3dCharge Density Distribution and Crystal Field in FeCl2: A Mössbauer Study

Available Mössbauer quadrupole-splitting (QS) data for Fe${\mathrm{Cl}}_2$ are analyzed in terms of crystal field and covalency parameters. It is shown that the temperature variation of QS in the paramagnetic phase is consistent with a large expansion of the $3d$ electronic charge density distribution (${\alpha }^2=\frac{〈r^{-3\mathrm{}}〉}{{〈r^{-3\mathrm{}}〉}_0}\approx 0.4$) as was previously predicted from the systematics of QS and isomer shift in the halides. The analysis yields a large ratio of 8.4 between the trigonal field component and the spin-orbit coupling constant, which is consistent with the Ising model proposed by Kanamori for the low-temperature magnetic properties of Fe${\mathrm{Cl}}_2$.