Fe Mössbauer and magnetic studies of ThCo Fe and ThNi Fe superconductors.

The noncentrosymmetric ThCo Fe and ThNi Fe superconductors were studied by means of Fe 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 [Formula: see text] 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 [Formula: see text] nor phonon softening were found. A comparison of Fe Mössbauer data of Ni- and Co-based superconductors with those of ThFe indicated that with decreasing 3d-electron numbers across the series ThNi Fe [Formula: see text] ThCo Fe [Formula: see text] ThFe, 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.