Nuclear Electric-Field Gradient Determination Utilizing the Mössbauer Effect (Fe57)

If a compound containing ${\mathrm{Fe}}^{57}$ exhibits a two-line Mössbauer absorption spectrum, as is the case for most paramagnetic ferrous salts, then this spectrum may usually be attributed to the electrostatic interaction between an electric-field gradient (EFG) and the quadrupole moment of the first excited state of the ${\mathrm{Fe}}^{57}$. Since the EFG may be represented as a traceless, symmetric tensor of the second rank, it is completely specified by five independent parameters. If the absorber is a single crystal, the two absorption peaks will generally have different intensities, the ratio of the intensities depending upon the orientation of the incident, unpolarized radiation relative to the crystal axes. In this paper, a general method is developed which utilizes this intensity ratio to determine the five EFG parameters. The possibility of an anisotropic recoilless factor is also considered. The method is then applied to Fe${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O.