Marcasite Structure Research Articles

First-Principles Study of X-ray Absorption Spectra of FeS2

It is well known for a long time that FeS2 has two stable crystal phases, cubic pyrite (space group Pa 3) and orthorhombic marcasite (Pnnm) structures. Surprisingly, local coordinations around Fe and S atoms are quite similar between the two phases. For example, Fe is almost octahedrally coordinated by six S atoms at a distance of 2.26 A in pyrite and at 2.23–2.25 A in marcasite (see Table I). Dumbbell structure of the nearest neighbor S atoms with a bond distance of about 2.2 A is commonly seen in the both phases. X-ray absorption spectroscopy (XAS) is a useful tool to investigate the element-specific electronic structure of condensed matter systems. In particular, local geometry and symmetry around a given site are also analyzed by combining electronic structure calculations with experiments. XAS spectra at S and Fe K-edges have been measured for NiAs-type FeS and pyrite FeS2 and the observed first peak at the S K-edge and the pre-edge structure at the Fe K-edge are attributed to a transition to the empty anti-bonding states formed by hybridization of Fe 3d and S 3p orbitals. Although several first-principles density-functional-theory (DFT) calculations have been reported for Fe sulfides including pyrite and marcasite FeS2 since then, no theoretical analysis on XAS spectra has been performed for Fe–S systems. Discharge and charge reactions in Li/FeS2 and Na/FeS2 were experimentally studied for ion-battery applications. Among them, Kitajou et al. have measured XAS spectra at Fe K-edge for pyrite FeS2 and Na discharged FeS2 and found some variations in the main peak region while the pre-edge structure retains its shape by discharge. In this note, we present some theoretical results of XAS spectra of FeS2 in the pyrite and marcasite phases by performing firstprinciples DFT calculations. Electronic-structure calculations are carried out within the generalized gradient approximation (GGA) by Perdew, Burke, and Ernzerhof (PBE). One-electron Kohn–Sham equations are self-consistently solved by the all-electron full-potential linearized augmented plane wave method (FLAPW) in a scalar relativistic scheme. Experimental crystal structure including lattice constants and internal parameters is assumed throughout the calculation. XAS spectrum can be computed by using Fermi’s golden rule for the transition probability from an initial state jii to a final state j f i by x-ray with the photon energy h ! as Ið!Þ 1⁄4 2 h X

Colloidal growth, characterization and optoelectronic study of strong light absorbent inexpensive iron pyrite nanomaterials by using amine ligands for photovoltaic application

ABSTRACTHighly pure iron pyrites (FeS2) cubic phased nanocrystals of diameter ∼ 20 nm were synthesized by colloidal method using only amines acting both as a coordinating and surfactant ligands. The details of synthetic condition at temperature 175 °C, 215 °C, 230 °C, 245 °C were compared and elucidated. The best synthetic conditions using an octylamine as a ligand at 230 °C for 2h have been optimized in an inert atmosphere. The XRD measurement shows diffraction peaks of pure cubic iron pyrite crystal structure without any detectable presence of marcasite, pyrrotite, greigyte and other impurity structures. The UV-Vis spectra depict clear absorption onset at 1200 nm in best sample with estimated band gap of ∼1.03 eV. These high pure and nanostructures based iron pyrite processed from solution route may offer excellent manufacturing scalability at very low cost since it can be used as inks for large scale fabrication. The morphological and optical characterizations are carried out by using XRD, UV-Vis, and SEM techniques.

