Temperature-Dependent Crystal Field and Charge Density: Mössbauer Studies of Fe${\mathrm{F}}_2$, KFe${\mathrm{F}}_3$, Fe${\mathrm{Cl}}_2$, and Fe${\mathrm{F}}_3$

A temperature-dependent crystal field at the ${\mathrm{Fe}}^{2+}$ site in Fe${\mathrm{F}}_2$ is inferred from anisotropic thermal-expansion coefficients and Mössbauer quadrupole-splitting data; the latter indicate that the splitting of the ferrous $T_{2g}$ state, due to the axial component of the crystal field, decreases from $E_{\mathrm{axial}}=1300\mathrm{}°$K at 300°K to $E_{\mathrm{axial}}=1000\mathrm{}°$K at 965°K. Thermal-shift and thermodynamic data for Fe${\mathrm{F}}_2$, KFe${\mathrm{F}}_3$, and Fe${\mathrm{Cl}}_2$ show that the electronic charge density at the nucleus is essentially independent of temperature; thus, the expected decrease in this density due to isothermal expansion must be approximately canceled by an increase due to thermal effects at constant volume. In contrast, Fe${\mathrm{F}}_3$ displays a significant decrease of electronic charge density at the nucleus with increasing temperature. The above conclusions result from Mössbauer studies of ${\mathrm{Fe}}^{57}$ in Fe${\mathrm{F}}_2$ (300-965°K), KFe${\mathrm{F}}_3$ (300-945°K), Fe${\mathrm{Cl}}_2$ (300-644°K), and Fe${\mathrm{F}}_3$ (370-810°K), reported in this paper. Also presented is a model which quantitatively fits the high-pressure Fe${\mathrm{F}}_2$ Mössbauer quadrupole-splitting data of Champion et al. below 60 kbar. The new feature of this model is a method for estimating the effect of pressure on the $3d$ radial wave function. A phase transition is proposed around 65 kbar.