Abstract
The noncentrosymmetric Th7Co2.5 57Fe0.5 and Th7Ni2.5 57Fe0.5 superconductors were studied by means of 57Fe transmission Mössbauer 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 K. The collected Mössbauer 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 nor phonon softening were found. A comparison of 57Fe Mössbauer data of Ni- and Co-based superconductors with those of Th7Fe3 indicated that with decreasing 3d-electron numbers across the series Th7Ni2.5 57Fe0.5 Th7Co2.5 57Fe0.5 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.
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