Electrical and Magnetic Properties of Sulfides
INTRODUCTION
THEORY AND MEASUREMENT OF ELECTRICAL AND MAGNETIC PROPERTIES
Electrical properties
Measurement of electrical properties
Magnetic properties
Measurement of magnetic properties
ELECTRICAL AND MAGNETIC PROPERTIES OF METAL SULFIDES
Metal sulfides of sphalerite and wurtzite (ZnS) structure-type
Chalcopyrite (CuFeS2)
Stannite (Cu2FeSnS4)
Cubanite (CuFe2S3)
Sulfides of pyrite (FeS2) and related structure-types
Pyrite (FeS2)
Cattierite (CoS2)
Vaesite (NiS2)
CuS2 and ZnS2
Sulfides with halite (NaCl) structures
Galena (PbS)
Alabandite (α-MnS)
Rare earth sulfides
Sulfides with niccolite (NiAs)-based structures
Troilite (FeS)
Pyrrhotite (Fe7S8)
CoS and NiS
Mackinawite, smythite, greigite and other thiospinels
PROPERTIES OF SULFIDE NANOPARTICLES
Synthesis of sulfide nanoparticles
Case study: pure and transition metal (TM) ion doped ZnS
Sulfide nanoparticles in the environment
APPLICATIONS
Electronics
Paleomagnetic investigations
Beneficiation of sulfi de ores
CONCLUDING COMMENTS
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Magnetic properties
All materials can be categorized into a number of general classes according to their response to magnetic fi elds. When a magnetization is induced which opposes the external magnetic field, the material is diamagnetic. When the induced magnetization is parallel to the external field, the material is paramagnetic. Paramagnetism results from the presence of atoms with permanent magnetic dipoles, and commonly occurs in atoms or molecules with unpaired electrons. This is because a single electron can be regarded, in terms of a classical model, as a magnet formed by the spinning of the negatively charged particle on its axis. Also, an electron traveling in a closed path around the nucleus produces a magnetic moment. The magnetic moments of atoms, ions and molecules are expressed as Bohr magnetons (BM or µB):
.............
In addition to simple paramagnetism, there are four other forms of paramagnetism, several of which are particularly important in the metal sulfi des. These are ferromagnetism, antiferromagnetism, ferrimagnetism, and Pauli paramagnetism, all of which arise through the interaction of unpaired electrons on neighboring “paramagnetic” ions. A spontaneous alignment of magnetic moments in the same direction occurs in ferromagnetic materials and is retained even after the external magnetic fi eld has been removed. The resulting permanent magnetization (as in metallic iron) is attributed to a quantum mechanical exchange interaction between the electrons and adjacent atoms. The efficiency of this interaction (and therefore the value of the susceptibility) decreases with increasing temperature, breaking down at the Curie temperature (Tc) above which such materials exhibit simple paramagnetic behavior (Fig. 6b).
In antiferromagnets, the moments spontaneously align themselves but are antiparallel on adjacent atoms, so that they cancel and result in no net moment. Such antiferromagnetic coupling frequently takes place between two paramagnetic metal cations via an anion intermediary. A number of mechanisms have been proposed (see Fig. 7) to account for antiferromagnetism and ferromagnetism in ionic solids such as metal sulfi des:
(1)Superexchange. For example, one of the two 3p valence electrons of S2− could be transferred to the half-filled shell of a transition metal ion (M, e.g., Mn2+ with five d electrons) and according Hund’s rule its spin would be antiparallel to the spins of the fi ve d electrons. The second sulfur p electron would have its spin antiparallel to the first (because of the Pauli exclusion principle) and would remain localized on the sulfur ion. Such a configuration can also be formed with the opposite metal ion, antiparallel alignment of unpaired electrons on the two metal ions shown in Figure 7 results. The same mechanism predicts ferromagnetic coupling for cations with less than half-filled shells. The superexchange interaction clearly depends on orbital overlap and is strongest for a linear M-S-M arrangement and weakest when the M-S bonds are at right-angles.
