I try to create structure for DFT and cannot understand, how to calculate concentration of impurities in crystal. For example, let's take crystal from group A3B:C. In primitive cell there are 6 atoms from A and 18 from B.
1. How many atoms of type C should I take to achieve 20% of impurities concentration?
2. What if A and C are atoms from same group, lanthanides for example. Shouls I calculate percent obly from number of lantanides in cell?