Marcasite Structure Research Articles

Investigation on the electrical and thermal properties of Cr-doped FeSe2 alloys

FeSe2 is a p-type semiconductor with a narrow band gap, which can be considered a potential thermoelectric material. Herin, the electrical and thermal properties of Cr-doped FeSe2, Fe1-xCrxSe2 (x = 0, 0.03, 0.06, and 0.09), polycrystalline alloys are investigated. All compositions show single-phase marcasite structures of FeSe2 with gradual decreases in lattice parameters with Cr doping x. With increasing x, the electrical conductivity gradually increases from 13 (x = 0) to 46 S/cm (x = 0.09), owing to a large increase in carrier concentration originated from the substitution of Cr2+ and Cr3+ at the Fe site. However, the magnitude of Seebeck coefficient decreases considerably at 300 K, resulting in decrease in power factor. In addition, the density-of-state effective mass decreases significantly from 6.0 m0–1.04 m0. At 700 K, the decrease in Seebeck coefficient is less severe. Although the total and lattice thermal conductivity decrease at all temperatures owing to additional phonon scattering of the extrinsic Cr ions, a thermoelectric figure of merit, zT, generally decreases compared to that of the pristine sample due to power factor decrease at 300 K. Meanwhile, a slight increase of zT to 0.11 is observed for x = 0.06 at 700 K.

Seven-Coordinated High-Pressure Phase of CrSb2 and Experimental Equation of State of MSb2 (M = Cr, Fe, Ru, Os).

The MSb2 compounds with M = Cr, Fe, Ru, and Os have been investigated under high pressures by synchrotron powder X-ray diffraction. All compounds, except CrSb2, were found to retain the marcasite structure up to the highest pressures (more than 50 GPa). In contrast, we found that CrSb2 has a structural phase transition around 10 GPa to a metastable, MoP2-type structure with Cr coordinated to seven Sb atoms. In addition, we compared ambient temperature compression with laser-heating experiments and found that laser-heating at pressures below and above this phase transition results in the known CuAl2-type structure. Density functional theory calculations show that this tetragonal structure is the most stable in the whole pressure interval. However, a crossing of the marcasite's and MoP2-like structure's enthalpies occurs between 5 and 7.5 GPa, which is in good agreement with the experimental data. The phase transition to the MoP2-type structure observed in this work opens up for discovering other compounds with this new transition pathway from the marcasite structure.

Efficient acidic hydrogen evolution in proton exchange membrane electrolyzers over a sulfur-doped marcasite-type electrocatalyst

Large-scale deployment of proton exchange membrane (PEM) water electrolyzers has to overcome a cost barrier resulting from the exclusive adoption of platinum group metal (PGM) catalysts. Ideally, carbon-supported platinum used at cathode should be replaced with PGM-free catalysts, but they often undergo insufficient activity and stability subjecting to corrosive acidic conditions. Inspired by marcasite existed under acidic environments in nature, we report a sulfur doping–driven structural transformation from pyrite-type cobalt diselenide to pure marcasite counterpart. The resultant catalyst drives hydrogen evolution reaction with low overpotential of 67 millivolts at 10 milliamperes per square centimeter and exhibits no degradation after 1000 hours of testing in acid. Moreover, a PEM electrolyzer with this catalyst as cathode runs stably over 410 hours at 1 ampere per square centimeter and 60°C. The marked properties arise from sulfur doping that not only triggers formation of acid-resistant marcasite structure but also tailors electronic states (e.g., work function) for improved hydrogen diffusion and electrocatalysis.

