First-principles investigations of electronic and mechanical properties for stable Ge 2 Sb 2 Te 5 with van der Waals corrections Baisheng Sa a,b , Jian Zhou c,b , Rajeev Ahuja a,d , Zhimei Sun c,b,⇑ a Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden b Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005 Xiamen, China c School of Materials Science and Engineering, Beihang University, Beijing 100191, China d Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden article info Article history: Received 12 August 2013 Received in revised form 10 September 2013 Accepted 12 September 2013 Keywords: Ge 2 Sb 2 Te 5 Phase-change materials Van der Waals-type bonding Hybrid functions abstract The Te–Te weak van der Waals-type bonding plays an important role in Ge 2 Sb 2 Te 5 , a widely investigated phase-change material and a potential topological insulator. In this work, we have studied the electronic and mechanical properties of stable Ge 2 Sb 2 Te 5 using ab initio calculations with the van der Waals correc- tions. The results show that the van der Waals corrections combined with hybrid functions improve the descriptions of the electronic structure of stable Ge 2 Sb 2 Te 5 . The band gap of 0.5 eV in very good agree- ment with the experimental value for stable Ge 2 Sb 2 Te 5 has been successfully reproduced. Furthermore, we have predicted the elastic constants and mechanical properties of stable trigonal Ge 2 Sb 2 Te 5 . Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Phase-change (PC) material Ge 2 Sb 2 Te 5 has been widely investi- gated since they have played important roles in the applications of optical storage and next generation non-volatile electronic mem- ory [1,2]. Ge 2 Sb 2 Te 5 has two crystalline states at ambient condi- tions, a stable trigonal phase and a metastable cubic (rock-salt type) structure [3,4]. Although most data storage applications using Ge 2 Sb 2 Te 5 as recording media is achieved by the reversible phase transition between the amorphous and metastable cubic states, it is worth noting that the phase change process from stable trigonal states to the amorphous or metastable cubic states is also very fast [5], and the crystallographic relationship of the two crys- talline phases is very close [3]. Recent studies demonstrate that tri- gonal Ge 2 Sb 2 Te 5 is a time-reversal-invariant topological insulator [6,7], showing the potential applications in quantum computers and spintronics [8]. Therefore, an investigation on the stable trigo- nal phase is of great interest and importance [9]. The trigonal Ge 2 Sb 2 Te 5 can be considered as a nounple layered structure with the space group of P  3m1, which consists of 9 atom layers in one nounple layer [10]. The stacking model for this nounple layer is still in debate. Based on experimental results, three possible stacking models have been proposed for stable Ge 2- Sb 2 Te 5 , i.e., Te–Ge–Te–Sb–Te–Te–Sb–Te–Ge– [10] (herein referred to as GST-I), Te–Sb–Te–Ge–Te–Te–Ge–Te–Sb– [11] (herein referred to as GST-II) and Te–Ge/Sb–Te–Ge/Sb–Te–Te–Ge/Sb–Te–Ge/Sb– [12] (the Ge and Sb atoms randomly occupy in the same layer, herein referred to as GST-III), the structure relations of which can be find elsewhere [7]. GST-III can be considered as a mixture of GST-I and GST-II, thus the physical properties of GST-III can be de- rived from GST-I and GST-II with the Vegard’s law. Therefore, in this work we investigated GST-I and GST-II. In Ge 2 Sb 2 Te 5 , The Te–Te weak bonding can be easily transferred to a so-called ‘‘va- cancy layer’’ by the movement of the nounple-layers along the [2 1 0] direction [3], which plays an important role in the fast phase transition process for Ge 2 Sb 2 Te 5 [13]. Hence, the study of stable Ge 2 Sb 2 Te 5 , as well as the Te–Te weak bonding with an advanced method can provide an extensive understanding of this material, and hence contribute to understanding the Ge–Sb–Te based ter- nary chalcogenides. In this work, we have investigated the electronic structures and mechanical properties of stable trigonal Ge 2 Sb 2 Te 5 using density functional theory (DFT) with Grimme’s semi-empirical correction [14] (DFT-D2 method) on the van der Waals interactions. The DFT-D2 method has been tested to provide very good results for Ge 2 Sb 2 Te 5 which predict the lattice parameters very close to the experimental results [15]. The electronic properties for stable Ge 2- Sb 2 Te 5 have been predicted with van der Waals corrections as well 0927-0256/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2013.09.026 ⇑ Corresponding author at: School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel.: +86 592 2186664. E-mail addresses: zmsun@buaa.edu.cn, zmsun@xmu.edu.cn (Z. Sun). Computational Materials Science 82 (2014) 66–69 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci