Oxygen vacancy order–disorder transition process during topotactic filament formation in a perovskite oxide tracked by Raman microscopy and transmission electron microscopy.

This article focuses on the investigation of chemical lattice strains and topotactic phase transitions in calcium-doped bismuth ferrite (Bi1-xCaxFeO3-δ, BCFO) thin films, a type of vacancy-ordered perovskite oxide. Using Raman spectroscopy and transmission electron microscopy (TEM), the study tracks the oxidation-induced formation of conducting filaments associated with the melting of oxygen vacancy ordering, revealing distinct lattice parameter changes in oxygen and vacancy layers during the transition. Notably, a negative Raman chemical shift during oxidation is observed, attributed to the expansion of the oxygen layer despite overall lattice contraction, highlighting complex electro-chemo-mechanical coupling at the atomic scale. The findings demonstrate that Raman microscopy can selectively probe local chemical lattice strains in vacancy-ordered oxides, providing insights relevant to the stability and functionality of electrochemical devices employing such materials.