Competing Pathways for Nucleation of the Double Perovskite Structure in the Epitaxial Synthesis of La2MnNiO6

Double perovskites of the form R2BB'O-6 (where R is a rare earth cation and B and B' are chemically distinct transition metal cations with half-filled and empty e(g) orbitals, respectively) are of significant interest for their magnetoelectric properties. La2MnNiO6 is particularly attractive because of its large expected ferromagnetic moment per formula unit (5 mu(B) f.u.(-1)) and its semiconducting character. If the ideal structure nucleates, superexchange coupling can take place via the B-O-B' bonds that form, and the moment per formula unit can attain its maximum theoretical value. However, we show that even in the case of layer-by-layer deposition via molecular beam epitaxy, the system can follow multiple reaction pathways that lead to deviations from the double perovskite structure. In particular, we observe a spatially extended phase in which B-site cation disorder occurs, resulting in Mn-O-Mn and Ni-O-Ni antiferromagnetic domains, as well as the formation of quasi-epitaxial, antiferromagnetic NiO nanoscale inclusions, surrounded by a Mn-rich double perovskite. The coexistence of the double perovskite and secondary phases in oxygen deficient conditions is supported by first-principles modeling. However, extended annealing in air restores long-range B-site order and begins to dissolve the NiO inclusions, yielding an ideal structure and an enhanced ferromagnetic moment. This study reveals fundamental structure property relationships that may not be apparent during the design phase of a multielement crystalline solid and illustrates how to engineer a synthetic path to a desired product.