INVAR and ELINVAR

The iron-nickel alloys called "invar" and "elinvar" were developed for use as metrical standards because of their "zero value" thermal expansion properties. The mechanism of their zero thermal expansion coefficients may be so-called "nickelide" bonding...where the iron atoms, in the alloyed lattice, donate a d electron, from the iron open d shell, in to the d shell of a neighboring Ni center bonded to the iron atoms through s-type delocalization. The inner d shell electron transfer completes the d shell of Ni as a closed dectet with zero angular momentum (Unsold's theorem) and creates a singly negatively charged nickelide center and a singly positively charged iron center (compare to CsAu with its auride anions). Heating such an alloy may possibly not be accompanied by lattice vibrations and thermal expansion...but perhaps maybe it instead may result in the addition of heat energy as it is deposited in an inner d shell electronic excitation within a continuum of states between the nickelide centers' d shell and the iron cationic centers' d shell. Nickelide bonding may also occur in diatomic molecules containing Ni and other 3d transition metals. This is just a hypothesis.