Publication Date:
2014-08-13
Description:
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 3 a c x 3 a c x 1 a 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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
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