It was recalled that a constrained state of solid matter was known to exist within the cores of crystal defects such as intercrystalline interfaces. This constrained state of solid matter differed in structure and properties from unconstrained solid states, such as perfect crystals and glasses, in terms of its atomic and electronic structure and its chemical composition. The basic concept of the nanocrystalline solid was thus to generate a novel type of material by incorporating a high density of defect cores, into a previously perfect crystal, to such an extent that the total volume of defect cores became comparable to the total volume of residual lattice regions between the defect cores. The resultant solids were termed nanocrystalline solids. Due to the large volume fraction of defect cores, nanocrystalline solids differed from other forms of solid (single crystals, coarse-grained polycrystals, glasses) in terms of their atomic and electronic structures, their chemical compositions and the fact that the size of the crystalline regions between neighbouring defects was reduced to a few interatomic spacings. Because the properties of solids depended on precisely the parameters of atomic structure, electronic structure, chemical composition and crystal size, the properties of nanocrystalline solids deviated from those of crystalline or glassy materials. Attention was focussed here on tuning the electronic structure of solids by varying their nanostructure. It was concluded that solids with nm-sized microstructures could constitute materials having an excess or deficit of electrons or holes of up to 0.3 electrons and holes per atom. That is, they effectively constituted elements that were electronically between the electrically neutral elements of the periodic table. Large deviations from charge neutrality could be obtained by using an externally applied voltage or by space charges at the interfaces between materials with mobile charge carriers (metals or semiconductors) and/or with differing chemical compositions. Because many properties of solid materials depended upon the electronic structure, large deviations from charge neutrality were expected to lead to materials having new and unexplored properties: such as modified electrical, ferromagnetic and optical properties.

Is there a Hidden World of New Materials and Effects “Between” the Elements of the Periodic Table? H.Gleiter: Materials Transactions, 2003, 44[6], 1057-67