Role of Cr- d states in the electronic and optical properties of the CdCr 2 X 4 (X = S, Se) normal ferromagnetic spinels
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We have investigated theoretically the role of Cr-d states in the electronic and optical properties of the CdCr2X4 (X 5 S, Se) normal ferromagnetic spinels using the framework of an all-electron fullpotential linearized augmented plane wave method. The calculations are performed using Coulomb corrected Perdew–Burke–Ernzerhof (PBE1U) and Tran–Blaha modified-Becke–Johnson (TB-mBJ) approximations with the adding of spin–orbit coupling in both schemes. The lattice parameters have been optimized and are in agreement with the existing experimental values. We found band gap values 1.606 eV and 0.972 eV of CdCr2X4 (X 5 S, Se), respectively, using the TB-mBJ scheme. Analysis of the site and momentum projected densities shows that the larger splitting of Cr-d states is responsible for the larger band gap by the use of the TB-mBJ scheme. Optical properties along the directions of lattice constants are studied on the basis of band to band transitions. We found the isotropic nature of the optical properties. Reflectivity stays low up to 1.6 eV, consistent with the energy gaps obtained using the TB-mBJ scheme in both the compounds. The refractive index, n(x), and the extinction coefficient, k(x), are also studied by the PBE and the TB-mBJ schemes.
I. INTRODUCTION
Investigation of spinel materials with higher transparency and conductivity is the recent field of research because of their use in optoelectronic devices such as flat panel displays and solar cells.1 The general formula of Cr based spinel materials family is ACr2X4, in which A 5 Mn, Fe, Co, Ni, Cu Zn, and Cd, which occupy the tetrahedral site having charge 12. The X site accommodates various elements of group VI (i.e., O, S, Se and Te). These compounds generally exist in the cubic spinel structure.2 Several of these compounds3–7 are wide band gap semiconductors. These wide band gap materials are transparent in the visible and infrared regions. This has motivated to search for new spinel materials in optoelectronic devices. The electronic structure methods with accurate potential propose new and improved transparent and conducting materials. Spinel compounds have been the subject of many experimental3–6 and theoretical7–17 works in the past few years, focused on their electronic and optical properties. The ternary spinel compounds CdCr2X4 (X 5 S, Se) with Cr as a transition metal are members of a group of chalcogenide semiconductors. These are ferromagnetic materials with Curie temperatures of 84.9 and 129 K, respectively. The structural properties such as lattice Contributing Editor: Amit Goyal a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.159
constant and anion parameter as well as magnetic and electrical properties of these compounds were studied in detail by Landolt-Börnstein.18 They were extensively studied due to a number of peculiar and interesting properties like giant magnetoresistance, photoferromagnetic effect, red shift of the optical absorption edge, and giant Faraday rotation apart from the coexistence of
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