Transport, thermal, and magnetic properties of the narrow-gap semiconductor CrSb2

Resistivity, Hall effect, Seebeck coefficient, thermal conductivity, heat capacity, and magnetic susceptibility data are reported for CrSb2 single crystals. In spite of some unusual features in electrical transport and Hall measurements below 100 K, only one phase transition is found in the temperature range from 2 to 750 K corresponding to long-range antiferromagnetic order below T_N ~ 273 K. Many of the low temperature properties can be explained by the thermal depopulation of carriers from the conduction band into a low mobility band located approximately 16 meV below the conduction band edge, as deduced from the Hall effect data. In analogy with what occurs in Ge, the low mobility band is likely an impurity band. The Seebeck coefficient, S, is large and negative for temperatures from 2 to 300 K ranging from ~ -70\muV/K at 300 K to -4500\muV/K at 18 K. A large maximum in |S| at 18 K is likely due to phonon drag with the abrupt drop in |S| below 18 K due to the thermal depopulation of the high mobility conduction band. The large thermal conductivity between 10 and 20 K (~350 W/m/K) is consistent with this interpretation, as are detailed calculations of the Seebeck coefficient made using the complete calculated electronic structure. These data are compared to data reported for FeSb2, which crystallizes in the same marcasite structure, and FeSi, another unusual narrow-gap semiconductor.

High quality FeSb2 single crystal growth by the gradient freeze technique

A single crystal of FeSb2 about 5mm in diameter and 10mm in length with a tapering spire was grown by the gradient freeze technique combining the Bridgman technique and flux method using Sb-rich melts in sealed quartz ampoules. X-ray powder diffraction pattern of single crystal FeSb2 indicates that it is a single phase marcasite structure. Clear Laue spots and scanning electron microscopy photographs indicate good quality and uniform of the FeSb2 single crystal. Magnetic susceptibility of FeSb2 along b-axis does not show the diamagnetic to paramagnetic crossover, indicating the intrinsic susceptibility of FeSb2 determined on large single crystals differs from that of the smaller samples prepared by other techniques. The new method of gradient freeze is of great significance for the single crystal growth of FeSb2 like materials with a peritectic point.

Pressure-induced structural transition and thermodynamic properties of RhN2 and the effect of metallic bonding on its hardness

The elastic constant, structural phase transition, and effect of metallic bonding on the hardness of RhN2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method. Three structures are chosen to investigate for RhN2, namely, simple hexagonal P6/mmm (denoted as SH), orthorhombic Pnnm (marcasite), and simple tetragonal P4/mbm (denoted as ST). Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure. On the basis of the third-order Birch—Murnaghan equation of states, we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa, respectively. Elastic constants, formation enthalpies, shear modulus, Young's modulus, and Debye temperature of RhN2 are derived. The calculated values are, generally speaking, in good agreement with the previous theoretical results. Meanwhile, it is found that the pressure has an important influence on physical properties. Moreover, the effect of metallic bonding on the hardness of RhN2 is investigated. This is a quantitative investigation on the structural properties of RhN2, and it still awaits experimental confirmation.

Synthesis and characterization of mono- and di-antimonide with the first transition metals in group 8B by mechanical alloying

Mono- and di-anitimonide compounds between antimony and the first transition metals in group 8B were synthesized by mechanical alloying method. All samples were investigated by the X-ray powder diffraction technique and differential thermal analysis. The single phase can be obtained only for CoSb, NiSb and CoSb 2 compounds. In this investigation, a single phase of a marcasite structure (FeSb 2 and NiSb 2) and Fe 0.56Sb 0.44 compound cannot be detected in the XRD patterns after 60 h of milling because of the instability of their structures at low temperature. They decomposed to their starting materials as seen by the characteristic peaks of the starting materials in the XRD patterns after 60 h of milling.

