Coexisting commensurate and incommensurate charge ordered phases in CoO

Abstract

The subtle interplay of strong electronic correlations in a distorted crystal lattice often leads to the evolution of novel emergent functionalities in the strongly correlated materials (SCM). Here, we unravel such unprecedented commensurate (COM) and incommensurate (ICOM) charge ordered (CO) phases at room temperature in a simple transition-metal mono-oxide, namely CoO. The electron difraction pattern unveils a COM (q1=1 2 (1, 1, 1¯) and ICOM (q2 = 0.213(1, 1, 1¯)) periodic lattice distortion. Transmission electron microscopy (TEM) captures unidirectional and bidirectional stripe patterns of charge density modulations. The widespread phase singularities in the phase-feld of the order parameter (OP) afrms the abundant topological disorder. Using, density functional theory (DFT) calculations, we demystify the underlying electronic mechanism. The DFT study shows that a cation disordering (Co1−xO, with x = 4.17%) stabilizes Jahn-Teller (JT) distortion and localized aliovalent Co3+ states in CoO. Therefore, the lattice distortion accompanied with mixed valence states (Co3+, Co2+) states introduces CO in CoO. Our fndings ofer an electronic paradigm to engineer CO to exploit the associated electronic functionalities in widely available transition-metal mono-oxides.