Electronic band structures of binary skutterudites Banaras Khan a, b , H.A. Rahnamaye Aliabad c , Saifullah a, b , S. Jalali-Asadabadi d , Imad Khan a, b , Iftikhar Ahmad a, b, * a Center for Computational Materials Science, University of Malakand, Chakdara, Pakistan b Department of Physics, University of Malakand, Chakdara, Pakistan c Department of Physics, Hakim Sabzevari University, Sabzevar, Iran d Department of Physics, Faculty of Science, University of Isfahan (UI), 81744 Isfahan, Iran article info Article history: Received 11 March 2015 Received in revised form 30 May 2015 Accepted 1 June 2015 Available online 10 June 2015 Keywords: Binary skutterudites Thermoelectric materials Electronic properties Theoretical condensed matter physics abstract The electronic properties of complex binary skutterudites, MX 3 (M ¼ Co, Rh, Ir; X ¼ P, As, Sb) are explored, using various density functional theory (DFT) based theoretical approaches including Green's Function (GW) as well as regular and non-regular Tran Blaha modified Becke Jhonson (TB-mBJ) methods. The wide range of calculated bandgap values for each compound of this skutterudites family confirm that they are theoretically as challenging as their experimental studies. The computationally expensive GW method, which is generally assume to be efficient in the reproduction of the experimental bandgaps, is also not very successful in the calculation of bandgaps. In this article, the issue of the theoretical bandgaps of these compounds is resolved by reproducing the accurate experimental bandgaps, using the recently developed non-regular TB-mBJ approach, based on DFT. The effectiveness of this technique is due to the fact that a large volume of the binary skutterudite crystal is empty and hence quite large proportion of electrons lie outside of the atomic spheres, where unlike LDA and GGA which are poor in the treatment of these electrons, this technique properly treats these electrons and hence reproduces the clear electronic picture of these compounds. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The skutterudites family has gained enormous attention as perfect materials in the renewable energy thermoelectric devices. The binary skutterudites display remarkable transport properties, including high degree of carrier's mobility [1]. The unusual trans- port properties of these compounds are a consequence of the high degree of covalent bonding because of the small electronegativity difference in their constituent elements and narrow bandgaps [1,2]. However, this strong bonding and simple order lead to high lattice thermal conductivity and thus, the issue with them is the reduction of lattice thermal conductivity, which can be resolved by the unique cage like open structure of skutterudites, where the open voids are filled with the rare-earth elements acting as phonon rattlers [1]. The bulk skutterudites with these unique physical properties are considered as ideal materials for thermoelectric applications [3]; although, higher Figure of Merit (ZT) values have been observed in superlattices as compared to bulk materials but superlattices are not useful in large-scale power production due to their high cost and heat transfer; therefore bulk skutterudites with improved ZT values are considered as ideal materials in high-temperature thermoelectric applications [3e7]. These skutterudites are repre- sented by MX 3 (M ¼ Co, Rh, Ir; X ¼ P, As, Sb) having body centered cubic structure with space group Im 3 [8] and crystallize in a cage- like crystal structure, where each transition metal atom is octahe- drally coordinated to six pnictide atoms [9,10]. The skutterudite structure is similar to the structure of ReO 3 , where perovskite (ABO 3 ) structure can be attained by introducing void on A site and also the octahedra of the ideal perovskite structure is somewhat tilted [11]. Binary skutterudites are extensively studied materials because of their potential technological applications and diverse physical properties but are challenging for experimentalists as well as theoreticians, due to their narrow bandgaps, complex crystal structure, large unit cell, low symmetry points and strong covalent bonding [12e14]. The complexity of the electronic structure of these compounds can be seen from the bandgap values of the CoSb 3 . For this compound, the experimentally measured bandgap value varies over a wide range (0.03, 0.04, 0.05, 0.55 and 0.63 eV) * Corresponding author. Center for Computational Materials Science, University of Malakand, Chakdara, Pakistan. E-mail address: ahma5532@gmail.com (I. Ahmad). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2015.06.018 0925-8388/© 2015 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 647 (2015) 364e369