The lines drawn parallel to the lines of intersection of any three faces of the Unit Cell which do not lie in the same plane are called Crystallographic Axis.

**Unit Cell**** **is defined as the smallest geometric figure or pattern, the repetition of which gives the actual crystal structure. This is also defined as undamental elementary pattern o minimum of atoms or molecules or groups of molecules, which represents total characteristics of the material or the crystal.

**Lattice Parameters or Unit Cell Parameters**

The lines drawn parallel to the lines of
intersection of any three faces of the Unit Cell which do not lie in the same
plane are called **Crystallographic Axis.**

The intercepts a,b, c (i.e., the distance
between the two lattices points ) are nothing but the edges of the unit cell,
which defines the dimensions of unit cell. These intercepts are known as its **primitives or
characteristic intercepts** on the axes.

These three quantities a, b, c is also called
the fundamental translational vectors or axial lengths. The angles (a,b), (b,c)
and (c,a) are denoted by α,ẞ,γ respectively. These three angles (α,ẞ,γ) are
called **Interfacial angle**.

But the intercept (a,b,c) and the interracial
angle (α,ẞ,γ) constitutes the lattice parameters o the unit cell.

The actual size o the unit cell can be
determined by knowing the values o intercepts and interracial angles.

**Primitive Cells.**

A **Primitive Cells**is the simplest type
of the unit cell, which contains only one lattice point per unit cell.
(Contains lattice points at its corner only)

Example: Simple Cubic

**Non Primitive Cell**

A Non Primitive Cell is one, which contains
more than one lattice point per unit cell.

Example: BCC and FCC contain more than one
unit cell.

If the number of lattice points per unit cell
is two (BCC), three and four(FCC), then the unit cell is called **doubly primitive,
triply primitive and Quadruple Primitive** respectively.

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