DFT calculation of the electronic structure and optical properties of two strontium germanium nitrides: a-Sr 2 GeN 2 and b-Sr 2 GeN 2 Zeyad A. Alahmed a,⇑ , Ali H. Reshak b,c a Department of Physics and Astronomy, King Saud University, Riyadh 11451, Saudi Arabia b Institute of Complex Systems, FFPW, CENAKVA, University of South Bohemia in CB, Nove Hrady 37333, Czech Republic c School of Material Engineering, Malaysia University of Perlis, P.O. Box 77, d/a Pejabat Pos Besar, 01007 Kangar, Perlis, Malaysia article info Article history: Received 18 December 2012 Received in revised form 10 January 2013 Accepted 11 January 2013 Available online 7 February 2013 Keywords: Inorganic materials Ternary nitride a-Sr 2 GeN 2 b-Sr 2 GeN 2 Ab initio calculations Optical constants abstract The existence of a-Sr 2 GeN 2 and b-Sr 2 GeN 2 were reported recently with their structural properties. In this paper, electronic and optical properties have been investigated using density functional theory. The Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) is used for the exchange and correlation potential to calculate the optimized parameters of the structures. Our results confirm the previous observation of the atomic positions and lattice parameters for both crystals. The values of the unit cell volume of a- and b-Sr 2 GeN 2 are in good agreement with the experimental values. The local den- sity approximation (LDA), (GGA), the Engel–Vosko GGA approximation (EV-GGA) and the modified Becke–Johnson method (mBJ) were used to calculate the electronic and optical properties. Our results show that a-Sr 2 GeN 2 has a small band gap of about 0.0 eV (LDA), 0.050 eV (GGA), 0.210 eV (EVGGA) and 0.268 eV (mBJ) while b-Sr 2 GeN 2 has energy band gap of about 0.10 eV (EVGGA) and 0.15 eV (mBJ). Additionally, optical dielectric constants were calculated for both ternary compounds. We believe that the current study can provide more information to understand the properties of these ternary nitrides. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Several ternary nitrides have been synthesized and developed in the past years [1]. They have gained more interest in recent years because they can expand several applications that are limited by the properties of the binary nitrides. Due to their properties, they can be used in optoelectronic application such as in the field of high performance light emitted diodes [2,3]. Recently, there is a considerable interest in the search for new material and properties by developing and investigating new ternary nitrides taking the advantage of the new preparation techniques [4,5]. New ternaries based on ternary germanium nitrides (GeN) have been synthesized recently with different composition which revealed new structures and interesting properties [6]. Thus one can obtain new ternary ni- trides with desired structures and properties. Additionally, consid- erable attempts have been focused on preparing a new ternary that can form more isolated anionic groups, which can extend anion network in 1D, 2D or 3D [1]. Some of these known ternaries are strontium germanium ni- trides a-Sr 2 GeN 2 and b-Sr 2 GeN 2 . They were synthesized by DiSalvo et al. with the use of Na melts which provides a very productive route to obtain single crystals [7,8]. These new ternaries can be used as substrates for GeN-based devices. More applications of these nitride compounds can be obtained from understanding their new structures and properties. Both structures contain isolated bent units of GeN 4 2 anions. They show several similarities in the volume, bond lengths and angles. However, the relative orientation of the GeN 4 2 units in the lattice makes a clear distinction between the two structures [8]. The available information about these strontium germanium nitrides focused only on their structures fea- tures in detail while there is no available report on their electronic and optical properties. In this paper, we investigate the electronic and optical properties of Sr 2 GeN 2 in both a and b phases using the density functional theory (DFT). We have used all electron full po- tential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. The paper is organized as follows. The computational methods are described in Section 2. In Section 3, structural properties, elec- tronic, and optical properties are presented. The last section sum- marizes the results of this work. 2. Computational details All electron full potential linearized augmented plane wave (FP-LAPW) method as implemented in WIEN2k package [9] has been employed to study the structural, electronic band structures and optical properties of a-Sr 2 GeN 2 and b-Sr 2 GeN 2 crys- tals. In FP-LAPW calculation, the minimum radius of the muffin-tin spheres (R MT ) values for Sr, Ge, and N atoms in a-Sr 2 GeN 2 (b-Sr 2 GeN 2 ) were assumed to be 2.36 (2.4), 1.8 (1.78), and 1.7 (1.78) a.u., respectively. These values were chosen in such 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.062 ⇑ Corresponding author. Tel.: +966 14676615; fax: +966 14673656. E-mail address: zalahmed@ksu.edu.sa (Z.A. Alahmed). Journal of Alloys and Compounds 559 (2013) 181–187 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom