The lattice parameters of crystals can be modified if a dopant is introduced. For instance, the unit cell parameters (a, с and β) of Bi_12.8Co_0.2Mo₅O_34±δ decrease from the parent Bi₁₃Mo₅O_34.5±δ structure because of either the smaller radius of cobalt (Co) or the absence of the ns² isolated electron pair in the cobalt atom.