Type of presentation: Poster

MS-12-P-1985 Effect of substrate-induced strain on the structure of LaNiO3 thin films

López-Conesa L.1, Rebled J. M.1,2, Pesquera D.2, Sánchez F.2, Dix N.2, Magén C.3,4, Serra R.5, Casanove M. J.5, Estradé S.6, Fontcuberta J.2, Peiró F.1
1Laboratory of Electron Nanoscopies (LENS-MIND-IN2UB), Departament d'Electrònica, Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain, 2Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08153 Bellaterra, Spain, 3Laboratorio de Microscopías Avanzadas - Instituto de Nanociencia de Aragón (LMA-INA), Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain, 4Fundación ARAID, 50018 Zaragoza, Spain, 5Centre d'Elaboration des Matériaux et d'Etudes Structurales (CEMES-CNRS), 29 Rue Jeanne Marvig, Toulouse, France, 6Centres Científics i Tecnològics de la Universitat de Barcelona (CCiT-UB), C/Lluís Solé i Sabarís 1, Barcelona, Spain
llopez@el.ub.edu

LaNiO3 (LNO) is a perovskite of great importance in complex oxide electronics. Its low resistivity at room temperature and high chemical stability make it an ideal electrode candidate for many applications in complex oxide-based devices. Strain, oxygen vacancies and their mutual interplay are key aspects to understand the transport properties of these oxides1,2, since they might affect the Ni-O hybridization. In this study, we perform a thorough analysis of these aspects with high spatial resolution TEM.
We have studied LNO thin films of different thicknesses (14 nm and 35 nm) grown on several substrates that allow studying a wide range of compressive (LAO and YAO) and tensile (LSAT and STO) strain states. Aberration corrected HRTEM, HAADF-STEM, atomic resolution EELS mapping and image simulation studies have been carried out. Strain states in the films have been studied by Geometric Phase Analysis (GPA) of the high resolution images.
The presence of brownmillerite phase has been detected in the LNO strained films (figures 1 and 2). This perovskite-related superstructure occurs when oxygen vacancies order along a given crystallographic direction. Contrast modulation in HRTEM images and Z contrast in HAADF images are consistent with this vacancy ordering. Image simulations (both HRTEM and HAADF) support these findings. We report on the effect of the strain state of the film on the occurrence and orientation of the brownmillerite superstructure.
Moreover, unexpected box-like defects are found in all the films (figure 3). Defect boundaries correspond to a displacement of 1/2 of the perovskite unit cell both in the in-plane and out-of-plane directions. High resolution STEM-EELS spectrum imaging confirms a missing Ni-O plane at these boundaries. Signals from the overlapping La and Ni edges have been separated and extracted using the Blind Source Separation (BSS) method in the Hyperspy advaced signal processing toolbox.

[1] J. Chakhalian, A. J. Millis, and J. Rondinelli, Nature Mater. 11, 92 (2012)
[2] I.V. Nikulin, M.A. Novojilov, A.R. Kaul, S.N. Mudretsova and S.V. Kondrashov, Mater. Res. Bull. 39, 775-791 (2004)

We acknowledge the financial support from the Spanish Ministry of Economy and Competitivity via projects Imagine-Consolider CSD2009-2013, MAT2010-16407 and FPI and JAE predoc grants. We acknowledge the Catalan Government for financial support via project CTP2011-00018.

Fig. 1: High resolution HAADF-STEM image of LNO on LAO (-1% lattice mismatch). The brownmillerite phase is visible in the contrast modulation perpendicular to the substrate/film interface and in the superstructure spots in the FFT in the inset.

Fig. 2: HRTEM image of LNO on LSAT (+1% lattice mismatch). The brownmillerite phase is visible in the contrast modulation parallel to the substrate/film interface.

Fig. 3: HAADF-STEM image and elemental EELS maps of a defect boundary. a) HAADF reference image b) HAADF image acquired simultaneosuly to the EELS spectra. c) Ni map. d) La map. e) Composite: La in yellow and Ni in red. Displacement of 1/2 of the perovskite unit cell is visible. EELS mappings show a missing Ni-O plane in the boundary.