Our paper, led by FINO Lab Ph.D. graduate Miles Blanchet, is out in the Journal of Vacuum Science and Technology A today! This work focused on the synthesis of spinel MnxCo3-xO4 films on MgAl2O4 substrates by MBE. These materials are highly relevant for electrocatalysis and energy storage, but there have been few studies of films in the range of x = 1 and a lot of inconsistencies in the characterization of powder and nanoparticle samples. By combining careful MBE synthesis with in vacuo XPS and ex situ X-ray diffraction, electron microscopy by Bethany Matthews and Steven Spurgeon, and hard X-ray absorption spectroscopy performed at the Advanced Photon Source by Steve Heald, we’re able to understand how oxygen vacancies in the films and the Jahn-Teller (JT) activity of Mn3+ ions on the octahedral site of the spinel affect phase stability and the propensity of the materials to phase segregate.
Because MnCo2O4 is an inverse spinel with an electron donated from Mn to Co to produce Mn4+ and Co2+ ions on the B site, any oxygen vacancies that are present will enhance the JT distortion in the material. We see signs of the JT distortion even in single phase films via polarized X-ray absorption spectroscopy, which shows that films with nominally ideal x = 1 stoichiometry have similar features to the tetragonal, JT-active CoMn2O4 spinel films we have studied previously. These distortions correlate well with phase segregation that we observe in many samples around that stoichiometry. Such phase segregation has been observed previously in nanoparticles, and we think that our studies of single-crystal films help to explain the disparities in the literature on this very interesting material.
Our work on this project was supported by our NSF Division of Materials Research grant, NSF-DMR-1809847.