In Situ Characterization of Cobalt Nanoparticle Exsolution During Topotactic Transformations in Lanthanum-Strontium-Cobaltite Thin Films.

The functional properties of complex oxides are strongly coupled to their crystal structure and their compositions, in which minute changes can be used to precisely tune their electronic and magnetic properties for a variety of purposes. These properties can be tailored via topotactic phase transformations, in which crystallographic relationships between phases are maintained through the transformation. Lanthanum-strontium-cobaltite (LSCO) is a complex oxide that possesses high oxygen vacancy conductivity and low oxygen vacancy formation energy, which makes it an ideal platform to study topotactic transformations [1]. This study investigates the properties of LSCO thin films during topotactic transformations via in-situ x-ray photoelectron spectroscopy (XPS) and grazing-incidence small-angle x-ray scattering (GISAXS). These methods allow for the simultaneous observation of changes in surface chemistry via XPS and of changes in surface topography via GISAXS [2]. With these methods, we observe the reduction of perovskite phase LSCO (La_0.7Sr_0.3CoO₃) to the Ruddlesden-Popper phase LSCO (La_1.4Sr_0.6Co_(1+��)O_(4−��), where 0≤��<1 and 0<��<1) in-situ. This transition differs from other common topotactic transitions, such as from perovskite LSCO to brownmillerite, square-planar, or Grenier phases, in that the Ruddlesden-Popper transformation necessitates the exsolution of Co cations in addition to the typical removal of O anions [2]. The in-situ XPS measurements observed an increase in the concentration of Co at the surface, and a shift in binding energies in the Co 2p spectra indicative of metallic Co exsolution. The in-situ GISAXS measurements observed an increase in surface roughness that corresponds to nanoparticle formation. These in-situ measurements are supported by a suite of ex-situ measurements, including x-ray diffraction, atomic force microscopy, x-ray absorption, and x-ray magnetic circular dichroism, which ascertain that the topotactic transformation has taken place, and that magnetic cobalt nanoparticles were exsolved from the structure. This work can help to inform future work in the development of devices that utilize topotactic transformations during operation, and can help to facilitate the development of LSCO based devices for applications such as neuromorphic computing or solid-oxide fuel cells.

[1] Chiu, I., Takamura, Y. et al.; Physical Review Materials, 5(6), 1–9. (2021).
[2] Heath Kersell, Slavomír Nemšák et. al.; Rev. Sci. Instrum. 92 (4), (2021).