Ligand-field splitting of Fe2+ in distorted octahedral sites of the magnesium-rich orthopyroxenes FexMg1-xSiO3: Correlation of magnetic susceptibility, Mossbauer, and optical absorption spectra - art. no. 144413

Orthopyroxenes, FexMg1-xSiO3, ranging in composition between the Fe and Mg end members, represent an interesting system from the physical point of vie w because of the strong site preference of Fe2+ for one of the two octahedr al sites in the crystal structure. For Mg-rich samples with x<0.5, a major portion of the Fe2+ ions reside in the distorted octahedral M2 site and the magnetic susceptibility and optical absorption spectra are dominated by th e effects of Fe2+ in this site. Two natural orthopyroxenes with x=0.12 and 0.20 have been investigated by magnetic susceptibility measurements and Mos sbauer spectroscopy. The thermal characteristics of the observed mean magne tic susceptibility <(chi )over bar> and the quadrupole splitting DeltaE(Q) were analyzed using ligand-field theory. A minimum number of approximations were made to modify the Hamiltonian for Fe2+ in the distorted octahedral M 2 site. The electronic energy-level pattern and the corresponding wave func tions were obtained from a best fitting of the experimental results with th e corresponding theoretical values. These results were used to calculate th e thermal characteristics of the crystalline magnetic susceptibilities chi (a), chi (b), and chi (c), arising from Fe2+ in the M1 and M2 sites in the ratio of their occupancies. The directional magnetic susceptibilities chi ( b)>chi (c)>chi (a) are in agreement with the corresponding values in orthop yroxenes with higher concentration of Fe2+. The energy-level diagram of Fe2 + at the M2 site agrees well with the optical absorption spectra resulting mainly from Fe2+ at the M2 site. There is very little variation in DeltaE(Q ) for Fe2+ at the M2 site as a function of temperature compared with that i n the regular octahedral M1 site which shows a large variation. The DeltaE( Q) of Fe2+ at the M2 site and the magnetic susceptibility show very little variation with chemical composition. Our analysis also suggests considerabl e overlap of the molecular orbitals of the ligand electrons with those of t he Fe2+ ions.