Thickness-Dependent Perovskite Octahedral Distortions at Heterointerfaces

Abstract

In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO₃ and LaAlO₃ grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO₃ and LaAlO₃ substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO₃/SrTiO₃ and LaAlO₃/SrTiO₃ interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO₃/LaAlO₃ interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.

Keywords: Perovskite; X-ray diffraction; heterostructure; nickelate; structure; thin film.

Figure 1

A simplified schematic of the response to biaxial strain through (a) pure bond length modifications and (b) pure oxygen octahedral rotations.

Figure 2

Pseudocubic c-axis lattice parameter as a function of total film thickness for the three heterostructure types, LaNiO₃/SrTiO₃, LaNiO₃/LaAlO₃, and LaAlO₃/SrTiO₃. Bulk lattice parameters for all materials are denoted by the black dashed line. The wider dashed lines show the film c-axis parameters that would be expected for a given heterointerface assuming only a volume change governed by the Poisson ratio (ν_LNO = 0.34 from ref (39) and ν_LAO = 0.22 from ref (40)). Error bars derive from the reliability of fitting the finite thickness fringes in InteractiveXRDFit, which increases with film thickness.

Figure 3

Diffraction intensity close to the reflection as an example of half integer Bragg peaks for the series of LaNiO₃/LaAlO₃ samples showing the evolution with film thickness. Solid lines are the fits to the data of a Gaussian function plus a linear slope and a constant background. The data was vertically displaced for clarity.

Figure 4

Oxygen octahedral tilt (a) and rotation (b) magnitudes as a function of total film thickness in pc u.c. for the three types of heterostructure system analyzed. Dashed lines indicate the octahedral tilt and rotation magnitudes for the bulk materials LaNiO₃ and LaAlO₃ (SrTiO₃ having no room-temperature octahedral distortions). The uncertainties are generated as described in the Supporting Information. An inset on each of the figures illustrates the relevant Glazer angles, α, β, and γ. Panels c and d are side view sketches of the octahedral tilt systems determined for, respectively, the LaNiO₃/SrTiO₃ and LaNiO₃/LaAlO₃ films, with the true measured angles. Three representative thicknesses of each series are shown. A strong evolution with thickness is seen for LaNiO₃/SrTiO₃ (panel c), whereas in panel d there is little variation with thickness for the LaNiO₃/LaAlO₃ series.