3dDensity Distribution and the Intrinsic Temperature Dependence of the Mössbauer Isomer Shift in Iron Compounds

Thermal-shift analysis for the trivalent iron compounds Fe${\mathrm{Cl}}_3$, HoFe${\mathrm{O}}_3$, and Fe${\mathrm{F}}_3$ reveals an intrinsic temperature dependence of the isomer shift of + (6 ± 1) × ${10}^{-5\mathrm{}}$ mm/sec °K. This is in contrast with the value of - (0.5 ± 1.0) × ${10}^{-5\mathrm{}}$ mm/sec °K, derived for the divalent compounds Fe${\mathrm{Cl}}_2$, Fe${\mathrm{F}}_2$, and KFe${\mathrm{F}}_3$. The apparent difference is discussed in terms of the opposite effects of temperature on the $3d\left(e_g\right)$ and $3d\left(t_{2g}\right)$ charge density distributions in the octahedral bonding configuration.