Magnetic superexchange involving the oxygen-sulfur-oxygen pathway of the sulfate ion: A Mössbauer spectroscopy and magnetic susceptibility study of Fe(2,9-di-CH3-phenanthroline) SO4

Zero-field Mössbauer spectra of powder samples of Fe(2,9-di-CH₃-phenanthroline) SO₄ over the range 1.7 to 300°K show a large (～ 3.6 mm/sec) temperature-independent quadrupole splitting corresponding to an orbital singlet ground term. The chemical isomer shift, δ_FE=O, is 1.16 mm/sec (source and absorber at 4.2°K) corresponding to six coordinate high-spin iron (II). Below 4.2°K, the compound exhibits magnetic hyperfine splitting suggesting slow relaxation and the possibility of long-range three-dimensional magnetic ordering with a critical temperature T_c such that 3.5°K < T_c < 4.2°K and an internal hyperfine field H_n = 325 kG at 1.69°K. High-field Mössbauer spectra at 300°K indicate that the principal component of the electric field gradient tensor is positive and axial. Similar spectra at 4.2°K show an absence of nuclear Zeeman splitting for applied fields up to 60 kG, and indicate that at 4.2°K the material is rapidly relaxing but with substantial magnetic polarization and a negative internal hyperfine field. The temperature dependence of the magnetic susceptibility confirms antiferromagnetic interactions with a broad maximum χ′_M at ～ 11.8°K presumably due to low-dimensionality exchange interactions (possibly one) along MOSOM chains. Least-squares fits of χ′_M^?1 versus T for T50°K indicate (Curie-Weiss behavior with C = 3.26 emu/mole, θ = -21.95°K, and μ_eff = 5.11.