Determination of the electric field gradient tensor of Fe³⁺ in the M1 site of aegirine by single crystal Mössbauer spectroscopy

Abstract

The electric field gradient (EFG) tensor of the ⁵⁷Fe Mössbauer nucleus is a basic physical property that is important in Mössbauer spectra measurements of a single crystal because the EFG provides a constraint on the intensity of quadrupole splitting peaks. To reveal the EFG tensor of Fe³⁺ in the octahedral M1 site of clinopyroxene, Mössbauer spectra of six crystallographically oriented single crystal of aegirine in thin sections were measured. The asymmetric parameter (η) of the EFG tensor of aegirine was almost equal to 1. The principal axes of the EFG tensor of aegirine were almost oriented along the b × c, b, and c axes, and the V_xx component of the EFG was along the b axis. To compare the experimental results with theoretical EFGs, three EFG tensors due to ligand oxygen ions were calculated based on atomic positions determined by X–ray structural analysis. The calculated EFG tensors were not necessarily consistent with the experimentally determined EFG tensors. This indicates that experimental determination of EFG is necessary for single crystal Mössbauer measurements because the calculated EFG may be inaccurate.