First-principles calculations of phase transition, elasticity, phonon spectra, and thermodynamic properties for CeO2 polymorphs

In this work, the structural transformation, elastic behavior, phonon spectra and thermodynamic properties of CeO2 under compressive and tensile loading were investigated by first-principles calculations based on density generalized function theory (DFT). Our calculated structural parameters and transition pressure under compressive condition are in good agreement with the experimental values. Under tensile condition, the phase transitions of CeO2 from fluorite-type (space group Fm3¯m) phase to cotunnite-type (space group Pnma), rutile-type (space group P 42/mnm) and marcasite-type (space group Pnnm) phases were predicted to occur at 23.2 ​GPa, −7.69 ​GPa, and −7.75 ​GPa respectively. The mechanical stability criteria are verified in terms of elastic constants and shows that the cotunnite and fluorite structures of CeO2 are mechanically stable in the studied pressure range, while the rutile and marcasite structures turn out to be mechanical unstable under high pressure. The calculated phonon spectra of CeO2 polymorphs demonstrate that all phases considered in present work are dynamically stable under ambient pressure. Furthermore, the calculated heat capacity, isothermal bulk modulus, Gibbs free energy, thermally expanding coefficient of CeO2 polymorphs as a function of temperature are discussed and compared with available experimental data.

Synthesis, Transfer, and Properties of Layered FeTe2 Nanocrystals.

Different from layered two-dimensional (2D) transition metal dichalcogenides (TMDs), iron dichalcogenides crystallize in the most common three-dimensional pyrite or marcasite structures. Layered iron dichalcogenides are rarely reported and little is known about their structures and properties. Here, layered hexagonal phase iron ditelluride FeTe2 (h-FeTe2) nanocrystals are grown on mica by atmospheric pressure chemical vapor deposition (APCVD) method and are fully characterized by various methods. Like other 2D layered TMD materials, the FeTe2 nanoflakes exhibit regular hexagon, half hexagon, or triangle shapes with a controllable thickness of 6-95 nm and lateral length from a few to tens of micrometers. A simple and effective method is used to transfer the FeTe2 nanoflakes from the mica substrate onto any other substrates without quality deterioration by using polystyrene (PS) as a support polymer, which can also be operated in ethanol or ethylene glycol in a glovebox to avoid contact with water and air. Temperature-dependent electrical transport demonstrates that the FeTe2 nanoflake is a semiconductor with a variable range hopping (VRH) conduction, and its nonsaturated linear magnetoresistance (MR) reaches up to 10.4% under magnetic field of 9 T at 2 K, both probably due to its structure disorders. No signature of magnetic ordering is observed down to 2 K. The CVD growth of this layered FeTe2 represents an addition to the extensive library of 2D materials, particularly iron chalcogenides or alloys. Synthesis, properties, and even doping of phase pure h-FeTe2 call for further study in the future.

High-Pressure Synthesis and Crystal Structure of the Sulfur-Richest Chromium Sulfide CrS3 Composed of Cr(III) and Disulfide Ions.

The new binary chromium sulfide CrS3 has been synthesized by reaction of Cr3S4 and sulfur mixtures at 800 °C and 13 GPa. CrS3 crystallizes in an orthorhombic unit cell with a = 4.6742(7) Å, b = 5.7315(8) Å, c = 10.603(2) Å, and V = 284.873(4) Å3. It has a novel structure composed of Cr2S10 edge-shared octahedral dimers, which share all of their corners to form a three-dimensional structure. All of the sulfur atoms form S22- disulfide ions with a S-S distance of 2.063(5) or 2.068(8) Å. The structure of CrS3 is a derivative of the crystal structure of marcasite FeS2, in which one in three metal sites of the marcasite structure is vacant in the CrS3 structure.

Chemical Bonding in Colossal Thermopower FeSb2.

FeSb2 exhibits a colossal Seebeck coefficient ( ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb2 are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb2 . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.

