Spin and charge density on iron nuclei in the BCC Fe-Mo alloys studied by 57Fe Moessbauer spectroscopy

Iron molybdenum alloys were prepared for the molybdenum concentration range 0-50 at.% by the arc melting method. X-ray diffraction patterns show single BCC phase for the Mo concentration up to 18 at.% with the lattice constant increasing upon addition of Mo. Sample with 21 at.% of Mo looks like composed of many BCC phases differing by the lattice constant. Finally, sample with 50 at.% of Mo looks like being composed of two BCC phases with various lattice constants and some amount of the non-stoichiometric λ-phase having symmetry P63/mmc. Moessbauer data indicate random solutions up to 12 at.% of Mo with magnetic order at room temperature. Room temperature paramagnetic phase appears for the sample with 18 at.% of Mo and its content increases with the increasing concentration of Mo. Traces of magnetically ordered phase (at room temperature) are seen for the sample with 40 at.% of Mo. Sample with 50 at.% of Mo is paramagnetic at room temperature. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and Mo impurity up to the third co-ordination shell within the single-phase random solution range. Mo atom as the nearest iron neighbor changes iron hyperfine field by -4.18 T, as the second neighbor makes change by -2.30 T and finally as the third neighbor changes the field by +0.51 T. Corresponding changes in the isomer shift are as follows: -0.033 mm/s, -0.005 mm/s and +0.003 mm/s. The average hyperfine field and the isomer shift decrease linearly versus Mo concentration at rates -0.383 T/at.% and -5.06 x 10-4 mm/(s at.%), respectively. Hence, addition of Mo increases the electron density on the iron nucleus at the rate +1.7 x 10-3 electron a.u.-3(at.%)-1.