(2) Indirect exchange is similar to superexchange but transfer (or “promotion”) of a sulfur p electron to the d shell of the overlapping cation is not involved. Here the two 3p electrons of the sulfur atom simultaneously participate in antiferromagnetic coupling by remaining in their orbits but occupying areas near the metal ions to which they are coupled in antiparallel alignment. The coupling is always antiferromagnetic.
(3) Double exchange is suggested for systems containing mixed valence states and involves a simultaneous transfer of electrons from cation to sulfur and sulfur to cation. Ferromagnetism always results.
(4) Semi-covalent exchange is a modification of superexchange which assumes appreciable covalent character in the metal-anion bond; distribution of electrons amongst hybridized orbitals leads to formation of electron pair bonds between the central anion and adjacent cations which are thus antiferromagnetically coupled.
Examples of exchange interaction in a sulfi de system are afforded by the magnetic structures of the three polymorphs of MnS—the NaCl structure-type (alabandite) and the sphalerite and wurtzite analogs. In each case, the Mn2+ cation has 12 nearest Mn2+ neighbors but different numbers of S2− anion neighbors. In alabandite, Mn2+ is bonded to next-nearestneighbor Mn2+ ions via 180° sulfur p orbitals, whereas in the other polymorphs the linkage is tetrahedral. The magnetic structures determined by neutron diffraction, after Corliss et al. (1956) are shown in Figure 8. In the alabandite structure, the nearest Mn2+ neighbors are antiferromagnetically coupled by superexchange via the sulfur intermediaries, whereas in the sphalerite and wurtzite structure-types, next-nearest-neighbors are coupled. The magnetic structure of chalcopyrite (CuFeS2) is closely related to sphalerite-type MnS.
It is characteristic of antiferromagnetic materials that their magnetic susceptibility increases with temperature over the range of antiferromagnetic behavior up to the Néel temperature (TN), above which normal paramagnetic behavior is exhibited with decreasing susceptibility as temperature increases (see Fig. 6c).
From the discussion of exchange mechanisms giving rise to ferromagnetism and antiferromagnetism, it can be seen that the possibility exists of materials in which both types of interaction occur simultaneously. Such materials do occur and are known as ferrimagnets. The best known material exhibiting this phenomenon is magnetite (Fe3O4). Greigite (Fe3S4), the sulfur analogue of magnetite, is also ferrimagnetic. Another important example is provided by monoclinic pyrrhotite (Fe7S8), in which metal atom vacancies on one of the magnetic sublattices result in imbalance and consequent ferrimagnetism.
The last type of paramagnetism, Pauli paramagnetism, is found only in metallic materials in which the outermost electrons are extensively delocalized. Here, in a partly fi lled band, application of an external fi eld causes imbalance between spin-up and spin-down electrons and hence a net magnetic moment. Such susceptibility values are very small and are independent of temperature.
Magnetic properties
All materials can be categorized into a number of general classes according to their response to magnetic fi elds. When a magnetization is induced which opposes the external magnetic field, the material is diamagnetic. When the induced magnetization is parallel to the external field, the material is paramagnetic. Paramagnetism results from the presence of atoms with permanent magnetic dipoles, and commonly occurs in atoms or molecules with unpaired electrons. This is because a single electron can be regarded, in terms of a classical model, as a magnet formed by the spinning of the negatively charged particle on its axis. Also, an electron traveling in a closed path around the nucleus produces a magnetic moment. The magnetic moments of atoms, ions and molecules are expressed as Bohr magnetons (BM or µB):
.............
In addition to simple paramagnetism, there are four other forms of paramagnetism, several of which are particularly important in the metal sulfi des. These are ferromagnetism, antiferromagnetism, ferrimagnetism, and Pauli paramagnetism, all of which arise through the interaction of unpaired electrons on neighboring “paramagnetic” ions. A spontaneous alignment of magnetic moments in the same direction occurs in ferromagnetic materials and is retained even after the external magnetic fi eld has been removed. The resulting permanent magnetization (as in metallic iron) is attributed to a quantum mechanical exchange interaction between the electrons and adjacent atoms. The efficiency of this interaction (and therefore the value of the susceptibility) decreases with increasing temperature, breaking down at the Curie temperature (Tc) above which such materials exhibit simple paramagnetic behavior (Fig. 6b).
