Simulation of STEM ‐ HAADF image contrast of Ruddlesden Popper faulted LaNiO3 thin films

Abstract

Perovskite nickelates are interesting candidates to be used as electrode materials or in the research towards artificial superconductors. Among them, LaNiO 3 (LNO) is of high interest because of its exceptional transport properties, with a resistivity lower than 100 µΩ·cm, and the potential tuning of electrical and magnetic interactions through appropriated strain engineering. A precise control of the stoichiometry, structural phases, lattice distortions and the presence/absence of dislocations/defects is critical to make LNO electrodes and related heterostructures suitable for electronic applications 1–3 . Frequent defects present in perovskite structures are Ruddlesden Popper (RP) faults which consist on the relative displacement of two perfect defect free perovskite ABO 3 blocks a distance of half‐ unit cell along the [111] direction. The earlier observations of RP faults were based on diffraction results, but they have also been imaged using advanced microscopy tools 4,5 . When observed along the [001] zone axis, this defect appears as a zig‐zag arrangement of the A cations with a BO 2 plane lost at the defect boundary. In the present work we focus our attention on the characterization of RP faults observed in LNO thin films 14 nm and 35 nm thick, grown by pulsed laser deposition on (001) oriented LaAlO 3 (LAO) single crystal substrates at an oxygen pressure P = 0.15 mbar and at a temperature T = 700°C. The preliminary characterization of the layers by high resolution transmission electron microscopy (HREM) and electron diffraction, confirmed the good pseudo‐cube‐on‐cube epitaxial growth with atomic‐sharp interfaces and the expected [010]LNO(001)//[010]LAO(001) epitaxial relationship, despite the compressive strain driven by the 1% mismatch between LNO (3.838 Å) and LAO (3.795 Å). Nevertheless, defects identified as potential RP faults oriented in both [100] and [010] directions were also observed. Detailed high angle annular dark field (HAADF) imaging of these defects enabled a better identification of the defects as Ruddlesden Popper type through the appropriated correlation of the contrast of the atomic columns with cationic La(A) and Ni(B) positions as shown in the figure 1. We will systematically address the interpretation of the contrast of these HAADF images through STEM‐HAADF simulations calculated through the multislice procedure 6 from atomistic models based on different arrangements of defect free perovskite blocks with octaetrahedral Ni 3+ coordination (Fig. 2). Gradual variation of Z‐contrast is interpreted in terms of the average of the atomic numbers of the La and Ni in A and B sites in RP displaced overlapping perovskite crystals 7 . The good agreement between the experimental images and the simulated ones (Fig. 3) validates the proposed geometrical configurations of the RP faulted crystals.