Rock salt-spinel structural transformation in anodically electrodeposited Mn-Co-O nanocrystals

Mn-Co-O nanocrystals, with a defective rock salt-type structure, were prepared by anodic electrodeposition. The structural and chemical stability of the as-deposited oxide nanocrystals were investigated using wavelength dispersive spectroscopy (WDS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The rock salt-spinel structural transformation occurred in the oxide nanocrystals annealed at a temperature of 500 degrees C. A cubic Fd (3) over barm spinel-type structure was obtained in the Co-rich, Mn-Co-O oxide nanocrystals, while a distorted tetragonal-type spinel phase with a space group of I4(1)/amd, induced by the Jahn-Teller effect, formed in their Mn-rich counterparts. It is noted that the structural transformation involves the migration of Mn/Co cations from the octahedral interstices to the tetrahedral interstices, which is accompanied by the reduction of Mn/Co cations and oxygen evolution. A novel mechanism for the structural transformation and reaction scheme is proposed in this work.