PNNL

The Science                                

Complex transition metal oxides have sufficient structural flexibility to produce a wide range of functional properties for electronics and energy storage applications. To control these properties, researchers grow thin layers of these materials seeking to control their crystal structure. Controlling crystal structure opens the door for tuning electronic structure and possibly achieving novel properties. Electronic structure in oxides is largely controlled by the charges on the cations. Ni⁴⁺ ions are energetically unstable and rarely found in chemical compounds. Now a team of researchers, led by scientists from Pacific Northwest National Laboratory, have stabilized Ni⁴⁺ in a carefully designed superlattice of SrNiO₃ and LaFeO₃. The cubic perovskite SrNiO₃, which is not observed in bulk form, was shown to form in these superlattices, but only at the thickness of one crystalline unit or unit cell.

The Impact

This work shows that accurate control of material structure through superlattice engineering may be generally useful for the synthesis and discovery of metastable, layered functional materials.

Summary

A team of researchers, led by scientists from Pacific Northwest National Laboratory, stabilized Ni⁴⁺ cations in a superlattice of SrNiO₃ and LaFeO₃. To do this, they deposited alternating layers of SrNiO₃ and LaFeO₃ with varying thicknesses for each and found that well-ordered SrNiO₃ nucleated only when a single unit cell of SrNiO₃ was included. By controlling the thickness of the adjacent LaFeO₃ layers, they could control the charges on the Fe and Ni cations.

If a single unit cell of LaFeO₃ is used, resulting in a (SrNiO₃)₁/(LaFeO₃)₁ superlattice, the Ni is trivalent (Ni³⁺) and the Fe is tetravalent (Fe⁴⁺). However, as the LaFeO₃ layer thickness increased, the Ni becomes tetravalent (Ni⁴⁺) and the Fe trivalent (Fe⁴⁺). It was found that additional layers of LaFeO₃ induce larger rotations of the FeO₆ octahedral cages in the LaFeO₃ layers. The result is that the structure of the LaFeO₃ layers approaches that of bulk LaFeO₃. This structural change modifies the densities of electronic states such that holes can localize in the SrNiO₃ layers, producing Ni⁴⁺ in the (SrNiO₃)₁/(LaFeO₃)₅ superlattice.

Contact

Scott Chambers
Pacific Northwest National Laboratory
scott.chambers@pnnl.gov

Funding

This work is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated by Argonne National Laboratory. This research was performed using the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research.