Charge transfer and hetero-bonding across the solid–solid interface at room temperature

Mechanisms of an incipient chemical reaction at the contact points of oxide particles are elucidated on the basis of various experimental analyses as well as molecular orbital calculation. By milling a mixture of metal oxides, homogeneity on nanometer scale increases, accompanied by partial or total amorphization. Simultaneously, charge transfer is promoted by OH groups at the state of low coordination. Both of them are predominantly caused by local plastic deformation. A primary consequence of charge transfer is an incipient chemical reaction at the contact points of dissimilar particles, by virtue of hetero-bridging bonds between dissimilar metal atoms mediated by an oxygen atom. A mixture at this stage serves as a precursor or an intermediate to accomplish complex oxides by subsequent heating at temperatures much lower than conventional reactions in solid state.