Ferrous ammonium sulphate hexahydrate Mossbauer revisited: a combined 57Fe Mossbauer and X-ray single crystal study

This paper investigates the electric-field gradient (efg) tensor for the Fe2+ ion in ferrous ammonium sulphate hexahydrate single crystals at 295 K from two different, but apparently equivalent, standpoints. Firstly, we calculate the expected angular dependence of the thin-crystal-limit reduced intensity ratios and total intensities of the quadrupole doublet utilizing earlier determined macroscopic intensity tensor data and X-ray-determined atomic-displacement parameters (ADPs) for the iron nucleus. The computer program used (MOSREF) utilizes matrix diagonalization in an arbitrary coordinate frame to calculate Mossbauer intensities for sites of any specified symmetry in crystals of any specified symmetry. Secondly we attempt to measure, directly by Mossbauer spectroscopy, the Lamb-Mossbauer factors that may, via the mean-squared-displacement tensor, be related to the Fe2+ ADPs. From the experimental angular dependence of the reduced intensities (areas), corrected to the thin crystal limit, and experimental Mossbauer-determined mean-squared displacements, we determine the microscopic electric-field-gradient tensors for the two symmetry-related sites in the unit cell. This is achieved using the refinement options of program MOSREF where reduced intensities and total intensities are refined simultaneously. Successful determination of microscopic tensors depends on anisotropy in the absorber recoilless fractions that are in turn related to anisotropic Lamb-Mossbauer factors.