Non-Native Pyrite Performs Better Than Native Marcasite CoSe2 for Electrochemical Oxygen and Hydrogen Evolution Reaction: A Theoretical Investigation

Efficient bifunctional electrocatalyst for water splitting is essential for replacing fossil-fuel energy sources with clean energy-dense hydrogen fuel (142 MJ/kg). Efficient electrocatalyst can be obtained by either increasing active site density or specific activity on individual active sites. The active site densities can be increased through roughening the potential energy surface or exposing the facets which has higher active site densities. The specific activity can be increased through modulation of strain or charge densities on active sites which can be achieved through introduction of dopants, defects or stabilization of “non-native phases” that are all the other crystalline and amorphous states that differ in terms of discrete translational symmetry in the sub-surface region from the “native” phase (or bulk ground-state). While for a given composition, there is a unique native state for a given set of thermodynamic condition while, there can be many non-native structures having different bond-angles, bond-distances and surface atom densities from the native phase, leading to different electrocatalytic properties. In this context, polymorphic engineering via stabilizing ‘non-native phase’ offers a potential approach for improving both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts and its activity. The beneficial effect of polymorphic engineering with regards to bifunctional electrochemical OER and HER is demonstrated by first principle calculation by taking CoSe2 as a model electrocatalyst which has marcasite (Space Group-58) and pyrite (Space Group-205) as the native (N) and non-native (NN) structures, respectively. The first principle computations predict pyrite (NN) structure of CoSe2 would have better electrochemical activity towards OER and HER than its marcasite (N) counterpart which is confirmed through experimental results in literature too. Though the co-ordination number of Co remains same in both the structures, the co-ordination symmetry surrounding Co atom varies. This results in differential charge distribution in constituting Co- and Se-atoms consequently resulting in variable density of state (DOS) near Fermi level (Figure 1) thereby affecting the binding energies (BE) of reaction intermediates of OER and HER. Pyrite (NN) phase of CoSe2 has a greater electron density near Fermi Level in comparison to its marcasite (N) counterpart due to differential co-ordination symmetry. A greater electron density near Fermi-level is indicative of lower work function and consequently lower polarization resistance during water splitting. A greater electron density near Fermi level is contributed by Co-3d orbitals which is the common active site for both OER and HER. The greater electron density and lower work function in Pyrite (NN) results in stronger metal-hydrogen BE (0.03 eV) resulting in lower overpotential of HER. Hydrogen adsorption on Se sites occurs only at higher HER overpotential due to weak Se-hydrogen BE (0.59 eV). This results in observation of twin Tafel slopes during HER on CoSe2 electrocatalyst as the potential determination step (PDS) switches from Volmer to Heyrovsky step with participation of Se during HER. The lower work function and higher electron density near Fermi level in Pyrite (NN) structure results in weaker metal-oxygen bond thereby promoting multi-electron OER activity. The OER intermediates (-OH, -O, -OOH) has a higher BE over Co- than Se-sites. The transformation of Oads à HOOads on Co-sites of CoSe2 (001) structure is the potential determination step with an onset potential of 1.66 V (vs RHE).The desorption of O2 from Se site is found to be the potential-determination-step (PDS) for OER (η=0.79 V). Furthermore, pristine CoSe2 acts as a precursor for OER which undergoes dissolution to form a surface Co-O structure which has a greater activity than pristine pyrite CoSe2 surfaces (η=0.31 V). This energetics is more favourable for pyrite (NN) structure than marcasite (N) structure for dissolution process to form surface Co-O structure due to stronger Co-Se bonds present in the latter case. Furthermore, point-defects which can aid both OER and HER, can be more easily formed in pyrite (NN) structure than marcasite (N) structure due to the aforementioned reason. The present study underlines the importance of stabilization of non-native structures which has a great potential to produce higher electrocatalytic activity thus providing greater options in search of better water splitting electrocatalyst. Figure 1

Fe-N system at high pressure reveals a compound featuring polymeric nitrogen chains

Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N–N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N2 in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe3N2, FeN2 and FeN4. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe3N2, synthesized at 50 GPa, is isostructural to chromium carbide Cr3C2. FeN2 has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN4, synthesized at 106 GPa, features polymeric nitrogen chains of [N42−]n units. Based on results of structural studies and theoretical analysis, [N42−]n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

High Pressure and High Temperature Synthesis of the Iron Pernitride FeN2.

