Structural, elastic, and lattice dynamical properties of Germanium diiodide (GeI 2 ) H. Ozisik a,⇑ , K. Colakoglu b , H.B. Ozisik a,b , E. Deligoz a a Aksaray University, Department of Physics, 68100 Aksaray, Turkey b Gazi University, Department of Physics, Teknikokullar, 06500 Ankara, Turkey article info Article history: Received 26 April 2010 Received in revised form 13 August 2010 Accepted 17 August 2010 Available online 15 September 2010 Keywords: Germanium diiodide Mechanical properties Lattice dynamical properties Ab-initio calculations abstract To investigate the structural, elastic, and lattice dynamical properties of the germanium diiodide, we have performed the first-principles calculations by using the local density approximation method based on density-functional theory. Some basic physical parameters such as lattice constant, bulk modulus and its first derivatives, elastic constants, shear modulus, Young’s modulus, and Poisson’s ratio are calculated. The phonon dispersion curves, electronic band-structures, and total and partial density of states have also been calculated for ground state C6 phase of GeI 2 . Our results show that this structure has got 1.72 eV direct band gap. Our secondary results on the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy are also presented for the same compounds. The obtained results are in good agreement with the available experimental and other the- oretical data. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Germanium iodides are widely used for synthesis pure single crystal Germanium, which is an important semiconductor material used in transistors and various other electronic devices [1–3]. It is used to obtain anhydrous proton conductors thioborate doped with Germanium diiodide [4]. Recently, the intercalated layer- structured heavy metal iodide systems have also had several pos- sible important uses. Being wide band gap semiconductors, they have potential as photo detectors or solar cells in the visible range [5,6]. The germanium has two or four oxidation states [7] in io- dides compounds such as germanium (II) iodide (GeI 2 ) and germa- nium (IV) iodide (GeI 4 ). GeI 2 crystallizes in the CdI 2 -type (trigonal x phase) structure with the P  3m1 space group symmetry (space number 164) [8] at the room temperature and zero pressure conditions. There are a lot of works on the structural phase transition under high pressure of CdI 2 -type layered-structure compounds. Bridgman studied the different phases of GeI 2 under pressure, and he did not determined its ‘‘the second phase” clearly. Beck [9] and Hiue et al. [10] have showed that the possibility of pressure induced phase transition from less dense AB 2 -type compounds of CaCl 2 , CdCl 2 and CdI 2 (C6) structures to the denser PbCl 2 , CaF 2 and SrBr 2 structures, so we have chosen these structures as a most probable phases to investigate the phase transition pressure of GeI 2 . There are only a limited number of experimental studies [11– 16] of the growth method and characterization of GeI 2 due to its sensitivity to air and the concurrent formation of GeI 4 [11]. In an early work, the high pressure induced phase transitions of GeI 2 was investigated by Bridgman [12]. Also, the thermodynamic properties of germanium-iodide compounds (gaseous and solid) were investigated by Zelenina et al. [13], experimentally. Some theoretical investigations were reported by Benedek and Frey [17] and references therein on the lattice dynamical behaviors of the CdI 2 -type of compounds, except GeI 2 itself. To our knowledge, electronic, elastic, lattices dynamical, and thermodynamical prop- erties, which are the important bulk properties for solids, have not been considered theoretically for this compound so far. In the present paper, we aim to complete these lacks of GeI 2 compound in detail and interpret the salient results of our calculations. The method of calculation is given in Section 2; the results are dis- cussed in Section 3. Finally, the summary and conclusion are given in Section 4. 2. Method of calculation The most probable six phases (C6, C22, C19, C35, C1, and C22) of GeI 2 are considered energetically and the detailed calculations have been made only on the most stable one, C6. The Ge ion is the center of D 3d symmetry in C6 structure [17], and the C6 lattice possesses a layered sandwich structure with a single molecule per unit cell. In this structure, a layer consists of a plane of Germanium atoms sandwiched between two planes of iodine atoms. Wyckoff positions of Ge and I are at (0, 0, 0) (1a) and (1/3, 2/3, ±z) (2d), respectively. All calculations have been carried out using the Vienna ab initio simulation package (VASP) [18–21] based on the density-functional 0927-0256/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2010.08.026 ⇑ Corresponding author. Tel./fax: +90 382 2801226. E-mail address: hacioz@aksaray.edu.tr (H. Ozisik). Computational Materials Science 50 (2010) 349–355 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci