Sign determination of the 57Fe electric field gradient in Al-based crystals and icosahedral quasicrystals

The icosahedral quasicrystals i-AlMn isomorphically substituted by 28 at.% Fe or by (CrFe) are studied or the first time by in-field Moessbauer spectroscopy (MS) in order to determine the sign and asymmetry parameter of the dominant EFG term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si phases are studied. We show that previous MS results are inadequate to determine if there are two sites in the i-AlMn structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with asymmetry parameter of about 0.6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. The β phase is found to have a very similar quadrupole effect as that in i-Al(MnFe). (orig.)