ELECTRICAL-PROPERTIES OF NIS2-XSEX SINGLE-CRYSTALS - FROM MOTT INSULATOR TO PARAMAGNETIC METAL

Electrical resistivity (rho) and Seebeck-coefficient measurements (alp ha) are reported for NiS2-xSex single crystals in the range 0 less tha n or equal to x less than or equal to 0.71. There is a general trend t oward increasing metallicity with increasing x. In the range 0.38 less than or equal to x less than or equal to 0.51 a pronounced rise of rh o with temperature (T) is observed where the antiferromagnetic insulat ing (or antiferromagnetic metallic) phase changes over to the paramagn etic insulating phase. The analysis of alpha vs T curves suggests that in the low-temperature insulating state both holes and electrons part icipate in charge transport. It is emphasized that the many changes in electrical characteristics occur without significant alterations in t he 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 less than or equal to x less than or equal to 0.55 are interpreted qualitatively on;the basis of an onset of the Hubbard splitting into subbands at the transition to the insulating (s emiconducting) state, which takes place in the temperature interval (5 0-100 K) upon heating the samples. The metallic state close to the met al-semiconductor boundary is viewed as being an antiferromagnetic semi metal, with an anisotropic Slater gap.