Tuning the probability of defect formation via substrate strains in Sr2FeMoO6 films

Abstract

Since oxide materials like Sr2FeMoO6 are usually applied as thin films, we studied the effect of biaxial strain, resulting from the substrate, on the electronic and magnetic properties and, in particular, on the formation energy of point defects. From our first-principles calculations, we determined that the probability of forming point defects -like vacancies or substitutions -in Sr2FeMoO6 could be adjusted by choosing a proper substrate. For example, the amount of anti-site disorder can be reduced with compressive strain in order to obtain purer Sr2FeMoO6 as needed for spintronic applications, while the formation of oxygen vacancies is more likely for tensile strain, which improves the functionality of Sr2FeMoO6 as a basis material of solid oxide fuel cells. In addition, we were also be able to include the oxygen partial pressure in our study by using its thermodynamic connection with the chemical potential. Strontium vacancies become for example more likely than oxygen vacancies at a pressure of 1 bar. Hence, this degree of freedom might offer in general another potential method for defect engineering in oxides besides, e.g., experimental growth conditions like temperature or gas pressure. arXiv:1805.02969v2 [cond-mat.mtrl-sci]

Challenges in Sr2FeMoO6−δ Thin Film Deposition

“…In accordance with a low AS defect formation energy, Fe/Mo nonstoichiometry in SFMO is mainly accommodated by Fe and Mo AS defects, that is, by Fe atoms on Mo sites and vice versa. A crude estimation of the lower energy limit for the formation of lattice defects in a cubic crystal consisting of an A‐atom in a host position B may be given by the elastic energy required for a volume change Δ V ≈ 3/2Δ r : where c ij are elastic constants, a the lattice constant, and Δ r the difference of the ion radii of the host and impurity atoms .…”

“…First‐principle calculations reveal that the most abundant point defects in SFMO are oxygen vacancies (Figure b) as well as antisite defect pairs Fe Mo and Mo Fe (Figure c) . They yield a diminution of half‐metallic character by Fe Mo antisite defects and Mo vacancies, that is, in Fe‐rich SFMO films, as well as by Fe Mo + Mo Fe pairs . According to other first‐principle calculations, the half‐metallic characters are maintained for SFMO containing Fe Mo and Fe vacancies (i.e., in Mo‐rich films), oxygen vacancies, or Sr vacancies .…”

“…First‐principle calculations for a Sr 8 Fe 4 Mo 4 O 24 supercell results in ASD formation energies of about 0.4…0.7 eV, where ASD along the diagonal of the cubic B‐site sublattice is predicted to be the most favorable while the formation energies of ASDs along the x − y plane and z directions are very similar . Another value of 0.77 eV was derived using the first‐principle approach for AS Mo Fe +Fe Mo pairs in stoichiometric SFMO . It is independent of oxygen partial pressure.…”

“…Section ). Note that at a high oxygen partial pressure (>100 Pa at 1050 °C), strontium vacancies are predicted to become the most abundant point defects in SFMO . However, such large oxygen partial pressures are not typical for film deposition conditions.…”

“…Inserting elastic constants from first‐principle calculations in ref. [5] and Δ r values between Fe 2+ and Mo 6+ ions taken from ref. [66] resulted in Δ H f (Fe Mo ) = Δ H f (Mo Fe ) = 0.22 eV.…”