The cooperative Jahn-Teller effect (CJTE) refers to the correlation of distortions arising from individual Jahn-Teller centres in complex compounds. The effect usually induces strong coupling between the static or dynamic charge, orbital and magnetic ordering, which has been related to many important phenomena such as colossal magnetoresistance and superconductivity. Here we report a Na5/8MnO2 superstructure with a pronounced static CJTE that is coupled to an unusual Na vacancy ordering. We visualize this coupled distortion and Na ordering down to the atomic scale. The Mn planes are periodically distorted by a charge modulation on the Mn stripes, which in turn drives an unusually large displacement of some Na ions through long-ranged Na-O-Mn(3+)-O-Na interactions into a highly distorted octahedral site. At lower temperatures, magnetic order appears, in which Mn atomic stripes with different magnetic couplings are interwoven with each other. Our work demonstrates the strong interaction between alkali ordering, displacement, and electronic and magnetic structure, and underlines the important role that structural details play in determining electronic behaviour.

Layered Na–metal oxides can form in different crystal structures, each with different electrochemical behavior. As a prototype system to better understand how each phase can be formed, we present the conditions under which different layered phases of Na x CoO2 can be stabilized in solid-state synthesis. Using a novel combination of ex situ XRD on as-synthesized samples, with in situ XRD to monitor the relation between Na content and lattice parameters, we are able to construct a phase diagram of Na x CoO2 between 450 to 750 °C in air and for Na:Co sample ratios ranging from 0.60 to 1.05. Four single phase domains of O3, O′3, P′3, and P2 are revealed based on the XRD analysis. In contrast to previous reports it is found that pure O3, O′3 and P′3 phase can only form at a fixed stoichiometry of x = 1.00, 0.83, and 0.67, respectively, while the P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging of O3–Na1.00CoO2 shows several flat and sloping regions on the voltage profile, which follows the sequence of O3–O′3–P′3–P3–P′3, with increasing interslab distances. Our results indicate that the electrochemically important P2 structure is likely stabilized by entropy.