Electronic and magnetic structure of pyroxenes: II. Orthoferrosilite

The electronic and magnetic structure of the chain silicate orthoferrosilite Fe ₂ ²⁺ Si₂O₆ has been investigated by electronic structure calculations in the local spin density approximation. All calculations are based on experimentally determined geometrical data at room temperature. The calculated spin-allowed d–d excitation energies and hyperfine parameters are in quantitative agreement with the respective experimental data from optical absorption and Mössbauer spectroscopy. Inside one ribbon that is parallel to the crystallographic c axis and contains two non-equivalent M1 and M2 sites, all iron spins are ferromagnetically coupled with coupling constants of about +16 cm^?1. Between these ribbons within the (b, c)-plane a weak ferromagnetic coupling of about +2 cm^?1 is obtained. Neighboured (b, c)-planes are coupled antiferromagnetically via chains of Si ^B -tetrahedra but ferromagnetically via chains of Si ^A -tetrahedra. Such a theoretically determined "double-plane antiferromagnetic" spin structure is at variance with an experimentally derived magnetic structure. This discrepancy is attributed to differences between the geometry at room temperature and at temperatures below the Néel temperature currently not available.