The high pressure chemistry of transition metals and nitrogen was recently discovered to be richer than previously thought, due to the synthesis of several transition metal pernitrides. Here, we explore the pressure-temperature domain of iron with an excess of nitrogen up to 91 GPa and 2200 K. Above 72 GPa and 2200 K, the iron pernitride FeN2 is produced in a laser-heated diamond anvil cell. This iron-nitrogen compound is the first with a N/Fe ratio greater than 1. The FeN2 samples were characterized from the maximum observed pressure down to ambient conditions by powder X-ray diffraction and Raman spectroscopy measurements. The crystal structure of FeN2 is resolved to be a Pnnm marcasite structure, analogously to other transition metal pernitrides. On the basis of the lattice's axial ratios and the recorded N-N vibrational modes of FeN2, a bond order of 1.5 for the nitrogen dimer is suggested. The bulk modulus of the iron pernitride is determined to be of K0 = 344(13) GPa, corresponding to an astounding increase of about 208% from pure iron. Upon decompression to ambient conditions, a partial structural phase transition to the theoretically predicted R3̅ m FeN2 is detected.

Sulfide Replacement Processes Revealed by Textural and LA-ICP-MS Trace Element Analyses: Example from the Early Mineralization Stages at Cerro de Pasco, Peru

The large Cerro de Pasco Cordilleran base metal deposit in central Peru is the result of three successive mineralizing stages comprising both low- and high-sulfidation mineral associations: (A) several pyrrhotite pipes grading outward to sphalerite and galena replacement bodies, (B) a massive, funnel-shaped pyrite-quartz replacement orebody, and (C) E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies. This superposition of hydrothermal events leads to complex replacement textures and crosscutting relationships. A detailed study of the textures and mineral composition of the up to 15-m-wide replacement front existing between the pyrrhotite pipes and the pyrite-quartz body allows for clarification of the relative chronology of the hydrothermal events. The results show that, in contrast to previous interpretations, the emplacement of the pyrrhotite pipes and their Zn-Pb mineralized rims precedes that of the pyrite-quartz body. The replacement textures affecting pyrrhotite and arsenopyrite and the nature of the newly formed minerals have been used as a qualitative way to track the evolution of fS2, fO2, and pH of the mineralizing fluids. Two steps of pyrrhotite replacement have been recorded. The first one takes place under moderate acidity and relatively reduced to moderately oxidized conditions and is marked by replacement of pyrrhotite by euhedral nonporous pyrite. The second step occurs under more acidic and oxidized conditions and is characterized by replacement of pyrrhotite by porous marcasite and replacement of arsenopyrite by pyrite. Subsequently, marcasite is partly replaced by fine-grained euhedral nonporous pyrite. LA-ICP-MS trace element analyses of the replaced pyrrhotite and arsenopyrite and of the newly formed marcasite and pyrite support dissolution-reprecipitation as the main mechanism for replacement. Positive correlations between some of the elements (e.g., Pb-Sb, Pb-Ag) are indicative of the possible presence of nanoscale solid inclusions as main carriers for those elements; however, coupled substitutions and incorporation of some of the elements at a ppm level into the pyrite and marcasite structures cannot be excluded. The obtained As, Sb, Pb, and Bi values in pyrite are systematically higher than published data of pyrite in epithermal and porphyry systems. Nature and trace element content of the newly formed minerals yield information on the physicochemical conditions during their precipitation, the initial trace element content of replaced minerals, and the subsequently dissolved neighboring phases. The results show that the metal concentration of the fluid is locally influenced by the composition of the dissolved minerals. This study leads to a simpler interpretation of the fluid evolution than previously proposed, with a progressive increase of fS2, fO2, and pH as a result of decreasing wall-rock buffering during the three successive mineralizing stages at Cerro de Pasco.

