Particle size dependency of ternary diagrams at the nanometer scale:: Evidence of TiO2 clusters in Fe-based spinels

The bulk (Fe,Ti,O) phase diagram is strongly modified at the nanometer scale: instead of the mixture of the rhombohedral and pseudo-brookite phases normally present in the micrometer range, a single fee phase is stabilized for grain size lower than 25 nm. The existence of a single phase permits the formation of small TiO2 clusters. which cannot happen in micrometer scaled compounds because of cluster growth until phase separation. Such clusters have been studied with techniques revealing order at different ranges. With diffraction experiments (revealing long range order), it has been shown that the crystallographic coherence between the fee phase and the clusters is broken. With absorption spectroscopy experiments (revealing short range order), it was evidenced that the clusters consist of a compact stack of 4 TiO2 entities. The presence of the fee matrix around the clusters leads then to a new magic number of 4. This corresponds to a very stable configuration of clusters constrained to reside in an embedding fee structure.