Mössbauer measurements of magnetite below the Verwey transition

Above the Verwey transition ( Tv = 119°K) Fe 3O 4 is a cubic inverse spinel. Below Tv it exists in a multiply twinned orthorhombic state. The lines in the Mossbauer spectrum below Tv are broadened due to the distribution of angles between the magnetic field and the electric field gradient. This angular ambiguity can be removed by measuring untwinned single crystals in a magnetic field. Twinning is elimenated by cooling the crystal through Tv in a magnetic field directed along [100]. The single crystal spectrum could then be resolved into five individual components corresponding to a tetrahedral Fe +3, two octahedral Fe +3 and two octahedral Fe +2 sites. The hyperfine fields, quadrupole shifts and isomer shifts are given for the five spectral components. To explain the observed five spectra, the unit cell proposed by Verwey must be at least doubled in size.