Twofold Coordinated Ground-State and Eightfold High-Pressure Phases of Heavy Transition Metal Nitrides MN2 (M = Os, Ir, Ru, and Rh)

Using ab initio evolutionary methodology for crystal structure prediction, a twofold coordinated ground-state hexagonal structure (P6/mmm, 1 f.u./cell) with peculiar double N horizontal lineN bonded N(2) units was uncovered for MN(2) (M = Os, Ir, Ru, and Rh) compounds. This structure exhibits an unusual incompressibility along the c axis, higher than that of diamond. At much higher pressures, an eightfold coordinated tetragonal structure (P4/mbm, 2 f.u./cell) containing MN(8) cuboids was unraveled and possesses also orientational ultra-incompressibility. A unified phase transition diagram for these nitrides was thus derived to reveal the intriguing chemistry of nitrogen at extreme conditions. Formation energy calculations demonstrate that the ground state phase is synthesizable at low pressure ( approximately 40 GPa) while the eightfold phase can be achieved through the phase transformation via the marcasite structure.

Theoretical study of the properties of [formula omitted] with pyrite and marcasite structures

Pyrite and marcasite structures are both potential structures of platinum nitride. The mechanical properties of these two structures are similar, so we further investigate the thermodynamic properties of platinum nitride under high pressure and high temperature by the quasi-harmonic Debye model, in which phonon effects are considered. In the paper the important thermodynamic parameters, Debye temperature and heat capacity, have been investigated. The results show that the melting point is higher and the interatomic force is stronger in the pyrite structure than in the marcasite and fluorite structures. In addition, the marcasite structure has a much higher value of C V at the same T and P , and the change in C V is much sharper than that of the pyrite structure at low temperatures. Moreover, we notice that pressure has less influence on C V .

Thermodynamic Ground States of Platinum Metal Nitrides

The thermodynamic stabilities of various phases of the nitrides of the platinum-metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir, and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.

Marcasite vs. arsenopyrite structural choice in MN2 (M = Ir, Os and Rh) transition metal nitrides

A first-principles density functional theory study of the recently reported IrN2 and OsN2 as well as the as yet not synthesized RhN2 transition metal nitrides is presented. It is found that although IrN2 has a slight preference for the arsenopyrite structure, OsN2 prefers the marcasite structure and RhN2 has no clear preference between these two structures. The electronic structure of these nitrides is discussed on the basis of a simple model for the octahedral MN4 chains present in both the marcasite and arsenopyrite structures. It is shown that the distortion from the marcasite to arsenopyrite structure in these nitrides is not a Peierls distortion, contrary to what is often claimed. Both metal–metal interactions along the MN4 chains and interchain interactions through the dinitrogen units compete in these compounds. IrN2 is found to be a semiconductor whereas OsN2 and RuN2 should be three-dimensional metals. It is predicted that RhN2 can exist in two metallic forms with the marcasite and arsenopyrite structures, which should exhibit different anisotropies.

Marcasite osmium nitride with high bulk modulus: First-principles calculations

The authors propose marcasite OsN2 as a structure of the experimentally synthesized orthorhombic OsN2 compound. From the first-principles calculations, they find that the marcasite structure is more stable than the fluorite and pyrite ones and its lattice constants are in good agreement with the experiment. The calculated elastic constants for marcasite OsN2 satisfy the stability condition. The band structure shows that marcasite OsN2 is metallic. There is a pseudogap around the Fermi level of the total density of states of marcasite OsN2, which may contribute to its stability. The appearance of the pseudogap is mainly caused by the anisotropic connectivity of OsN6 octahedra.

Os N 2 : Crystal structure and electronic properties

Osmium nitride belongs to a family of nitrides synthesized recently at high pressures from their parent elements. Here we show, based on first-principles calculations, that the crystal structure of osmium nitride is isostructural to marcasite. Excellent agreement is obtained between the authors’ results and x-ray, Raman, and compressibility measurements. In the OsN2 marcasite structure single-bonded N2 units occupy the interstitial sites of the Os close-packed lattice, giving rise to a metallic compound. A comparison between the formation energies of OsN2 and PtN2 explains the similar thermodynamic conditions of formation reported experimentally for the two compounds.

