Onset of phase separation in the double perovskite oxide La2MnNiO6

Abstract

Identification of kinetic and thermodynamic factors that control crystal nucleation and growth represents a central challenge in materials synthesis. Here we report that apparently defect-free growth of La 2 MnNiO 6 (LMNO) thin films supported on SrTiO 3 (STO) proceeds up to 1-5 nm, after which it is disrupted by precipitation of NiO phases. Local geometric phase analysis and ensemble-averaged X-ray reciprocal space mapping show no change in the film strain away from the interface, indicating that mechanisms other than strain relaxation induce the formation of the NiO phases. Ab initio simulations suggest that oxygen vacancies become more likely with increasing thickness, due to the electrostatic potential build-up associated with the polarity mismatch at the film-substrate interface; this, in turn, promotes the formation of Nirich regions. These results suggest that the precipitate-free region could be extended further by increasing the oxygen chemical potential through the use of an elevated oxygen pressure or by incorporating electron redistributing dopants to suppress the built-in potential.1 arXiv:1710.08535v2 [cond-mat.mtrl-sci] 15 Apr 2018The formation of undesirable and uncontrolled phases during thin film nucleation and growth represent a fundamental obstacle to the atomically-precise synthesis of materials with targeted properties. As highlighted in recent reviews, 1,2 it is extremely difficult to observe and harness kinetic processes, hindering our efforts to control film growth. 3 The conditions associated with different techniques, such as pulsed laser deposition (PLD) and molecular beam epitaxy (MBE), lead to vastly different energy landscapes that govern the synthesis process. Furthermore, it is understood that film deposition is a dynamical process-namely, conditions at the growth front change during deposition resulting in complex synthesis outcomes. There is a pressing need to understand the factors that govern this dynamical behavior and how they can trigger the formation of inhomogeneities, from isolated defects to parasitic phases. [4][5][6] Coherently strained epitaxial thin films offer a highly controlled environment in which to identify the signatures and guiding mechanisms of phase separation events. The ability of perovskite oxides to phase separate has already been exploited to produce complex nanostructured materials; 7-11 however, predictive control of nanocomposites remains elusive, and more insight is needed into the details of elementary processes and species that mediate phase separation. For example, the polarity mismatch at polar / non-polar interfaces is thought to induce NiO phase separation in LaNiO 3 / LaAlO 3 superlattices grown on SrTiO 3 (STO). 12 Lazarov et al. have demonstrated that polar Fe 3 O 4 / MgO (001) interfaces can be stabilized through the formation of Fe nanocrystals, rather than through interface reconstruction, faceting, or intermixing. 13 These studies illustrate that oxide thin film systems form through complex synthesis routes dictated by chemical compos...

Electronic Structure and Band Alignment of LaMnO3/SrTiO3 Polar/Nonpolar Heterojunctions

“…As plotted in Figure S5a of the Supporting Information, as the LMO layer thickness increases, we find that the V O formation energy decreases. This is consistent with previous results for other polar/nonpolar heterojunctions, such as La 2 MnNiO 6 /STO and LAO/STO . The presence of a surface V O is shown in Figure S5b of the Supporting Information to redistribute electronic charge within the LMO layer, including reducing a neighboring surface Mn 3+ to Mn 2+ …”

“…By contrast, the additional electronic, magnetic (in some cases), and chemical degrees of freedom in other complex oxides compared to LAO can lead to alternative mechanisms to alleviate the polar catastrophe. One example is manifested as NiO precipitation in La 2 NiMnO 6 films 1–5 nm from the STO interface and at the LaNiO 3 /STO interface . The formation of an NiO secondary phase appears to be driven by the formation of oxygen vacancies; at these polar/nonpolar interfaces, oxygen vacancy formation is favorable to reduce the magnitude of the built‐in electric field .…”

“…One example is manifested as NiO precipitation in La 2 NiMnO 6 films 1–5 nm from the STO interface and at the LaNiO 3 /STO interface . The formation of an NiO secondary phase appears to be driven by the formation of oxygen vacancies; at these polar/nonpolar interfaces, oxygen vacancy formation is favorable to reduce the magnitude of the built‐in electric field . Another mechanism to alleviate the polar catastrophe is a charge redistribution mechanism in which the interfacial charge remains on the L M O ( M = transition metal) side of the L M O/STO interface, reducing the valence charge of interfacial cations without forming a 2DEG.…”

Formation of Nickel Oxide Nanocuboids in Ferromagnetic La2Ni1−xMn1+xO6

“…To improve the magnetic properties of LNMO samples post-growth annealing processes are commonly employed [ 15 , 16 , 17 ]. Nonetheless, recent studies report the formation of the NiO phase impurities in LNMO thin films grown by MBE technique with the post-growth annealing, with apparently defect-free growth [ 18 , 19 ]. Detailed cross-sectional high-angle annular dark-field studies revealed an inverted “pyramid-like” shape morphology of the NiO precipitate, which progresses from a 2–3 nm base to a ∼10 nm wide mouth at the film surface [ 19 ].…”

“…Nonetheless, recent studies report the formation of the NiO phase impurities in LNMO thin films grown by MBE technique with the post-growth annealing, with apparently defect-free growth [ 18 , 19 ]. Detailed cross-sectional high-angle annular dark-field studies revealed an inverted “pyramid-like” shape morphology of the NiO precipitate, which progresses from a 2–3 nm base to a ∼10 nm wide mouth at the film surface [ 19 ]. The coexistence of double perovskite and NiO secondary phase is supported by first principles modeling of growth in oxygen deficient conditions with expected dissolution of inclusions during annealing [ 18 ].…”