Structural study of SrIrO₃

L. Horák¹, X. Martí¹, D. Kriegner¹, C. Frontera²

¹Department of Condensed Matter Physics, Charles University in Prague, Czech Republic

²Magnetic Materials and Functional Oxides, Institut de Ciencia de Materials de Barcelona, Spain

horak@karlov.mff.cuni.cz

We present a method that allows to determine atom positions in the thin layer epitaxially grown on the substrate whose structure is well known. Usually, the unit cell diameters of the layer and the substrate is very similar which results in the overlap of the diffraction maxima of the layer and the substrate in the reciprocal space. Since, it is not possible to determine the intensity of the layer peak, and consequently, to determine the atomic positions from the structure-factor amplitude. On the other hand, the knowledge of the atomic arrangement in thin strained layers is very important as it is related to the material properties which can be diametrically different from those in a bulk.

The measured diffraction curve along the crystal truncation rod is a result of the interference between the wave diffracted in the substrate and in the layer. From the atomic arrangement in the substrate unit cell, which is usually well-known for the bulk single crystal, it is possible calculate exactly the diffracted wave by the substrate. We use the dynamical theory of diffraction to calculate the diffraction curve [1]. Here, the known substrate wave serves as a reference and the atomic arrangement in the layer is refined to fit the experimental data.

We demonstrated this method on a set of epitaxial layers of SrIrO3 grown on different substrates (DyScO3, NdScO3, GdO3). Several tenths of the diffractions for each sample were measured and all experimental data were simultaneously fitted. The first guess was derived from the structure of the substrate as all materials in this class have a similar atomic arrangement and the same space group (PnmB). From the fitting we were able to refine atomic arrangement. The sensitivity of this method is demonstrated by the figure bellow which shows that the interference pattern significantly differs for the guessed (started) arrangement and for the refined one.

Figure 1. On the left, there is shown the unit cell of SrIrO3 (sp.gr. Pnmb) simultaneously for both considered atom configurations: the substrate-like atomic arrangement and the refined atomic arrangement. The positions of the heavy atoms are very similar for both cases whereas the positions of the oxygen atoms differ. Their positions are indicated by the green (substrate-like) and blue spheres (refined arrangement). The plot on the right shows the diffraction curve measured (red points) at SrIrO3/NdScO3 (111) diffraction maxima. We simulated the diffraction curve based on the substrate-like (green curve) and the refined (blue curve) atomic arrangement.

References

1. Pietsch, U., Holý, V. and Baumbach, T., High-resolution X-ray Scattering from Thin Films to Lateral

Nanostructures (New York: Springer), 2004