Spinels renaissance: The past, present, and future of those ubiquitous minerals and materials. Bond-length fluctuation in the orthorhombic 3 × 3 × 1 superstructure of LiMn2O4spinel

Single-crystal synchrotron X-ray diffraction experiments are conducted on spinel-type LiMn2O4at 230 and 320 K to investigate the effect of charge disproportionation of Mn ions on phase transition near room temperature. The orthorhombic 3ac× 3ac× 1acsuperstructure 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 Mn4O4heterocubane 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. © 2014 by Walter de Gruyter Berlin/Boston.