Preparation, thermal stability and electrical transport properties of vaesite, NiS2

Ferreira, Helena M.; Lopes, Elsa B.; Malta, José F.; Ferreira, Luís M.; Casimiro, Maria H.; Santos, Luís; Pereira, Manuel F.C.; Pereira Gonçalves, Antonio

Key information

Authors:

Ferreira, Helena M.; Lopes, Elsa B. (Elsa Maria Simões Branco Lopes); Malta, José F.; Ferreira, Luís M. (Luís Miguel Mota Ferreira); Casimiro, Maria H. (Maria Helena Freitas Casimiro); Santos, Luís; Pereira, Manuel F.C. (Manuel Francisco Costa Pereira); Pereira Gonçalves, Antonio (António Cândido Lampreia Pereira Gonçalves)

Published in

06/26/2019

Abstract

Vaesite, a nickel chalcogenide with NiS2 formula, has been synthetized and studied by theoretical and experimental methods. NiS2 was prepared by solid-state reaction under vacuum and densified by hot-pressing, at different consolidation conditions. Dense single-phase pellets (relative densities >94%) were obtained, without significant lattice distortions for different hot-pressing conditions. The thermal stability of NiS2 was studied by thermogravimetric analysis. Both as-synthetized and hot-pressed NiS2 have a single phase nature, although some hot-pressed samples had traces of the sulfur deficient phase, Ni1-xS (<1%vol), due to the strong desulfurization at T > 340 °C. The electronic band structure and density of states were calculated by Density Functional Theory (DFT), indicating a metallic behavior. However, the electronic transport measurements showed p-type semiconductivity for bulk NiS2, verifying its characteristic behavior has a Mott insulator. The consolidation conditions strongly influence the electronic properties, with the best room-temperature Seebeck coefficient, electrical resistivity and power factor being 182 µVK−1, 2,257 µΩ m and 14.1 µWK−2 m−1, respectively, pointing this compound as a good starting point for a new family of thermoelectric materials.