Thickness-dependent perovskite octahedral distortions at heterointerfaces

The plethora of exotic physics found in transition metal oxides is no accident, it directly reflects the particularly fascinating chemistry of the d-orbitals. The d wavefunctions are highly directional and electronically very localized, leading to a strong coupling between the lattice, electronic and spin degrees of freedom. This means, therefore, that fine details of the crystal structure can have important consequences for the material properties. One famous example of this remarkable sensitivity is the family of perovskite rare earth nickelates where the temperature of their electronic and magnetic phase transitions are greatly modified by small changes of the Ni-O-Ni bond angle. The sublattice of corner connected O₆ octahedra is therefore essential in driving the physics of these materials and many other perovskites like them.
Within the field of perovskite oxide heterostructures, how the oxygen sublattice reacts to the heterointerfacial constraints, as well as the contstraint of keeping the octahedra connected, is of great interest [1]. Complicating matters, however, is the difficulty in probing a light element such as oxygen when the sample volume is - by design - small and the distortions themselves may be minute.
Here we have demonstrated that the synchrotron x-ray diffraction approach successfully developed to extract the oxygen positions from half-order Bragg peaks [2] can be applied to films as thin as 2 nm [3]. Our study looks at LaNiO₃ and LaAlO₃ films on SrTiO₃ and LaAlO₃ substrates. It reveals that biaxial strain alone cannot account for the accommodation of the heterointerface when the thickness approaches the ultrathin limit and, thus, highlights layer thickness as a control parameter to adjust the lattice on the picometer scale.



[1]. J. M. Rondinelli, S. J. May and J. W. Freeland, MRS Bulletin 37 (2012)
[2]. S. J. May et al, Phys. Rev. B 82 (2010)
[3]. J. Fowlie et al, ACS Nano Lett. 19 (2019)