Study of Mg_2Ni_(1-x)Fe_x alloys by Moessbauer resonance

摘要

After preparing Mg_2Ni_(1-x)Fe_x (x = 0.005, 0.01, 0.02 and 0.08) alloys, they were studied by Moessbauer resonance. The Moessbauer spectra of x = 0.005 and 0.01 alloys exhibit two doublets (doublet 1, 2). That of x = 0.02 alloys shows two doublets (doublet 1, 2) and one sextuplet, and that of x = 0.08 alloy has only one sextuplet. Doublet 1 for x = 0.005, 0.01 and 0.02 alloys is considered to result from a fraction of Fe in excess showing a superparamagnetic behavior. Doublet 2 is considered to result from the Fe substituted for Ni in the Mg_2Ni phase. The values of isomer shift 0.25-0.29 mm/s suggest that the iron exists in the state Fe~(+3). The result that the quadrupolar splitting of doublet 2 is not zero shows that the distribution of electrons around the iron is asymmetric. Their values for the doublet 2, 1.21-1.39 mm/s, approach the value of quadrupolar splitting for the oxidation number +3. The sextuplet showing the magnetic hyperfine interactions results from the iron, which has not substituted the nickel in the Mg_2Ni phase. The Moessbauer spectrum of the x = 0.01 alloy hydrided at 723 K shows one doublet and sextuplet. The segregation of Fe is considered to result from the reaction of the Mg_2Ni phase with the oxygen contained in the hydrogen as impurity. A part of the segregated iron remains in the form of small particles and gives the doublet of the Moessbauer spectrum. The other part of this iron reacts with the nickel to form an Fe-Ni alloy magnetically ordered at room temperature.