Octahedral tilting in strained LaVO${}_3$ thin films

The effect of biaxial strain on the oxygen octahedra rotations in a LaVO${}_3$ thin film is investigated using synchrotron radiation. First, we find that the film adopts a distorted orthorhombic structure under the compressive stress induced by the SrTiO${}_3$ substrate. Second, we separate the contribution to the superstructure peaks arising from cation displacement and VO${}_6$ rotations in order to quantify the rotation angles. Finally, we find an original $a^{-}{a}^{+}{c}^{-}$ tilt system, which is induced by the biaxial strain imposed by the substrate. These quantitative results may open up new directions for understanding the modification of electronic properties of engineered oxide films.

Figure 1

$\theta -2\theta$ XRD pattern of LaVO${}_3$ film grown on a (001)-oriented SrTiO${}_3$ substrate, recorded along the [002] direction of SrTiO${}_3$. The presence of the Laue fringes around the (202) diffraction peak of the LaVO${}_3$ film attests to the high quality of the sample, both in terms of surface and interface roughness. The simulation of the spectra was done with diffax taking into account only the film contribution.[40] The matching between the experimental spectra and the simulation confirms the thickness of the film closed to 734 Å, corresponding to 186 pseudocubic unit cells.

Figure 2

XRD $\phi$ scans recorded along the (103) direction of the (001)-oriented SrTiO${}_3$ substrate (bottom panel) and the (103)${}_p$ pseudocubic direction of the LaVO${}_3$ film (top panel). The alignment of both scans confirms a cube-on-cube epitaxy of LaVO${}_3$ thin film grown on a (001)-oriented SrTiO${}_3$ substrate, considering a pseudocubic description for LaVO${}_3$ (see text for details).

Figure 3

LaVO${}_3$ XRD patterns recorded around the asymmetric diffraction peaks (204)${}_o$, (402)${}_o$, (323)${}_o$, and (3-23)${}_o$ corresponding to the 103${}_p$ family. The matching between the intensities and the positions of the four peaks indicates that $c_p$ is normal to the substrate plane.

Figure 4

Schematic representation of the orientation of an epitaxial LaVO${}_3$ thin film grown under compressive stress on a cubic (001)-oriented SrTiO${}_3$ substrate. $a_s$ is the substrate lattice parameter (3.905  Å); $a_p$, $b_p$, and $c_p$ are the pseudocubic axes [$a_p=b_p=3.91\left(3\right)$ Å, $c_p=3.945\left(1\right)$ Å] and $a_o$, $b_o$, and $c_o$ are the orthorhombic axes; and ${\beta }_o$ is the angle between $a_o$ and $c_o$ [$a_o=c_o=5.55\left(2\right)$ Å, $b_o=7.82\left(6\right)$ Å, and ${\beta }_o=89.489{\left(6\right)}^{\circ }$].

Figure 5

XRD patterns of the half-order peaks produced by in-phase octahedral rotations.

Figure 6

Theoretical powder diffraction patterns of LaVO${}_3$ produced by the structure with: (1) neither La displacement nor octahedral rotation, (2) La displacements but no octahedral rotation, (3) no displacement but with octahedral rotations, and (4) octahedral rotation and La displacement.

Figure 7

XRD patterns of equivalent half-order peaks. $A$, $B$, and $C$ correspond to the peak family ${\left\{0\frac{1}{2}\frac{3}{2}\right\}}_p$, ${\left\{\frac{1}{2}\frac{1}{2}\frac{3}{2}\right\}}_p$, and ${\left\{1\frac{1}{2}\frac{3}{2}\right\}}_p$, respectively; see text for details.