Electronic structure, elastic and optical properties of MnIn2S4

The elastic constants and various optical properties of MnIn2S4 including the dielectric constant, absorption coefficient, electron energy loss function and reflectivity were calculated as a function of incident photon energy. Those results are discussed in this study and compared with available experimental results.
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Electronic structure, elastic and optical properties of MnIn2S4 JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 135-143 This paper is available online at http://stdb.hnue.edu.vn ELECTRONIC STRUCTURE, ELASTIC AND OPTICAL PROPERTIES OF MnIn2 S4 Nguyen Minh Thuy and Pham Van Hai Faculty of Physics, Hanoi National University of Education Abstract. The electronic, elastic, and optical properties of MnIn2 S4 were investigated using first-principle calculations based on density functional theory (DFT) with the plane wave basis set as implemented in the CASTEP code. Our study revealed that MnIn2 S4 has indirect allowed transitions for both DFT and DFT + U (U = 6 eV) with energy band gaps of 1.57 eV and 2.095 eV, respectively. The elastic constants and various optical properties of MnIn2 S4 including the dielectric constant, absorption coefficient, electron energy loss function and reflectivity were calculated as a function of incident photon energy. Those results are discussed in this study and compared with available experimental results. Keywords: Inorganic compounds, Ab initio calculations, electronic structure. 1. Introduction Recently, MnIn2 S4 which are ternary compounds of the AB2 X2 type have received much attention as materials which have potential for optoelectronic application and as magnetic semiconductors [1]. In the literature, physical properties of MnIn2 S4 have been reported [1, 2]. Recently, the optical absorption spectra of MnIn2 S2 single crystals have been measured and it was found that the fundamental absorption edge is formed by direct allowed transitions [3, 4]. However, Bodnar et al. showed that MnIn2 S4 has both direct and indirect transitions [5]. Therefore further calculations of MnIn2 S4 are needed to clarify the origin of its band gap structure. Density functional theory (DFT) has been the dominant method used when making electronic structure calculations in solid state physics. In this work we report on the band structure, optical and elastic properties of MnIn2 S4 using density functional theory. The calculated results can provide a good model for understanding and predicting other behaviors of this material. Received August 26, 2014. Accepted October 23, 2014. Contact Nguyen Minh Thuy, e-mail address: thuynm@hnue.edu.vn 135 Nguyen Minh Thuy and Pham Van Hai 2. Content 2.1. Calculation models and methods MnIn2 S4 is a spinel-type compound and crystallizes in the space group Fd3m with lattice parameters a = b = c = 10.722 A ˚ [4]. In this structure, the Mn atoms share the tetrahedral sites, while the In atoms share the octahedral sites, as shown in Figure 1. Figure 1. Crystal structure of cubic MnIn2 S4 First principle calculations were performed using the CASTEP module in Materials Studio 6.0 developed by Accelrys Software, Inc.. Electron-ion interactions were modeled using ultrasoft pseudopotentials. The wave functions of the valence electrons were expanded through a plane wave basis set and the cutoff energy was selected as 380 eV. The Monkhorst-Pack scheme k-points grid sampling was set at 8 × 8 × 8. The convergence threshold for self-consistent iterations was set at 2 × 10−6 eV/atom. In the optimization process, the energy change, maximum force, maximum stress and maximum displacement tolerances were set at 10−5 eV, 0.03 eV/A,˚ 0.05 GPa and 0.001 A,˚ respectively. 2.2. Results and discussion 2.2.1. Electronic structure We used density functional theory (DFT) to calculate the band structure and the density of states (DOS) of MnIn2 S4 . The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional were used to describe the exchange-correlation effects. The core electrons were replaced by the ultrasoft core potentials. Electron configurations were 3p6 4s2 3d5 for Mn, 4d10 52 5p1 for In and 3s2 3p4 for S atoms. Both the lattice parameter and the atomic position are optimized. The optimized lattice constants calculated by GGA + PBE (10.854 A) ˚ show good 136 Electronic structure, elastic and optical properties of MnIn2 S4 agreement with experimental details 10.722 A ˚ [4] in which the difference value is about 3 - 5 percent. As is well known, the GGA structural results are somewhat overestimated in comparison with experimental values. Calculated band structures of MnIn2 S4 are shown in Figure 2a. Coordinates of the special points of the Brillouin zone area are as follow (in terms of unit vectors of the reciprocal lattice): W (0.5, 0.25, 0.75), L (0.5, 0.5, 0.5), G (0, 0, 0), X (0.5, 0, 0.5) and K ( ...