ELECTRONIC CRYSTALLIZATION IN A LITHIUM BATTERY MATERIAL - COLUMNAR ORDERING OF ELECTRONS AND HOLES IN THE SPINEL LIMN2O4

LiMn2O4 presents a first order structural transition at 290 K that was known to perturb the functioning as cathode in rechargeable Li batter ies. We have solved the structure at 230 K and deciphered unambiguousl y the nature of this phase transition. The analysis of valence bond su ms shows that the transition results from a partial charge ordering: t wo of the five Mn sites correspond to well-defined Mn4+ and the other three sites are close to Mn3+ ions. Charge ordering is accompanied by simultaneous orbital ordering due to the Jahn-Teller effect in Mn3+ io ns. The microscopic details obtained from the structure are crucial fo r understanding the electron hopping persisting below the transition. [S0031-9007(98)07667-4].