First-principles study of defective and nonstoichiometric Sr ₂FeMoO₆

The influence of disorder and stoichiometry-breaking point defects on the structural and magnetic properties of Sr₂FeMoO₆ have been investigated with the help of electronic structure calculations within the spin-polarized GGA+U approach. Defining the chemical potentials of the constituent elements from constitutional defects, we calculate the energetics of the possible point defects in nonstoichiometric Sr₂FeMoO₆ and find transition-metal-ion antisites and oxygen vacancies to be dominant. In nonstoichiometric Sr₂Fe_1+xMo_1-xO₆ with -0.75 ≤ x ≤ 0.25, both Fe_Mo antisites (for Fe-rich samples or x < 0) and MoFe antisites (for Mo-rich samples or x > 0) lead to a systematic decrease in saturation magnetization. OnlyMo_Fe antisites destroy the half-metallic character of the electronic structure, since their t_2g band crosses the Fermi level for x ≤ -0.125. This leads to a decrease of spin polarization from 100% for x > -0.125 to 0 at x ≈ -0.75. Oxygen vacancies also reduce the saturation magnetization, but the half-metallic character and, hence, 100% spin polarization is retained. The optimized unit-cell lattice parameter remains within a relatively narrow range (7.96 Å for x = +0.25 to 8.00 Å for x = -0.75), despite large changes in composition. In stoichiometric Sr₂FeMoO₆, the saturation magnetization decreases linearly as the Fe/Mo antisite disorder increases, and the half metallicity is lost, because of the t_2g states on both MoFe and Fe_Mo. The spin polarization remains ̃100% only for very small amounts of disorder. The calculated disorder formation energies suggest that short-range ordering is favorable in Sr₂FeMoO₆. The calculated results are in excellent quantitative agreement with experimental values, where available.