Simple and versatile one-step synthesis of FeS2 nanoparticles by ultrasonic irradiation

The synthesis of stable pyrite nanocrystals (NCs) in a simple way is very difficult. A facile and single-stage ultrasonic synthesis route has been successfully developed for the preparation of pyrite (FeS2) NCs in 10min and 70°C. In this study, the influences of reaction time, temperature, and acoustic power on the formation of the target compound were considered. In addition, a comparison was made of the synthesis of pyrite NCs by sonication with 20kHz apparatus and by the classical method (without ultrasound). The as-prepared pyrite was characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), Raman spectrophotometry and energy-dispersive X-ray spectroscopy (EDS). The XRD of the sample prepared by classical method indicated a typical cubic FeS2 with space group Pa3 and the average size of 16nm. Sulfur as an impurity in the sample was verified by XRD analysis. The sample prepared by ultrasound showed two phases of pyrite, cubic and marcasite structures. The space groups were pa3 and pnnm, respectively and the average size was 28.8nm. According to the SEM images, the morphology of the synthesized pyrite using the two methods are closely the same. The FT-IR and Raman spectra presented the FeS, FeS and SS functional groups. In addition, the sono-synthesis of pyrite was done under milder conditions and in shorter time with better features than classical method.

Effect of Vacancies on Magnetism, Electrical Transport, and Thermoelectric Performance of Marcasite FeSe2−δ (δ = 0.05)

The marcasite structure FeSe2−δ was synthesized using a simple solvothermal method. Systematic study of the electrical transport properties shows that the transport is dominated by variable-range hopping (VRH), with a changeover from Mott VRH at higher temperature to Efros-Shklovskii VRH for temperatures lower than the width of the Coulomb gap. This also confirms the presence of a Coulomb gap in the density of states at the Fermi energy. We observe that Yttrium doping increases the electrical conductivity dramatically without significantly reducing the Seebeck coefficient. This results in remarkably high power factors for thermoelectric performance in the regime where the mean hopping energy shifts from defect dominated to Coulomb repulsion dominated. High resolution transmission electron microscopy, in combination with theoretical calculations, proves the narrowing of the band gap by introducing Se vacancies. This leads to a good conductivity and is responsible for the excellent thermoelectric performanc...

Physical properties of osmium dinitride with fluorite, pyrite, marcasite, and monoclinic structures under high pressure: First-principles study

The structure, phase transition, elastic, and thermodynamic properties of OsN2 have been studied via first-principles calculations. It is shown that the CoSb2 structure is more stable than other structures. By the calculated H-P relations at 0 K, we found that the phase transition of OsN2 from CoSb2 structure to marcasite structure (ε → δ) occurs at 16.8 GPa, while the phase transition pressure between pyrite structure and fluorite structure (γ → α) is 80 GPa. The results of obtained phase transitions are also confirmed by bond length, sound velocity, and thermal expansion coefficient under different pressures. The pressure dependences of the elastic constants, mechanical stability, and mechanical anisotropy of four structures of OsN2 have been investigated by finding that the fluorite (pyrite and marcasite) structure OsN2 is mechanically stable under hydrostatic pressure (up to 60 GPa). However, the monoclinic structure is mechanically unstable under pressure from 0 to 60 Gpa. The calculated elastic anisotropic factors show that OsN2 possesses high elastic anisotropy under pressure. Moreover, the calculations on total density of states show that OsN2 of different structures has a metallic character, in agreement with previous theoretical results. The thermodynamic properties and sound velocity under diverse pressures of OsN2 of the four structures have been also investigated successfully.

Facile Preparation of State-of-the Art Thermoelectric Materials by High-pressure Synthesis

We describe the HP preparation of some state-of the art thermoelectric materials: chalcogenides and pnictides. Those include Bi2Te3 and materials with skutterudite structure (CoSb3). Some present a huge preferred orientation and nanostructuration, resulting from the uniaxial pressure applied during the synthesis, which may be beneficial to enhance the thermoelectric properties. Besides pnictides (group Va) like CoSb3 or AuSb2, we have also prepared semi-metal chalcogenides (group VIa) with NiAs structure (CoTe) or marcasite structure (FeTe2). All these materials have been structurally characterized by XRD and NPD (neutrons are useful to prevent preferred orientation problems in the structural resolution) and the three involved magnitudes in the ZT figure of merit have been preliminary characterized in as-grown pellets of the bulk materials.

