Kenji Tateishi

The effect of mixed Mn valences on Li migration in LiMn2O4 spinel: A molecular dynamics study

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.

Structural Evolution of Corundum at High Temperatures

The evolution of the corundum (α-Al2O3) structure was investigated at 297, 573, 873, and 1173 K using the single-crystal X-ray 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 = 4.75385 (1.0 + 5.05841×10-6T + 1.69084×10-9T2) A, and c = 12.9782 (1.0 + 4.61344×10-6T + 2.63802×10-9T2) A.

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

Abstract 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.

Synchrotron X-ray study of noncentrosymmetric Tb(3)RuO(7) with partial structural disorder.

Single-crystal synchrotron X-ray diffraction reveals partial structural disorder of Tb atoms at 293 K in flux-grown Tb(3)RuO(7) (triterbium ruthenium heptaoxide) crystals. The structure is noncentrosymmetric and composed of infinite single chains of corner-linked RuO(6) octahedra embedded in a Tb(3)O matrix. Two Tb atom sites out of the six crystallographically independent Tb sites are split into two positions. The split sites are separated by approximately 0.3-0.4 Angstrom, with slightly different coordination environments. The RuO(6) octahedra in the present P2(1)nb modification have two tilt systems about the a and c axes, in contrast with a single tilt about c in the other Cmcm modifications of Ln(3)RuO(7) (Ln = lanthanoid elements).

The effect of Fourier series truncation errors on the electron density distribution of LiMn2O4

C423 sets. Therefore, there are only two reports of high-resolution magnetic Compton experiments [2, 3] in this decade. A new setup of the Cauchois-type x-ray spectrometer for Compton scattering experiments installed on BL08W at SPring-8 allows us to perform high-resolution experiments within a reasonable beam time. This spectrometer employs an X-ray image intensifier as a position sensitive detector. Using this spectrometer, a high-resolution magnetic Compton profile of iron single-crystal was measured with a momentum resolution of 0.14 atomic units. The statistical accuracy is approximately 4% at pz=0. The experiment shows a good agreement with the theoretical prediction [1].