Elastic and Optoelectronic Properties of Cs 2 NaMCl 6 (M = In, Tl, Sb, Bi)
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https://doi.org/10.1007/s11664-020-08603-y Ó 2020 The Minerals, Metals & Materials Society
ORIGINAL RESEARCH ARTICLE
Elastic and Optoelectronic Properties of Cs2NaMCl6 (M = In, Tl, Sb, Bi) IMAD KHAN ,1,3 SHAHAB,1 IZAZ UL HAQ,1 AKBAR ALI,1 ZAHID ALI,1 and IFTIKHAR AHMAD1,2 1.—Center for Computational Materials Science, Department of Physics, University of Malakand, Chakdara Dir(L), Pakistan. 2.—Gomal University, Dera Ismail Khan, Pakistan. 3.—e-mail: [email protected]
In this article, elpasolite perovskites, Cs2NaMCl6 (M = In, Tl, Sb, Bi), are investigated using density functional theory (DFT). Structural properties like lattice constants and bond lengths are in agreement with the available experimental data. Electronic properties are calculated by several DFT exchange-correlation approximations, and it is found that a modified Becke– Johnson (mBJ) approximation along with the inclusion of spin orbit coupling (SOC) gives the most promising results. The M-site cation decides the nature of the band gap; i.e. direct band gaps are obtained for group IIIA elements (In, Tl), and indirect band gaps are experiential for group VA elements (Sb, Bi). Narrow discrete energy bands are observed in the valence and conduction bands, which make these compounds suitable for scintillation applications. SOC induces splitting of Bi/Sb p orbitals in the conduction band and reduces the band gaps of these double perovskite halides. Obtained values of mechanical parameters confirm that these compounds are ductile and anisotropic. Optical properties, i.e. dielectric functions, energy loss function and refractive index, are also calculated, and interesting variations are found which can play a important role in scintillation and other optoelectronic applications of these materials. Key words: Halide perovskites, discrete electronic bands, SOC, semiconductors, scintillation
INTRODUCTION Organic and inorganic lead (Pb) halide perovskites have attracted much interest due to their high efficiency in the conversion of solar radiation into electricity.1–4 These compounds are characterized by long diffusion length of charge carriers, high absorption coefficient, defect tolerance, efficient luminescence and remarkable light-energy harvesting ability.1–4 Lead-based halide perovskites have stability problems, such as moisture and temperature sensitivity, and the existence of toxic lead limits their applications.5 To overcome these
(Received June 16, 2020; accepted November 3, 2020)
shortcomings, lead-free metal halide perovskites having the desired absorption capacity were designed.6 Replacing Pb2+ by isovalent Sn2+ and Ge2+ results in instability, because they are both easily oxidized to Sn4+ and Ge4+5.7 Alternatively, Pb2+ can be replaced by monovalent M+ and a trivalent M3+, forming double perovskites.5 The general formula of double perovskites is A2M+M3+X6, where ‘‘A’’ generally symbolizes alkaline metals, and ‘‘M’’ sites are occupied by two different elements. Two ions of the same metal having different oxidation states have also been re
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