Effect of Precursor Concentration on Structural and Morphological Properties of Iron Pyrite Thin Films

The cubic FeS2 (Pyrite) films are prepared by chemical bath deposition method (CBD) at different iron ion precursor concentrations that varying from 2.0M to 0.5M. X-ray diffractograms revealed that the presence of (110) plane, corresponding to marcasite structure, and (106) and (304) planes, corresponding to FeS (Triolite) phase are observed along with the pure pyrite phase (200) at higher concentrations of iron. The films deposited at 1.0M concentration has strong (200) and (023) planes related to pure pyrite phase. This is confirmed by the Raman measurements. It is observed that the molar concentration of the precursor plays an important role in controlling the morphology of the films. FTIR spectra shows the existence of –OH, -CH stretching of –CH2 vibrations modes. The optical absorption coefficient is > 104 cm-1 in the visible spectrum, indicating that this material is exploitable as solar cell absorber and found to be optically isotropic. The studies revealed that the precursor concentration of Iron has a significant effect on the behaviour of FeS2 films. A detailed analysis of the results with appropriate discussion is presented.

Crystal growth and characterization of the narrow-band-gap semiconductors OsPn₂ (Pn = P, As, Sb).

Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

Electrolytic synthesis of FeS2 for thin-layer lithium battery

Purposeful synthesis of iron sulfide FeS2 films for a lithium battery were purposefully synthesized on a 18N12Kh9T steel cathode from a solution containing Mohr’s salt and Na2S2O3. Various aspects of synthesis were studied. Thin-film Fe-sulfide synthesis products were tested in a prototype lithium battery and also in a lithium-ion system. The synthesized electrolytic iron sulfide FeS2 with a marcasite structure is capable of reversible electrochemical transformation with output of 390 mA h g−1 in negative electrodes of the lithium-ion system with a LiMn2O4 counter electrode.

First-principles studies of FeS2using many-body perturbation theory in theG0W0approximation

We present a theoretical study on iron pyrite using density-functional theory (DFT) and the $GW$ approximation to many-body perturbation theory. The fundamental band gap of iron pyrite is determined by iron 3$d$ states at the valence band edge and a sulfur 3$p$-dominated conduction band at $\ensuremath{\Gamma}$. The gap is quite sensitive to structural changes as well as to the applied electronic structure method. We found that this $p$-dominated band does not play a significant role for the optical absorption, leading to a large difference between the optical and fundamental band gaps of iron pyrite. As a consequence the $GW$-corrected energies result in no considerable change of the optical band gap as compared to standard DFT, both being in reasonable agreement with experiment. However, we show that the fundamental band gap is reduced to about $0.3$ eV in $GW$, which may contribute to the low open-circuit voltage of about $0.2$ V observed in iron pyrite solar cells, representing a serious bottleneck for photovoltaic applications. To demonstrate that this unconventional reduction of the $p$-$d$ gap is not unique for iron pyrite, similarities for FeS${}_{2}$ in the marcasite structure are presented.

A novel layer-structured PtN2: First-principles calculations

The potential structures of platinum nitride with a chemical composition of PtN2 have been examined by utilizing a widely adopted evolutionary methodology for crystal structure prediction. Except reproducing the previously proposed phases, a Pmmm symmetric novel layer structure with a low formation enthalpy that is slightly lower than those of marcasite and CoSb2 structures but slightly higher than that of pyrite structure has also been identified. The elastic constants and the lattice dynamical calculations show that this layer-structured PtN2 is mechanically and dynamically stable. The calculated band structures suggest this new phase together with the simple tetragonal phase are metallic, while other phases are insulators. In addition, it has been found by the phonon spectrum calculations that the fluorite structure is dynamically unstable, although it is mechanically stable as suggested by calculated elastic constants.