57Fe Mössbauer and magnetic studies of Th7Co2.5 57Fe0.5 and Th7Ni2.5 57Fe0.5 superconductors

The noncentrosymmetric Th7Co2.5 (57)Fe0.5 and Th7Ni2.5 (57)Fe0.5 superconductors were studied by means of (57)Fe transmission Mossbauer spectroscopy (TMS), ac-magnetic susceptibility and magnetization measurements. The low-temperature data showed that both compounds are type-II superconductors with transition temperature close to [Formula: see text] K. The collected Mossbauer spectra of the studied samples consist of one well-resolved doublet over the temperature range measured. Neither abnormal behaviour of the hyperfine parameters at or near [Formula: see text] nor phonon softening were found. A comparison of (57)Fe Mossbauer data of Ni- and Co-based superconductors with those of Th7Fe3 indicated that with decreasing 3d-electron numbers across the series Th7Ni2.5 (57)Fe0.5 [Formula: see text] Th7Co2.5 (57)Fe0.5 [Formula: see text] Th7Fe3, the s-electron density around the Fe nucleus and as well as the electric field gradient strength increases. The latter finding is associated with increasing asymmetricity of 3d-electron distribution and this fact presumably gives arise to an exotic superconductivity in the Co- and Fe-based compounds.