First-Principles Prediction on Geometrical and Electronic
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Journal of Physics and Chemistry of Solids 104 (2017) 56–61 Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs First-principles prediction on geometrical and electronic properties MARK of K-doped chrysene ⁎ Xiaohui Wanga,b, Guohua Zhongb,c, , Xunwang Yand, Xiaojia Chenb,e, Haiqing Linb a Department of Physics, University of Science and Technology of China, Hefei 230026, China b Beijing Computational Science Research Center, Beijing 100094, China c Shenzhen Institutes of Advance Integration Technology, Chinese Academy of Sciences, Shenzhen 518055, China d School of physics and electrical engineering, Anyang Normal University, Anyang 455000, China e Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China ARTICLE INFO ABSTRACT Keywords: The significant discovery of superconductivity in potassium (K) doped polycyclic aromatic hydrocarbons (PAHs) Chrysene provides a novel system to understand the superconducting mechanism. Here, we focus on K-doped chrysene Polycyclic aromatic hydrocarbons which is possibly a superconductor. Chrysene contains four benzene rings, however, the superconductivity Electronic structures induced by the K doping has not been discovered. Based on the first-principles calculations with the Van der Superconductor Waals functional correction, we predicted the geometrical and the electronic structures of K -doped chrysene First-principles calculations x (x=1, 2, 3 and 4). We found that the K doping results in the phase transition from C2/c to P21 symmetry. The result of the formation energies shows that K2-doped chrysene is the most stable and can easily be fabricated. K2chrysene is still a semiconductor, but K2chrysene with small charge fluctuation can behave as a metal and is thermodynamically stable. Our results provide a route to experimentally obtain the stable K-doped chrysene with metallic feature. 1. Introduction promising candidates for high Tc superconductors. It was also indi- cated that Tc of PAH superconductors increases with the number of Since the first theoretical prediction of organic superconductivity by benzene rings [10,11]. Hence, PAHs containing different number of W.A. Little in 1964 [1], much efforts have been put to look for organic benzene rings have been observed by intercalating K atoms, such as superconducting materials. In the following year, the superconductivity three, five, six and seven benzene-ring systems [7–10]. However, there was discovered in graphitic compounds doped by alkali metals [2].In is still lack of study of K-doped chrysene (consisting of four benzene 1980, the first experimental evidence of superconductivity in organic rings), KxC18H12. In 2015, G.A. Artioli et al. pointed out that Sm-doped metals named (TMTSF)2PF6 with Tc equaling 0.9 K under 1.2 GPa [3] chrysene is superconductive with Tc–5K[12]. Though this is not a K- was found. Then in 2003, the superconducting transition temperature doped superconductor, we believe that the K metal is a good dopant for was raised to 14.2 K in the BEDT-TTF salt family under 8.2 GPa [4].In PAH superconductors which had been proved in these systems of 1991, the superconductivity was also observed in the potassium-doped phenanthrene [7], picene [8], coronene [9] and 1,2:8,9-dibenzopenta- fullerene with Tc–18 K [5]. Furthermore in 2008, the Tc in cesium- cene [10]. Therefore, it is natural to investigate whether and how a doped fullerene reached to 38 K [6]. In addition to these materials, typical K-doped chrysene is superconducting and what is its electronic polycyclic aromatic hydrocarbon (PAH) is a new family of organic behavior. In this work, we will firstly examine the crystal structures of superconductor with different numbers and arrangements of benzene KxC18H12, secondly will analyze the thermodynamic stability, thirdly rings. Recently the discovery of superconductivity in PAH was reported will investigate variations of electronic properties induced by doping, by putting dopant metal atoms into the interstices of the aromatic and finally will estimate the possible superconductivity. This work can molecules. For example, potassium-doped phenanthrene [7], picene fill the vacant study of alkali-metal-doped chrysene within the frame- [8], coronene [9] and 1,2:8,9-dibenzopentacene [10], were proved work of PAH materials as well as help to understand its superconduct- experimentally superconducting with Tc as high as 4.95 K, 18 K, 15 K ing mechanism, which is well worthy for clear and detailed exploration. and 33 K, respectively. PAH compounds are light and low-dimensional, making them ⁎ Corresponding author at: Beijing Computational Science Research Center, Beijing 100094, China. E-mail address: [email protected] (G. Zhong). http://dx.doi.org/10.1016/j.jpcs.2017.01.004 