Overlap Contribution to the Isomer Shift of Iron Compounds

The electron density Ψ²(0) at the iron nucleus in oxides is calculated as a function of the Fe-O distance, by taking into account the overlap of the inner shells of iron with the oxygen $2p$ wave functions. The calculation shows the importance of the intershell terms, which effectively reduce the overlap-induced electron density. By comparing these results with the high-pressure isomer-shift measurements of divalent iron in CoO, the relative change of the ⁵⁷Fe nuclear radius is found to be less than -4.0×{}10^-4. The overlap effect provides an important amplification mechanism for the contribution of $4s$ bonding to Ψ²(0). On the basis of this mechanism, large increases in Ψ²(0) observed on going from Fe²⁺ to Fe³⁺ salts can be explained by a reasonable increase in the amount of $4s$ bonding.