Control of octahedral rotations in (LaNiO${}_3$)${}_n$/(SrMnO${}_3$)${}_m$ superlattices

Oxygen octahedral rotations have been measured in short-period (LaNiO${}_3$)${}_n$/(SrMnO${}_3$)${}_m$ superlattices using synchrotron diffraction. The in-plane and out-of-plane bond angles and lengths are found to systematically vary with superlattice composition. Rotations are suppressed in structures with $m>n$, producing a nearly unrotated form of LaNiO${}_3$. Large rotations are present in structures with $m<n$, leading to reduced bond angles in SrMnO${}_3$. The metal-oxygen-metal bond lengths decrease as rotations are reduced, in contrast to behavior previously observed in strained, single-layer films. This result demonstrates that superlattice structures can be used to stabilize nonequilibrium octahedral behavior in a manner distinct from epitaxial strain, providing a novel means to engineer the electronic and ferroic properties of oxide heterostructures.

Figure 1

Scans along (00$L$) in three samples exhibiting distinct satellite peaks due to the coherent superlattice structure (a). The magnitude of the ($1/2$ $1/2$ $3/2$) peak decreases as the ratio of $m$ to $n$ increases, indicating the magnitude of the octahedral rotations is reduced (b). Inset of (b), the L4S2 data shown on a log scale; negligible satellite features are observed, consistent with coherent rotational behavior along the $c$ axis.

Figure 2

Scan through $L$ of the ($1/2$ $1/2$ $3/2$) peak in the L1S2 superlattice (a). The measured peak consists of a broad contribution from the STO substrate (curve centered at $L$ $=$1.5) and a narrow contribution of the octahedral rotations within the superlattice (curve centered at $L$ $=$1.533). The substrate peaks obscure the weaker superlattice peaks, such as the ($1/2$ $1/2$ $7/2$) (inset, a). The calculated intensity of the ($1/2$ $1/2$ $3/2$) peak as a function of the $\alpha$ rotation angle (b). At low $\gamma$ values, the ($1/2$ $1/2$ $3/2$) intensity is independent of $\gamma$.

Figure 3

The measured and calculated peak intensities for the L4S2 (a) and L2S2 (b) samples. In (c), the measured intensities of the ($1/2$ $1/2$ $3/2$) and ($1/2$ $3/2$ $3/2$) peaks, normalized by the L4S2 intensities, are given for the three samples.

Figure 4

Bond angles (a) and bond lengths (b) presented as a function of superlattice composition. The atomic structures of the L1S2 and L4S2 samples are shown in (c). The top structure is the L1S2 superlattice, while the bottom structure is the L4S2 superlattice.