Crystallography: ordered forms

**Crystal** - a regular geometric solid bounded by smooth
surfaces.

**Crystalline** - homogeneous solid containing long-range
three dimensional order. Order implies repetition in space (i.e.,
found in intervals of equal distance).

**Motif - **the unit of pattern (e.g., the flower repeated on
wall paper or wrapping paper design). In two dimensional space, it
is easy to think of the translation of an object in a pattern in
any X and Y direction.

2-dimensional symmetry (Plane Groups)

*Examples*: **2-D square lattice**. To translate a motif
(the snail) in space we employ rules of symmetry.

**Rules of symmetry** are the way we describe the orderly
repetition of a motif in space. In the 2-D example of a square or
isoclinic plane lattice, we use the rule or **geometric
operation** of a **translation **to move the snail an
equal distance in an XY coordinate system. The coordinate system
is defined by increments whose unit lengths (*a* and *b*)
are equal and whose axes are at right angles to each other (The
Cartesian coordinate system is a square lattice whose unit lengths
are equal to one (γ = 90°).

Points or nodes connecting each other collectively form a net
that is called a **lattice**.

Square or Isoclinic (*a = b, *γ
= 90°)

The area (or volume in the case of 3-D) that comprises the unit
length's distances between lattice points is known as the **unit
cell**.

The **primitive** (*p*) unit cell is that which contains
nodes only at the corners of the unit cell and has the smallest
possible repeat distance that can be translated along the x and y
directions to produce the entire pattern.

Orthoclinic (*a* not equal to *b*, *γ = *90°)

The choice of a unit cell is a manner of convention. A primitive
lattice may also be described as a rectangular lattice with a **centered**
(*c*) unit cell. The reason for chosing the convention of a
non-primative unit cell is to enable description of pattern using
higher symmetry operations (more on this later).

Symmetry elements without translations.

**Symmetry element** - locus of points or point that aid
visualizing the symmetrical repeat of a motif.

**Symmetry operation** - is the act of moving the motif about
the element.

One type of a geometric operation that reproduces a motif is a **rotation**.
The object is to create a pattern by moving the motif about an
imaginary axis through some angle ( a).
The axis is the element and rotation is the operation.

The amount of rotational symmetry is designated by an integer (n) from 1 to infinity. n is defined by 360° divided by the number of rotational degrees in the operation. Remember, you must return to the original position of the motif.

A good example can be seen by viewing a beer can. The bottom appears with an infinte amount of rotational symmetry while the top view appears to have an n equal to one.

Because of geometrical constraints (all the polyhedron must be
space occupied) the internal order of a crystal is limited to
1-fold, 2-, 3-, 4-, 6-fold axis (*i.e.*, n = 1, 2, 3, 4, or
6).

An example of a 4-fold rotation operation is given here

Other symmetry operations include the creation of new pattern through a

The example below combines two operations, including a mirror
plane reflection and a translation that is known as a glide plane
translation (*g*). The elements for a glide symmetry (a plane
and an axis in this case) must occupy a common place in 3-D space.

An

Operations and element without translations are summarized below

**Symmetry operations - **rotation, reflection and inversion

**Symmetry elements** - rotation axis, mirror plane (m) and
symmetry center (i)