Mössbauer Studies of Inequivalent Ferrous Ion Sites in Ferrous Formate

The iron ions in ferrous formate, Fe${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O, are located at two sites with different environments. The nearest-neighbor symmetry of the type-1 site is nearly octahedral, while the type-2 site has lower symmetry. Our Mössbauer experiments with a powder absorber of ferrous formate show a spectrum composed of four lines resulting from the inequality of the effective electric-field gradients at the two sites. The lines are grouped in two pairs having the same center of gravity located at the usual ferrous-ion isomer shift. The quadrupole splittings at the two sites are 2.97 mm/sec (type 2) and 0.59 mm/sec (type 1) at room temperature, and 3.36 mm/sec (type 2) and 1.48 mm/sec (type 1) at 6°K. The larger splitting has naturally been assigned to the more asymmetrical site (type 2). Calculations based on the temperature dependence of the quadrupole splittings indicate that the energy separation between the ground and first excited ion orbital states corresponds to 850°K (type 2) and 250°K (type 1). The method used for the calculations is based on known properties of spherical harmonics.