Transition metal sulfides

The metal sulfides exhibit a great diversity of electrical and magnetic properties with both scientific interest and practical applications. These properties apply major constraints on models of the electronic structure (or chemical bonding) in sulfides (Vaughan and Rosso 2006, this volume). The pure and doped synthetic equivalents of certain sulfide minerals have actual or potential applications in the electronics industries (optical devices, photovoltaics, photodiodes and magnetic recording devices). Sulfides are also components of many thin film devices and have been extensively investigated as part of the nanotechnology revolution. Certain electrical and magnetic properties of sulfide minerals mean they contribute to geomagnetism and paleo-magnetism, and provide the geophysical prospector with exploration tools for metalliferous ore deposits. To the mineral technologist, these same properties provide methods for the separation of the metal-bearing sulfides from associated waste minerals after mining and milling and before extraction of the metal by pyrometallurgical or hydrometallurgical treatment.

In this chapter, the theory and measurement of electrical and magnetic properties are outlined along with spectroscopic and diffraction studies that can provide insights into magnetic behavior are discussed. A brief review of electrical and magnetic studies of major sulfide minerals includes some examples of the applications of sulfide electrical and magnetic properties, including special consideration of the properties of sulfide nanoparticles. Most of the available data presented are for pure synthetic binary and ternary sulfides, as very small concentrations of impurities can dramatically affect electrical properties leading to problems of interpreting data from natural samples. Although data for several of the commonly found sulfides are discussed in this chapter, no attempt is made at a comprehensive coverage.

The section below on theory and measurement of electrical and magnetic properties draws on the account given in Vaughan and Craig (1978) to which readers are referred for further details. It …

Electrical and magnetic properties of sulfides

The wide variety of metal sulfide structures and their accommodation of atomic substitution, non-stoichiometry and metal-metal ( M - M ) and ligand-ligand interactions allows for diverse physical, chemical and electronic properties. The energy band structure of 3 d transition-metal sulfides, in particular, is strongly influenced by the covalence of metal-S bonds, which results in hybridization of S 3 p and metal 3 d bonding states and direct or indirect M - M bonding interactions in favorable cases. Differences in the phase relations of isostructural metal sulfides are often attributable to subtle changes in electronic states. The literature on metal sulfide phase relations relevant to the earth sciences is very extensive and could not possibly be summarized in a single chapter. Therefore, following Craig and Scott (1974), this chapter focuses on the base metal (Fe, Co, Ni, Cu, and Zn) sulfides, with the literature for other metal chalcogenides and pnictides and some sulfosalts summarized in a single table (Table 1⇓). The relevant phase relations of the platinum-group element (PGE) chalcogenides and pnictides have been comprehensively reviewed in Makovicky (2002). A section on the halite structure sulfides (niningerite, alabandite and oldhamite) is also included in this chapter. The material presented here relates closely to that discussed in other chapters, in particular the chapter on sulfide thermochemistry (Sack and Ebel 2006). The importance of understanding phase equilibria in the context of electronic and magnetic properties and, hence, electronic structure is also emphasized, thereby reinforcing material presented in several other chapters (Pearce et al. 2006; Vaughan and Rosso 2006).

View this table:

 Table 1. 
Summary of studies on sulfide phase relations relevant to Earth sciences.



Abbreviations used for mineral/phase names include: tr- troilite; po- pyrrhotite; hpo- hexagonal pyrrhotite; mpo- monoclinic pyrrhotite; py- pyrite; pn- pentlandite; hpn- high pentlandite; vs- vaesite; mss - (Fe,Ni) monosulfide solid …

Phase equilibria at high temperatures

Hexagonal nickel sulphide shows a transition at 260 K from an antiferromagnetic to a paramagnetic state which is accompanied by an abrupt change in the conductivity. It is suggested that this is not a Neel temperature, that is a destruction of long-range order, but a first-order disappearance of the moments, and that it occurs because of the screening out of the Hubbard intra-atomic electron-electron interaction by the electrons in the sulphur 3p band. It is also shown that the low temperature phase is probably a semimetal.

The metal-non-metal transition in nickel sulphide (NiS)

Magnetic susceptibility of single-crystal Fe1−xS

Hexagonal nickel sulphide shows a metal to semiconductor first order phase transition at a temperature TN which depends on sulphur content. Thermoelectric power, conductivity, Hall and magnetic susceptibility measurements have been made on five single crystals of compositions ranging from NiS-NiS1.02 in the temperature range 82-300 K. The results suggest that below TN the Ni 3d states are essentially localized or form a narrow band. The predominant mechanism of conduction is by holes in a wide band. The discontinuity in conductivity at TN is most likely due primarily to a change in mobility. Impurity band conduction occurs at low temperatures. The susceptibility curves have a markedly different shape from that normally expected for an antiferromagnet. This results from the first order character of the phase transition at TN.

https://iopscience.iop.org/article/10.1088/0022-3719/4/5/010/pdf

L J Ripley

Papers

Metal-semiconductor transition in single crystal hexagonal nickel sulphide