Bond-length fluctuation in the orthorhombic 3 x 3 x 1 superstructure of LiMn2O4 spinel

Single-crystal synchrotron X-ray diffraction experiments are conducted on spinel-type LiMn2O4 at 230 and 320 K to investigate the effect of charge disproportionation of Mn ions on phase transition near room temperature. The orthorhombic 3ac × 3ac × 1ac superstructure of the low-temperature form, where “ac” is the ideal cubic cell edge, has a network of Mn4+ ions at the vertices of a slightly distorted truncated square tessellation comprising one square and two octagonal prisms; the square prism and one type of octagonal prism house Mn3+ ions with Jahn-Teller (JT) elongated Mn-O bonds almost parallel to the c and b axes, respectively, whereas the other octagonal prism houses Mn ions with JT-induced bond-length fluctuation for the Mn-O bonds lying almost parallel to the a axis. The Mn ions in the latter octagonal prism are assumed to exchange their oxidation states dynamically between 3+ and 4+ in a time ratio of -3:1, forming a polaron centered at a Mn4O4 heterocubane cluster with orbital and spin orders. The high-temperature cubic form contains an inherent positional disordering of oxygen ions. The effect of the molecular polarons on the phase transition mechanism is discussed on the basis of a spin blockade in the form of truncated square tessellation.