Atomic-scale investigation of nickelate-based perovskite superlattices

Abstract text (incl. figure legends and references)

Nickelate-perovskite heterostructures offer outstanding oppportunities to design technologically relevant structures with functionalities such as unconventional magnetic, electric, catalytic features and even the possibility of superconductivity [1]. The wide variety of fuctional properties stems from the strong hybridisation between the Ni d and oxygen p orbitals. Targetted manipulation of the oxygen environment of the Ni can be realised by structural distortions, which are associated with the deformation or rotations of oxygen octahedra of the perovkites [2].

Superlattices consisting of dissimilar perovskite-oxides offer several tuning parameters for direct control over the octahedral network. Besides the epitaxial strain, spatial confinement, octahedral connectivity and symmetry mismatch at the heterointerfaces can alter the crystal structure and induce sizable changes in the electronic properties. However, targetted design of properties through the orbital-lattice interaction is impeded by poor understanding of the heterostructuring parameters on the structure and their resultant effect on the orbital response. In order to disentagle the effects of heterostructuring parameters and understand the underlaying mechanisms, atomic scale investigation of the oxygen octahedral network is necessary.

In this work, we investigated structural and transport properties of LaNiO₃/LaGaO₃ (LNO/LGO) superlattices (SLs). SLs with stacking sequences of LNO/LGO: 8/4, 4/4, and 4/8 pseudocubic unit cell were grown on (001) SrTiO₃ substrate by pulsed-laser deposition. Cross-sectional HRTEM specimens were prepared by focused ion beam. Negative-spherical-aberration imaging was used to resolve the oxygen atomic columns using C_s-corrected HRTEM (FEI TITAN) operated at an electron accelerating voltage of 300 kV. Electrical transport measurements were performed in van-der-Pauw geometry.

The results revealed, that the three stacking sequences of LNO/LGO stabilised different non-equilibrium states of the LNO. Within the LNO layers, we found significant variation in octahedral rotations and distortions in the vicinity of the LGO-LNO interface and away from the interface. We explained these results based on the varying contribution degree of epitaxial strain and spatial confinement by LGO on the structure evolution. The relative thickness ratio of the components also has an influence on the "strength" of the interfacial octahedral connectivity determining different structural distortions under the same epitaxial-strain condition. Thus the contribution of octahedral connectivity to epitaxial strain became tunable. Associated tunability of the electronic properties was comfirmed by transport measurements. The results show, that superlattices are effective platforms to optimize perovskite nickelate functionalities and manipulate the material properties in a controllable way.

Acknowledgements

We are grateful for financial support from DFG 323667265. We gratefully acknowledge Manuel Mundszinger for TEM sample preparation.

[1] S. Catalano, M. Gibert, J. Fowlie, J. Íñiguez, J.-M. Tricone, and J. Kreisel, Rep. Prog. Phys., 2018

[2] J.M. Rondinelli, S.J. May, and J.W. Freeland, MRS Bulletin, 2012