The electric field gradient in synthetic fayalite α-Fe 2 SiO 4 at moderate temperatures

Abstract

This work reports on the evaluation of the electric field gradient (efg) in synthetic fayalite α-Fe2SiO4 using three different procedures: (1) experimental, with single-crystal Mossbauer spectroscopy (SCMBS) on the three principal sections of an α-Fe2SiO4 sample for several temperatures in the range 20 °C ≤ T ≤ 300 °C; (2) semiquantitative, with the on-line calculation of the efg from three-dimensional difference electron densities, and (3) a fully quantitative method with cluster molecular orbital calculations based on the density functional theory. The experimental efg is in good agreement with our previously published low-temperature data as well as with the results obtained by the fully quantitative approach. In the latter case, large cluster sizes (up to 97 atoms) are necessary to obtain quantitative agreement with experiment. This is also considered as being responsible for the lack of correspondence with the semiquantitative efg on M1. Previously reported symmetry violations on the M2 site are also confirmed by our SCMBS measurements.