In antiferromagnets, the moments spontaneously align themselves but are antiparallel on adjacent atoms, so that they cancel and result in no net moment. Such antiferromagnetic coupling frequently takes place between two paramagnetic metal cations via an anion intermediary. A number of mechanisms have been proposed (see Fig. 7) to account for antiferromagnetism and ferromagnetism in ionic solids such as metal sulfi des:
(1)Superexchange. For example, one of the two 3p valence electrons of S2− could be transferred to the half-filled shell of a transition metal ion (M, e.g., Mn2+ with five d electrons) and according Hund’s rule its spin would be antiparallel to the spins of the fi ve d electrons. The second sulfur p electron would have its spin antiparallel to the first (because of the Pauli exclusion principle) and would remain localized on the sulfur ion. Such a configuration can also be formed with the opposite metal ion, antiparallel alignment of unpaired electrons on the two metal ions shown in Figure 7 results. The same mechanism predicts ferromagnetic coupling for cations with less than half-filled shells. The superexchange interaction clearly depends on orbital overlap and is strongest for a linear M-S-M arrangement and weakest when the M-S bonds are at right-angles.
(2) Indirect exchange is similar to superexchange but transfer (or “promotion”) of a sulfur p electron to the d shell of the overlapping cation is not involved. Here the two 3p electrons of the sulfur atom simultaneously participate in antiferromagnetic coupling by remaining in their orbits but occupying areas near the metal ions to which they are coupled in antiparallel alignment. The coupling is always antiferromagnetic.
(3) Double exchange is suggested for systems containing mixed valence states and involves a simultaneous transfer of electrons from cation to sulfur and sulfur to cation. Ferromagnetism always results.
(4) Semi-covalent exchange is a modification of superexchange which assumes appreciable covalent character in the metal-anion bond; distribution of electrons amongst hybridized orbitals leads to formation of electron pair bonds between the central anion and adjacent cations which are thus antiferromagnetically coupled.
Examples of exchange interaction in a sulfi de system are afforded by the magnetic structures of the three polymorphs of MnS—the NaCl structure-type (alabandite) and the sphalerite and wurtzite analogs. In each case, the Mn2+ cation has 12 nearest Mn2+ neighbors but different numbers of S2− anion neighbors. In alabandite, Mn2+ is bonded to next-nearestneighbor Mn2+ ions via 180° sulfur p orbitals, whereas in the other polymorphs the linkage is tetrahedral. The magnetic structures determined by neutron diffraction, after Corliss et al. (1956) are shown in Figure 8. In the alabandite structure, the nearest Mn2+ neighbors are antiferromagnetically coupled by superexchange via the sulfur intermediaries, whereas in the sphalerite and wurtzite structure-types, next-nearest-neighbors are coupled. The magnetic structure of chalcopyrite (CuFeS2) is closely related to sphalerite-type MnS.
It is characteristic of antiferromagnetic materials that their magnetic susceptibility increases with temperature over the range of antiferromagnetic behavior up to the Néel temperature (TN), above which normal paramagnetic behavior is exhibited with decreasing susceptibility as temperature increases (see Fig. 6c).
From the discussion of exchange mechanisms giving rise to ferromagnetism and antiferromagnetism, it can be seen that the possibility exists of materials in which both types of interaction occur simultaneously. Such materials do occur and are known as ferrimagnets. The best known material exhibiting this phenomenon is magnetite (Fe3O4). Greigite (Fe3S4), the sulfur analogue of magnetite, is also ferrimagnetic. Another important example is provided by monoclinic pyrrhotite (Fe7S8), in which metal atom vacancies on one of the magnetic sublattices result in imbalance and consequent ferrimagnetism.
The last type of paramagnetism, Pauli paramagnetism, is found only in metallic materials in which the outermost electrons are extensively delocalized. Here, in a partly fi lled band, application of an external fi eld causes imbalance between spin-up and spin-down electrons and hence a net magnetic moment. Such susceptibility values are very small and are independent of temperature.
Hello, my name is Dr Abderrahmane Reggad. I'm a 51 year old and I am a researcher in materials' sciences.
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