2.9 Units for Defect Concentration

Defect concentration fraction, n/N , is nothing but the ratio of number of defects, n, relative to number of occupied lattice sites N i.e. defect concentration fraction. The denominator should actually be n+N but since, N>>n, it can be approximated as n+N ~ N.  Commonly used units for concentration is  #/cm³ or cm^-3

Typical defect concentration in ceramics ~ 1 ppm.

So, if the density of atoms in a solid ~10²³ cm^-3, 1 ppm concentration would be equivalent to 10¹⁷ cm^-3.
Conversion of mole fraction to number per unit volume can be the following:

No. of formula units per unit volume =