From the unit cell dimensions, it is possible to calculate the volume of the unit cell. Knowing the density of the metal. We can calculate the mass of the atoms in the unit cell. The determination of the mass of a single atom gives an accurate determination of Avogadro constant.
Suppose edge of a unit cell of a cubic crystal determined by X-Ray diffraction is a, d is the density of the solid substance and M is the molar mass, then in case of cubic crystal
Volume of a unit cell = a3
Mass of the unit cell = no. of atoms in the unit cell × mass of each atom = Z × m
Here Z = no. of atoms present in one unit cell
m = mass of a single atom
Mass of an atom present in the unit cell = m/NA