The electronic, magnetic and optical properties of Co doped marcasite FeS2
Graphical abstract
Spin charge density at isosurfaces level of 0.0038 e/Å3: (a) Fe7CoS16 compound, (b) Fe6Co2S16 compound. Yellow and cyan isosurfaces denote positive and negative charge density, respectively.
Introduction
Iron disulfide (FeS2) has been researched extensively in many respects. It can be used as photovoltaic materials [1], anode material for lithium ion batteries [2], counter electrode materials for dye-sensitized solar cells [3], thermo-electric materials [4], spintronics [5] and so on. FeS2 possesses glorious electron mobility [6], large photon absorption coefficient of 105 cm−1 and it is made up of natural abundant and environmental friendly elements [[7], [8], [9], [10]]. There are two polymorphs for FeS2 in naturally: pyrite FeS2 (p-FeS2) and marcasite FeS2 (m-FeS2). The structure of p-FeS2 belongs to space group in cubic crystal system, while m-FeS2 possesses orthorhombic crystal system in space group Pnnm. Very earlier research have provided band gap of 0.34 eV for m-FeS2 [11], so that it always be considered as a non-convenient impurity of p-FeS2 in photovoltaic materials. Recent published studies have predicted that m-FeS2 have a band gap quite similar to that of p-FeS2 [[12], [13], [14]], which made researchers began to study m-FeS2 seriously. L. F. Wu and coworkers [15] reported the advantageous role of m-FeS2 in photo-electrochemical applications of iron sulfide-based materials. Their research indicated that the FeS2 films mixed pyrite-marcasite phase not only improve photo-currents and photo-response considerably, but also increase the stability of the films against photo-corrosion and reduce the dark current prominently. T. T. Li et al. [16] synthesized the marcasite FeS2 nanoparticles for the first time in colloidal solution via a hot-injection protocol. Their research indicated that compared with p-FeS2 and other iron sulfides, the m-FeS2 exhibit more excellent lithium ion storage and more stable charge-discharge performance as the anode materials in lithium ion battery application. D. G. Moon et al. [17] investigated the influence of marcasite on iron pyrite thin films, they found that marcasite should have a band gap of approximately 0.85–0.88 eV with a higher absorption coefficient than pyrite, and the pyrite/marcasite film have a larger photocurrent density than pure pyrite. The pioneer's works clearly suggested that m-FeS2 should be a useful semiconductor in photovoltaic materials.
It is a universal approach that introducing the impurities into a material artificially to tune the electronic, magnetic and optical properties of the material. Semiconductors doped with magnetic impurities are called Diluted magnetic semiconductors (DMSs) [18]. DMSs have attracted extensive attention in recent years due to their potential applications as a new type of functional materials. It can be used in magnetic sensors, spin-polarized light-emitting diode and so on. Those kinds of materials continue to challenge our understanding. The purpose of the research is to find DMSs with high Curie temperature for the applications of spintronic devices. Room-temperature ferromagnetism has been reported in Mn-doped CdGeP2 [19], Co-doped anatase TiO2 [20], Co-doped rutile TiO2 [21], V-doped rutile TiO2 [22] and so on. To the best of our knowledge, the electronic, magnetic and optical properties of Co doped m-FeS2 have not been reported in detail so far. In present work, we investigate the electronic, magnetic and optical properties of m-FeS2 and Co doped m-FeS2 compounds. The calculation details are given in section 2, the results and discussions are shown in section 3. Finally, the summary and conclusions are given in section 4.
Section snippets
Computational details
First-principles calculations are performed with the Vienna ab-initio simulation package (VASP) [[23], [24], [25], [26]] based on pseudo-potential plane wave method within the frame work of the density function theory (DFT). The projector augmented wave (PAW) potentials [27] are used to describe the electron-ion interactions. The exchange and correlation terms are described with the generalized gradient approximations (GGA) [28] of Perdew-Burke-Eruzerh (PBE) formulation, as well as taking into
Results and discussions
Through the theoretical method we have described, all atoms have been fully relaxed until the required accuracy is satisfied. The optimized lattice constants of marcasite Fe8S16 compound are 8.883 Å, 5.422 Å, 6.804 Å (2a, b, 2c, respectively), which are good consistent with experimental results [34,35], showing that our calculation is reasonable and acceptable. The calculated volume of Fe8S16, Fe7CoS16 and Fe6Co2S16 compounds are 327.80 Å, 330.42 Å and 333.25 Å, respectively. The volume
Conclusions
In conclusion, the electronic, magnetic and optical properties of Co doped m-FeS2 are investigated by using the first-principles. The following conclusions are obtained:
- (1)
There have no imaginary frequency over the whole Brillouin-zone, indicating that the Fe7CoS16 and Fe6Co2S16 compounds are dynamical stable.
- (2)
The marcasite Fe8S16 is a nonmagnetic semiconductor with band gap of 1.17 eV, while Fe7CoS16 is a magnetic semiconductor with very narrow band gap of 0.106 eV and Fe6Co2S16 is a half-metal.
Acknowledgement
The authors would like to acknowledge the Fundamental Research Funds for the Central Universities (Grant No. GK201804001) and the Fundamental Research Funds of Ningxia Medical University (Grant No. XM201306) for providing the financial support for this research.
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