M6ssbauer measurements on synthetic iron orthosilicate Fe2SiO 4 (fayalite) were carried out in the antiferromagnetic spin state below TN-~65K. The M6ssbauer parameters isomer shift c~, inner magnetic field H(0), angle O between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter r/were determined as a function of temperature, These parameters could be attributed to the two crystallographic sites MI and M2.
The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments./-/(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T= 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous M6ssbauer refinements published elsewhere.
Single crystal M6ssbauer spectroscopy revealed the orientation of the crystallographic c-axis (Pnma) with respect to the axes of the efg. A second parameter set which also holds true for the powder refinement yields the angle between c and H(0) on M1 to be temperature-dependent in quantitative agreement with previous neutron diffraction results and solves an old discrepancy discussed elsewhere. On M2, the angle between c and H(0) is found to be much smaller than 90 ~ in contradiction to symmetry requirements.