Gamma-ray resonance study of adsorbed FeCl2monolayers

A Mössbauer resonance study of Fe${\mathrm{Cl}}_2$ monolayers deposited on oriented basal planes of graphite (Grafoil) is reported. Samples with fractional monolayer coverages between 0.2 and 0.9 were studied between 300 and 80°K. The spectra show two distinct quadrupole doublets; one with a room-temperature splitting of ∼ 2 mm/sec (designated $L$) and the other ∼ 0.8 mm/sec ($B$), which is identical to the splitting of bulk Fe${\mathrm{Cl}}_2$ and has the same shift. There is a marked difference between the intensities of the two lines of each doublet: For each sample, the ratio $\frac{I_{+}}{I_{-}}$ depends on the orientation of the planes of the Grafoil sheets relative to ${\overrightarrow{\mathrm{k}}}_{\gamma }$. The intensity ratio for ${\overrightarrow{\mathrm{k}}}_{\gamma }$ normal to the plane of the foils progresses from $\frac{I_{+}}{I_{-}}>1\mathrm{}$ to $\frac{I_{+}}{I_{-}}<1\mathrm{}$ with reduction of coverage. The total intensity of the $B$ doublet relative to $L$ is reduced with decreasing coverage. The velocity shift of $L$ and the temperature dependence of the intensity of both $L$ and $B$ doublets differ markedly from those of bulk Fe${\mathrm{Cl}}_2$ and other known ${\mathrm{Fe}}^{++}$ compounds. The results suggest the existence of at least two distinct phases of Fe${\mathrm{Cl}}_2$ in the film samples, and that the Fe${\mathrm{Cl}}_2$-graphite samples involve monolayer surface states rather than bulk Fe${\mathrm{Cl}}_2$ aggregates. A calculation of the adsorption binding energy from the temperature dependence of the Debye-Waller factor yields about 80 kcal/mole for $B$ and about 60 kcal/mole for the $L$ phase; both values are larger than the heats of fusion (10 kcal/mole) and evaporation (30 kcal/mole) of bulk Fe${\mathrm{Cl}}_2$, and are comparable with the heat of formation (80 kcal/mole). Possible models for surface arrangements of ferrous chloride molecules are discussed.