Received 23 June 2016; Received in revised form 28 December 2016; Accepted 5 January 2017 Available online 05 January 2017 0022-3697/ © 2017 Elsevier Ltd. All rights reserved. X. Wang et al. Journal of Physics and Chemistry of Solids 104 (2017) 56–61 2. Computational details K atoms occupy respectively on the first and third benzene rings of chrysene molecules as shown in Fig. 1(c). As similar to K1C18H12,K For K doped chrysene, KxC18H12, we have considered different atoms also deviate from the center of the ring in K2C18H12. For x=3, doping concentrations, i.e., x=1, 2, 3 and 4, as well as fractional doping. K3C18H12, one of the three K atoms is located at the center of chrysene During the optimization, we checked various initial crystal structures molecule and the other two K atoms are moved to the ends of chrysene for each doping concentration. Finally, we selected the most stable molecules to hold respectively the first and fourth benzene rings from configuration of every doping level to analyze the thermodynamic the top view as shown in Fig. 1(d). For these three doping levels stability. In the first-principles calculation, we employed the Vienna mentioned above, all of the K atoms are in the intralayer region of Ab-initio Simulation Package (VASP) [13] based on the projector organic molecules seen from the top view. However, when four K atoms augmented wave (PAW) [14] method with a cutoff energy of 600 eV. are added for each chrysene molecule, K4C18H12, two of them are All of configurations were fully optimized using a conjugate-gradient pushed into the interlayer region of the organic molecules, as shown in algorithm. The Monkhorst-Pack k-point grids are generated according Fig. 1(e). In Table 2, we summarize the lattice parameters of the most to the specified k-point separation of 0.02 Å−1 and the convergence stable structures for each doping level. In general, the crystal volume thresholds are set as 10−6 eV in energy and 10−3 eV/Å in force. For this increases with the doping level. In order to visually comprehend the low-dimensional system, we tested several different functionals to most stable structures shown in Fig. 1, we have additionally presented investigate the influence of the dispersion interactions on the optimiza- the metastable configurations for each doping level in Supplementary tion of crystal structures, which include the local density approxima- Fig. S1 and the corresponding lattice parameters in Supplementary tion (LDA) [15], the generalized gradient approximation of Perdew- Table S1. Within the same doping level, the energy difference between Burke-Ernzerhof version (GGA-PBE) [16], and the Van der Waals the most stable and the metastable configurations are about 0.055, (vdW) density functional in the versions of vdW-DF [17] and vdW-DF2 0.011, 0.009 and 0.028 eV/f.u. for x=1, 2, 3 and 4, respectively. [18], as well as the semi-empirical DFT method of Grimme (DFT-D2) The stable configuration for every doping level can be distinguished [19]. The optimized crystal lattice parameters of pristine chrysene from the total energy, but it is not possible to determine which doping based on different functionals are summarized in Table 1. Clearly, we concentration is easy to be fabricated in experiment. Therefore we also found that the vdW-DF2 functional produces better result compared calculated the formation energy (Ef ) of doped system to analyze the with experimental one [20]. So we adopted the vdW-DF2 functional in thermodynamic stability as follows the following calculations. Ef=−−()−−(),E KCHx 18 12 E CH 18 12 xμKKK bulk x[] μ μ bulk (1) 3. Results and discussion where EKCx 18 H 12 and ECH18 12 are the total energy of the doped and host The pristine chrysene crystallizes as the C2/c (No.15) structure with solid chrysene respectively. Here x is the number of doped K atoms and four chrysene molecules in the unit cell. As shown in Fig. 1(a), the μK is the chemical potential of the K element. μK (bulk) can be obtained organic molecules form the herringbone pattern. Based on the vdW- from the energy per K atom in the K bulk metal with the bcc structure. DF2 functional, the optimized crystal lattice parameters are obtained as When μKK=(μ bulk), it means that the element is so rich that the pure 3 a=22.673 Å, b=6.063 Å, c=8.456 Å and V=1156.8 Å , which are in element phase can form. Ef <0indicates that the doped compound can good agreement with the experiment values [20]. This test verifies that exist stably. From the calculated results shown in Fig. 2, these four our method is well reliable. With the help of pristine chrysene, we kinds of doped systems considered are existent in a certain range of K intercalated K atoms into unit cell to realize the doping cases of C2/c chemical potential. However, K2C18H12 has lower formation energy symmetry.