Q. There is no end centered tetragonal lattice in the bravis list, but there is an end centered orthorhombic lattice. Explain why this is so?
Ans. A tetragonal system is defined by three mutually perpendicular vectors, just two of which are equivalent in magnitude. There is no face centered tetragonal space lattice. Any array o9f lattice points that can be represented by an FCT cell can equally well be described by a BCT cell.
When there are two such alternatives of the same crystal system available to describe the same array of lattice points, the unit cell which has the smaller no. of lattice point is chosen for the bravais list. An orthorhombic cell is defined by three unequal but mutually perpendicular translation vectors. There are four orthorhombic space lattices.