Electron Correlation Effects in the Nickel-Sulfide-Selinide System

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Abstract 9612130 Honig This project is the continuation of research completed under terms of the predecessor grant on the physical characteristics of the NiS2-x SeX system. As x is increased there is a continuing shift away from the strongly insulating characteristics of NiS2 toward the strongly metallic properties of NiSe2. One thus anticipates a compositional region in which the system exhibits interesting electrical anomalies. This is indeed the case for 0.3 x 0.5. Further, over the entire x range, the cation sublattice is left intact. Moreover, there is almost no change in the crystal structure except for the continuing increase of lattice parameter with x. Thus, the system lends itself very well to a study of changes in electron correlation effects in a system with essentially the same structure and with fixed valency of the cation. Using single crystals that have been grown from a Te flux and annealed for control of stoichiometry, it is proposed to conduct detailed X-ray and neutron scattering studies, magnetization measurements, AC conductivity investigations, specific heat studies, optical refectivity measurements, and studies on the effects of applying pressures sufficient to alter the phase transformations that are encountered in this system. The experimental results, as well earlier measurements obtained under terms of the predecessor grant, are to be rationalized in terms of existing theories dealing with electron correlation phenomena. %%% There is continuing interest in the characterization of metal-insulator transitions and related phenomena. What sets the NiS2Sex system apart from almost all others in the category is the remarkable alterations and neardiscontinuities in electrical properties that can be achieved by changes of composition in the anion sublattice while keeping the cation sublattice intact. This comes about because NiS2 is a very good insulator whereas NiSe2 is a metal; these materials can be mixed in all proportions. T hus, it is anticipated that interesting electrical anomalies might be encountered over some intermediate composition range of the alloy. This is indeed the case for the range 0.3 x 0.5. It was on this basis that funding was requested for initiation of research in this area. The above variations occur in the absence of significant alterations in crystal structure when the material passes through various phase transitions with changing temperature in the relatively complex phase diagram. Any crystallographic changes that might become detectable in very careful studies devoted to this topic are not likely to play a significant role in the various electronic phase changes that are the object of the proposed study. One can thus investigate the effect of changing electron correlations without the complications of large scale concomitant structural alterations or of any compositional changes in the cation sublattice array. It is important to search for and study singlevalent system that display transitions from a weakly to a strongly correlated electron regime - a problem that is generally conceded to be of fundamental importance. These requirements are met by the proposed system.