedra relative to a direction normal to, say, the (111) plane, which of course would be the direction of indentation if a (111) surface or a (TIT) surface was indented. The slip plane tetrahedra are mirror images i.e., the triangular base of the tetrahedron lies on (111) or (TIT) and the apex is then below the surface, or of course the apex can be a (111) or (ITT) atom with the triangular base lying beneath the surface. These two glide polyhedra are sketched in Figure 3.31. 

Plastic deformation by the irreversible shear displacement (translation) of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane. Sometimes called glide. 

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