Effect of Fe Doping on Thermoelectric Properties of Mechanically Alloyed CoSb<sub>3</sub>

Fe doped skutterudite CoSb3 with a nominal composition of FexCo1-xSb12 (0≤x≤2.5) have been synthesized by mechanical alloying (MA) of elemental powders, followed by vacuum hot pressing. Phase transformations during mechanical alloying and vacuum hot pressing were systematically investigated using XRD. Single phase skutterudite was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. However, second phase in the form of marcasite structure FeSb2 was found to exist in case of x≥2, suggesting the solubility limit of Fe with Co in this system. Thermoelectric properties as functions of temperature and Fe contents were evaluated for the hot pressed specimens. Fe doping up to x=1.5 with Co in FexCo4-xSb12 appeared to increase thermoelectric figure of merit (ZT) and the maximum ZT was found to be 0.78 at 525K in this study.

기계적 합금화에 의한 Skutterudite계 FexCo4-xSb12의 합성 및 열전특성

Fe-doped skutterudite <TEX>$CoSb_3$</TEX> with a nominal composition of <TEX>$Fe_{x}Co_{4-x}Sb_{12}(0\;{\le}\;x\;{\le}\;2.5)$</TEX> has been synthesized by mechanical alloying (MA) of elemental powders, followed by hot pressing. Phase transformations during mechanical alloying and hot pressing were systematically investigated using XRD. Single phase skutterudite was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. However, second phase in the form of marcasite structure <TEX>$FeSb_2$</TEX> was found to exist in case of <TEX>$x\;{\ge}\;2$</TEX>, suggesting the solubility limit of Fe with Co in this system. Thermoelectric properties as functions of temperature and Fe contents were evaluated for the hot pressed specimens. Fe substitution up to x=1.5 with Co in <TEX>$Fe_{x}Co_{4-x}Sb_{12}$</TEX> appeared to increase thermoelectric figure of merit (ZT) and the maximum ZT was found to be 0.78 at 525K in this study.

Structural and Electrical Properties of Cr1‐xRuxSb2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural and electrical properties of Cr1−xRuxSb2

Electrical resistivity, Hall coefficient, thermoelectric power, and crystal structure were studied for the pseudo-binary system Cr1−xRuxSb2. The results of powder X-ray diffraction measurements showed that all the prepared samples of Cr1−xRuxSb2 (0≤x≤1) were in a single phase of the orthorhombic marcasite structure. The electrical measurements show that Cr1−xRuxSb2 (0≤x≤1) are all semiconductive in spite of the gradual change of valence electron number by substituting Ru for Cr. It was found that logρ of the samples for x<0.7 exhibits T−1/2 temperature dependence in certain temperature range. These temperature changes of ρ are the same as those of the variable range hopping (VRH) conduction with Coulomb gap in the impurity conduction range of usual semiconductors.

Surface states and reactivity of pyrite and marcasite

The sulphur surface sites of pristine marcasite surfaces and their reactivity during initial air oxidation were investigated for the first time using synchrotron radiation excited photoelectron spectroscopy (SRXPS). Both S 2p and Fe 2p spectra were collected and compared to those of the polymorph pyrite. A new sulphur surface component is found additionally to the two known pyrite surface signals. Due to the non-isotropic character of the orthorhombic marcasite structure, its peak intensity depends strongly on the surface orientation. Its binding energy is close to the region of short-chained polysulphides. The signal is assigned to sulphur trimers produced by electron transfer between surface near sulphur dimers and S − formed after ruptures of SS bonds which results in S 2− and S 3 2−. In the cubic crystal arrangement of pyrite the stabilisation of S − to S 2− is realised by an electron transfer from iron to sulphur only. A confirmation of this assumption is found in the Fe 2p spectra. The contribution of Fe 3+ species — a result of the electron transfer process — is considerably less pronounced for marcasite. The mechanism of initial oxidation is different for marcasite and pyrite. The sulphur species responsible for the new spectroscopic component are the most reactive sites at the marcasite surface, while S 2− is oxidised first in case of pyrite. The signals indicating the products of marcasite oxidation in the S 2p spectrum are less intense as expected from the intensity decreasing of the trimer peak. Therefore, an oxidation of sulphur trimers to elemental sulphur is assumed which could not be detected because of its volatility in the vacuum of the air lock system.

