Mössbauer Spectra of Some Tetrahedral Iron (II) Compounds

Abstract

The 57Fe Mössbauer-effect spectra of a series of tetrahedrally coordinated Fe(II) compounds of general formula R2FeX4 have been measured over a wide temperature range. The absorption showed approximately a tenfold increase on cooling from 300° to 4.2°K, showing that the effective Debye temperature for the Fe(II) ion is low. The isomer shifts (1.0–1.1 mm/sec relative to iron) and quadrupole splittings (2.7–3.3 mm/sec at 4.2°K) reflect the degree of covalency in the anions and their distortion from cubic symmetry. The distortion was present in all the compounds studied, and was dependent both upon the ligand X and the cation R. For large cations the temperature variation of the quadrupole splitting can be understood in terms of a static distortion which splits the dz2 and dx2—y2 states by an energy Δ, where, e.g., Δ=470 cm−1 for R2=(N,N′-dimethyl-4,4′-dipyridyl)++. For (NMe4)2FeCl4 the quadrupole splitting cannot be accounted for simply by a temperature-independent ligand field splitting Δ. Measurements of the spectrum in a magnetic field showed that the sign of the electric field gradient is negative and hence that the groundstate orbital is dz2. This corresponds to the distortion being a compression along the z axis. Magnetic-susceptibility measurements were in accord with a splitting of this orbital state according to the spin Hamiltonian H=D[Sz2−13S(S+1)] where D=+9.0 cm−1, and with g‖=2.05 and g⊥=2.27. The component of the hyperfine magnetic field perpendicular to the distortion axis z was found to be −27 kG.