Low-Temperature Anharmonicity in FeCl2

Mössbauer spectra of ${\mathrm{Fe}}^{57}$ in thin anhydrous ferrous chloride powdered absorbers have been investigated at several temperatures between 300 and 10°K. The quadrupole splitting, velocity shift, and Mössbauer fraction have been determined. The Mössbauer fraction of Fe${\mathrm{Cl}}_2$ shows an anomalously weak temperature dependence which cannot be explained by any harmonic model for the interatomic forces or by the usual hightemperature anharmonicity. This anomalous behavior implies a low-temperature anharmonicity which is qualitatively explained in terms of the relative sizes of the ions involved. The interpretation is consistent with the finding of essentially harmonic behavior for Fe${\mathrm{F}}_2$. The results of reexamination of other Mössbauer $f$ studies are also consistent with these size arguments.