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

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      Spurgeon, S.R. et al. Competing Pathways for   Nucleation of the Double Perovskite Structure in the Epitaxial   Synthesis of La2MnNiO6. Chemistry of Materials (2016).   DOI:10.1021/acs.chemmater.6b00829


        Double perovskites of the form R2BB'O6 (where R is   a rare earth cation and B and B' are   chemically distinct transition metal cations with half-filled and   empty eg 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 μB f.u.-1)   and its semiconducting character. If the ideal structure   nucleates, superexchange coupling can take placevia 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 viamolecular   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   whichB-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 multi-element   crystalline solid and illustrates how to engineer a synthetic   path to a desired product.

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