Bravais lattice is a basic concept in the crystalline solid. This gives information of the periodic array in which the repeated units of the crystal are arranged. The units themselves may be an atom, groups of atoms, molecules, ions, etc. But the Bravais lattice summarizes only the geometry of the underlying periodic structure, regardless of what the actual units may be.
Bravais lattice is the lattice in which the atom or all the atoms at lattice points are identical or if the surroundings of each lattice point is the same.
Before studying Bravais Lattice, let us first discuss the meaning of space or crystal lattice.
In the case of a solid crystalline material, the atoms or molecules are arranged in regular and periodic manner in all three dimensions. Thus, Crystal structure is the atomic arrangement in a crystal.
To make crystal symmetries more clear, let us assume an atom or a group of atoms that repeats in three dimensions in the crystal as a unit. Now replace each atom or unit of atoms in a crystal by a point in space. Thus the resultant points in space are the space lattice.
Lattice point is each point in a space lattice. Whereas the basis or pattern is each atom or unit of atoms.
A space lattice is representing the geometrical pattern of crystal in which the surroundings of each lattice point is the same. And as we discussed before, if the surroundings of each lattice point is same or if the atom or all the atoms at lattice points are identical. Then we name such a lattice Bravais lattice.
On the other hand, if the atom or the atoms at lattice points are not the same, then it is a Non-Bravais lattice.
It is important to note that not only the arrangement but also the orientation must appear the same from each and every point in Bravais lattice.
Classification of Bravais lattice
For representing the type of distribution of lattice points in space, seven different coordinate systems needed. These coordinate systems are called crystal systems.
Also, We can give names to the crystal systems on the basis of geometrical shape and symmetry . The seven crystal systems are:
(6) Trigonal (or Rhombohedral)
Bravais in 1948 showed that 14 lattices are sufficient to describe all crystals. These 14 lattices are known as Bravais lattices. And these are classified into 7 crystal systems based on cell parameters or lattice points present per unit cell.
Bravais lattices are as follow
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