Question

Define primitive cell. are all unit cells are primitive?

Answer 1

In geometry, crystallography, mineralogy, and solid state physics, a primitive cell is a minimum volume cell (a unit cell) corresponding to a single lattice point of a structure with discrete translational symmetry.
No,not all unit cells are primitive.Non-primitive unit cells, however, contain extra lattice points not at the corners. 

Answer 2

Answer:

Primitive unit cells contain only one lattice point, which is made up from the lattice points at each of the corners. Non-primitive unit cells contain additional lattice points, either on a face of the unit cell or within the unit cell, and so have more than one lattice point per unit cell.