Nanoarchitectonics of self-assembled chessboard-like structures by recurrent phase separation and coalescence of nano domains in CoFeMn oxide

Abstract

Self-assembled chessboard-like (CB-like) nanostructures in oxides are potential candidate materials for high density memory and energy storage devices. The evolution of CB-like nanostructures has been studied by combining transmission electron microscopy and 3D atom probe tomography in spinel-forming CoFeMn oxide. CB-like nanostructures evolve through a complex process of twinning, subdomain formation through recurrent pseudo-spinodal decomposition and domain coalescence along specific crystallographic axes. It initiates with the cross-penetration of compound deformation twins with subtle chemical composition separation between different domains, which ultimately results in the polymorphic transformation of cubic and tetragonal phases. The requirement of interfacial strain minimization leads to the formation of two tetragonal and two cubic spinel domains of different compositions which are rotationally orientated. In the CB-like nanostructure, cubic and tetragonal domains are connected along 〈022〉 directions. Cubic with cubic domains, and tetragonal with tetragonal domains, are connected along 〈040〉 or 〈004〉 directions. Recurring pseudo-spinodal decomposition within the CB-like domains leads to the formation of finer CB-like nanodomains. The formation of cuboid shaped domains of the cubic and tetragonal phases to rod shape morphology with four {220} and two {001} faces can give rise to 2D CB-like nanostructures. Combined transmission electron microscopy and atom probe tomography analysis indicates that CB-like nanostructures can also form in 3D, and a new model for this has been proposed based on the octahedral/cuboidal shape of the compositionally separated domains. The coalescence of such nanostructures along 〈004〉 directions gives rise to a rod-like morphology. The proposed model is expected to lead to a paradigm shift in the current understanding of CB-like nanostructures.