Structural and electrical properties of Co 1− xRu xSb 2

Electrical resistivity, Hall coefficient, thermoelectric power and crystal structure were measured for the pseudo-binary system Co 1− x Ru x Sb 2. By the results of X-ray analysis, the monoclinic distortion from the orthorhombic marcasite structure decreases gradually and disappears at about x=0.14. The electrical measurements show that Co 1− x Ru x Sb 2 (0≤ x≤1) are all semiconductive, in spite of the gradual change of valence electron number, by substituting Ru atom for Co. It was found that log ρ of the samples for 0.1≤ x≤0.9 exhibit T −1/4 and T −1/2 temperature dependence in the lower temperature range. These temperature changes of ρ are the same as those of the variable range hopping (VRH) conduction in the impurity conduction range of the usual semiconductors.

Oxidation state of gold and arsenic in gold-bearing arsenian pyrite

XANES measurements on gold-bearing arsenian pyrite from the Twin Creeks Carlin-type gold deposits show that gold is present as both Au 0 and Au I+ and arsenic is present at As I- . Au 0 is attributed to sub-micrometer size inclusions of free gold, whereas Au I+ is attributed to gold in the lattice of the arsenian pyrite. STEM observations suggest that As I- is probably concentrated in angstrom-scale, randomly distributed layers with a marcasite or arsenopyrite structure. Ionic gold (Au I+ ) could be concentrated in these layers as well, and is present in both twofold- and fourfold-coordinated forms, with fourfold-coordinated Au I+ more abundant. Twofold-coordinated Au I+ is similar to gold in Au 2 S in which it is linearly coordinated to two sulfur atoms. The nature of fourfold-coordinated Au I+ is not well understood, although it might be present as an Au-As-S compound where gold is bonded in fourfold coordination to sulfur and arsenic atoms, or in vacancy positions on a cation site in the arsenian pyrite. Au I+ was probably incorporated into arsenian pyrite by adsorption onto pyrite surfaces during crystal growth. The most likely compound in the case of twofold-coordinated Au I+ was probably a tri-atomic surface complex such as S pyrite -Au I+ -S bi-sulfide H or Au I+ -S-Au I+ . The correlation between gold and arsenic might be related to the role of arsenic in enhancing the adsorption of gold complexes of this type on pyrite surfaces, possibly through semiconductor effects.

Thermoelectric properties of the skutterudite Co1−xFexSb3 system

We have examined the phase equilibrium and thermoelectric properties of Co1−xFexSb3 ternary system up to high iron context x=0.40. Traces of Sb were observed in the hot-pressed samples with x⩾0.06, and FeSb2 (Fe0.73Co0.27Sb2) compound with marcasite structure was also observed in the samples with x⩾0.25 by x-ray diffraction. The lattice parameter of Co1−xFexSb3 is slightly larger than that of the binary compound CoSb3. The Seebeck coefficient and the electrical resistivity are generally reduced by the substitution for Co by Fe. The thermal conductivity is also reduced by the substitution especially at high iron content region. These behaviors of the thermoelectric properties in the samples with low iron content are ascribed to the substituted Fe, while those in the samples with high iron content are ascribed to the precipitated FeSb2 (Fe0.73Co0.27Sb2) compound. For x⩽0.04, the figure of merit for Co1−xFexSb3 decreases with increasing x. However, above x=0.06 the figure of merit increases with x and as a result, Co0.75Fe0.25Sb3 has a maximum value of figure of merit of 4.8×10−4 K−1 at 773 K. This value is much larger than the maximum value for CoSb3 3.2×10−4 K−1 at 482 K.