Lattice Expansion of (Ga,Mn)As: The Role of Substitutional Mn and of the   Compensating Defects
  We apply the density-functional technique to determine the lattice constant of GaAs supercells containing Mn_Ga, Mn_int, and As_Ga impurities, and use a linear interpolation to describe the dependence of the lattice constant a of Ga_{1-x}Mn_xAs on the concentrations of these impurities. The results of the supercell calculations confirm that Mn_Ga does not contribute to the lattice expansion. The increase of a is due to both Mn_int and As_Ga, that are both created in the as-grown (Ga,Mn)As in proportion to x, and that are most probably present in a remarkable amount also in the best annealed materials. 
J. Masek, F. Maca