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Lattice and basis

What is a lattice?

A lattice is a hypothetical regular and periodic arrangement of points in space. It is used to describe the structure of a crystal. Lets see how a two-dimensional lattice may look.

Image latt2

A basis is a collection of atoms in particular fixed arrangement in space. We could have a basis of a single atom as well as a basis of a complicated but fixed arrangement of hundreds of atoms. Below we see a basis of two atoms inclined at a fixed angle in a plane.

Image basis2

Let us now attach the above basis to each lattice point (in black) as follows.

Image lattplusbas

Next remove the lattice points in black (remember that the lattice is an abstract entity). Lets see what we have got?

Image cryst2

We have got the actual two-dimensional crystal in real space. So we may write:

lattice + basis = crystal

How do we define a lattice mathematically?

If r be the coordinates of a lattice point from any origin, then if we apply a lattice translation thought the lattice translation vector

\begin{eqnarray*}
{\mathbf {T}} & = & u{\mathbf {a}}+v{\mathbf {b}}+w{\mathbf {c}}
\end{eqnarray*}

then we would arrive at any other lattice point r$\scriptstyle \prime$ around which the environment would look exactly the same as around r.

\begin{eqnarray*}
\mathbf {r^{\prime}} & = & \mathbf {r} + \mathbf {T} \\
\; & = & \mathbf {r}+ u \mathbf {a} + v \mathbf {b} + w \mathbf {c}
\end{eqnarray*}

a, b and c are the primitive translation vectors or basis vectors which form the primitive cell of the lattice and u, v, w are integers.


next up previous
Next: Primitive and unit cells Up: Crystallography Previous: Crystallography
Abhijit Poddar 2008-01-07