We have found that spinel AlV2O4 shows a phase transition at ∼700K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V2.5+ ions are separated into three V2.5-δ ions and one V2.5+3δ ion. This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of "charge frustration.".
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