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 batteries. We have solved the structure at 230 K and deciphered unambiguously the nature of this phase transition. The analysis of valence bond sums shows that the transition results from a partial charge ordering: two 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+ ions. The microscopic details obtained from the structure are crucial for understanding the electron hopping persisting below the transition. [S0031-9007(98)07667-4].