An interstitial-electron model for the structure of metals and alloys, 5

Abstract On the basis of the interstitial-electron model for metals the essential feature of the metallic state is the relatively free movement of electrons, (\bare), between interstices in a close-packed array of metal ions (positive cores). For a solid solution of one metal in another in which the metal ions have nearly the same size and core charge, the interstitial-electron structures which incorporate spatial and spin correlation of electrons are the same as for metals. Metallic properties can also result when the ratios of two less similar metals in an alloy are such as to give interstices with a constant ratio of the two metals surrounding them. Occupancy of such "homogeneous interstices" is the basis for intermetallic compounds and accounts for the Hume-Rothery rules. Intermetallic compounds such as γ phases (e.g. Cu5Zn8) are unique in that \bare and electron pairs must be assigned to two or more types of interstices which have different ratios of the two metal ions around them. This localization of \bare density explains the very poor conductivity, the hardness, the brittleness and the large diamagnetic susceptibility of γ phases. The structural framework of the interstitial-electron description accounts for the large composition range of the interstitial phases and can naturally include effects of ion core size and charge. Interpenetration of d10 ion cores by itinerant electrons which gives a balancing of the positive fields of the 2 ion cores accounts for the preponderance of d10 metals among intermetallic phases.