Electrical resistivity (

ρ

) and Seebeck-coefficient measurements (

α

) are reported for

Ni S_{2 - x} {Se}_{x}

single crystals in the range

0 <~ x <~ 0.71

. There is a general trend toward increasing metallicity with increasing

x

. In the range

0.38 <~ x <~ 0.51

a pronounced rise of

ρ

with temperature (

T

) is observed where the antiferromagnetic insulating (or antiferromagnetic metallic) phase changes over to the paramagnetic insulating phase. The analysis of

α

T

curves suggests that in the low-temperature insulating state both holes and electrons participate in charge transport. It is emphasized that the many changes in electrical characteristics occur without significant alterations in the pyrite crystal structure, and that physical properties are greatly altered solely by adjustment of the anion sublattice while the cation sublattice remains intact. The results concerning electrical transport for the samples with

0.38 <~ x <~ 0.55

are interpreted qualitatively on the basis of an onset of the Hubbard splitting into subbands at the transition to the insulating (semiconducting) state, which takes place in the temperature interval (50-100 K) upon heating the samples. The metallic state close to the metal-semiconductor boundary is viewed as being an antiferromagnetic semimetal, with an anisotropic Slater gap.

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