Re-examining the nature of ordering in CaMnO2: The role of Mn-O covalency in the local structure

Considering the proposed structural anomaly in the "topotactically" reduced compound CaMnO₂, we investigated its local structure through extended x-ray absorption fine structure (EXAFS) spectroscopy. The in-depth analysis of the EXAFS results unraveled an unforeseen local chemical order of Mn and Ca, which helps to advance the previous understanding of average disordered scenario, obtained based on diffraction probes like x-ray and neutron. We established the microscopic origin of such locally ordered structure on the basis of Mn-O covalency through analysis of density functional theory calculations. Further, fluorination is attempted to explore the influence of modification of the anionic network and bond covalency. We found that the more ionic Mn-F bonds, compared to Mn-O bonds, drive a local phase separation between Mn-rich phase and Ca-rich phase.