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The evolution of the corundum (-Al 2 O 3 ) structure was investigated at 297, 573, 873, and 1173 K using the single-crystal Xray diffraction method and molecular dynamics simulation. The results were smoothly extrapolated to the previously reported high-temperature data of the compound at 2170 K [Ishizawa et al.: Acta Crystallogr., Sect. B 36 (1980) 228], revealing a significant shift of the fractional coordinate z of the Al atom as a function of temperature. The Al atom shift indicates that the Al•••Al intermetallic repulsion force across a pair of face-sharing octahedra becomes conspicuous over other interatomic forces at high temperatures. The increasing Al•••Al repulsion is relieved by both the preferential expansion of a unit cell along the c-axis and the change in the z coordinate of the Al atom. The results of molecular dynamics simulation experiments qualitatively agreed with the X-ray diffraction data. The thermal expansion of the crystal was well described by the quadratic functions, a

Previous molecular dynamics (MD) simulations have shown that, with good choice of potential shapes and parameters, the structure and properties of stoichiometric LiMn2O4 can be reproduced to good accuracy. In particular, the experimentally observed self-diffusion of Li ions was shown to occur only for a discrete, mixed Mn valence model. Here, those MD studies have been extended, demonstrating that periodically switching the Mn valence states, reflecting an electron hopping model, greatly facilitates the Li ion self-diffusion. The interaction is mediated by the O atoms, which coordinate both cations and temporarily adopt local distortions based on the three coordinating Mn valences. Although the O atoms continue to vibrate harmonically about a displaced mean, permitting the Li atoms to migrate, the time and spatial average remains that of the ideal spinel with distributed Li.

Single-crystal synchrotron X-ray diffraction experiments are conducted on spinel-type LiMn 2 O 4 at 230 and 320 K to investigate the effect of charge disproportionation of Mn ions on phase transition near room temperature. The orthorhombic 3a c × 3a c × 1a c superstructure of the low-temperature form, where "a c " is the ideal cubic cell edge, has a network of Mn 4+ 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 Mn 3+ 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 Mn 4 O 4 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.

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