The low-temperature specific heat of FeS and M0.875X (M = Fe, Co; X = S, Se) with a NiAs-like structure

We have measured the specific heat of FeS, Fe0.875X, and Co0.875X (X = S, S e) compounds with a NiAs-like structure over the temperature range from 2 t o 30 K. It was found that the ground state of FeS has an insulating charact er. The opening up of a gap in the density of states is probably caused by electron correlation. Two types of superstructure are known to occur due to ordering of vacancies at iron sites in Fe0.875X; one is a 3c structure and the other a 4c structure. The gamma-values obtained for 3c structure are a bout 10% larger than those of 4c structures. Accordingly, this result indic ates that the electronic structure of Fe0.875X should be sensitive to the o rdered structure of the vacancies. Since the gamma-values obtained for non- stoichiometric compounds are larger than the calculated ones, there is elec tron effective-mass enhancement in non-stoichiometric compounds. Moreover c omparison of the observed and calculated electronic contributions to the lo w-temperature specific heat suggests that the mass enhancement in Pauli-par amagnetic Co0.875X is larger than that in ferrimagnetic Fe0.875X.