First-principles calculations on structural, magnetic and electronic properties of oxygen doped BiF3

First-principles calculations on structural, magnetic and electronic properties of oxygen doped BiF3

Computational Materials Science 50 (2011) 3131–3135 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www...

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Computational Materials Science 50 (2011) 3131–3135

Contents lists available at ScienceDirect

Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci

First-principles calculations on structural, magnetic and electronic properties of oxygen doped BiF3 Zhenhua Yang a,b,c, Xianyou Wang a,b,c,⇑, Li Liu a,b,c, Shunyi Yang b,c, Xuping Su c a Faculty of Materials, Optoelectronics and Physics, Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China b School of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China c Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University, Xiangtan 411105, Hunan, China

a r t i c l e

i n f o

Article history: Received 23 April 2011 Received in revised form 25 May 2011 Accepted 26 May 2011 Available online 29 June 2011 Keywords: First-principles calculations O doping Electronic structures

a b s t r a c t First-principles calculations based on density function theory within the generalized gradient approximation (GGA) have been carried out to investigate the effects of O doping on the structural, magnetic and electronic properties of BiF3. Based on the calculated cohesive energies, O impurities prefer substituting F atom at tetrahedral sites (0.25, 0.25, 0.25). And the geometry of BiF3 changes little due to similar radius of O and F atoms. By analyzing density of states (DOS) of Bi4OF11 (II), it has been found that Bi4OF11 (II) presents magnetic character and half-metallic state, implying its potential applications in Li-ion batteries. Finally, the character of bond in Bi4OF11 (II) was discussed by analysis of charge density and bader charge. The result shows that O doping weakens ionic bond in BiF3. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Li-ion batteries have been widely used in portable electronics due to their high energy density and power density [1–3]. To date, intercalation materials such as LiCoO2 and spinel-LiMn2O4 have been commercially used [4–6]. Although these cathode materials provide excellent cycling performance, their capacities are limited. What is more, these intercalation materials bring about the problem of safety because they can release oxygen gas and react with organic solvent at elevated temperature. To prepare Li-ion batteries with higher performance, metal fluorides based on conversion reactions have been considered as cathode materials, gradually entering a prominent place of energy storage and conversion. The reversible conversion reaction can be expressed as follows: þ

nLi þ ne þ MeFn () nLiF þ Me where Me is metal, such as Fe, Co, Ni, and Bi. Opposite to the intercalation reaction, all the oxidation states of the metal can be utilized in the reversible conversion reaction, which will provide a large theoretical energy density. Moreover, the metal fluorides can provide high operating voltage due to their high iconicity and electro-negativity between Me–F [7–10]. In the recent years, BiF3 has been an attractive cathode material due to its high operation voltage ⇑ Corresponding author at: School of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang). 0927-0256/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2011.05.040

(3.21 V vs. Li/Li+) and considerable gravimetric energy density (969 W h/kg) [11]. However, owing to the character of large band gap, the BiF3 suffers from poor electronic conductivity. Therefore, it could not be used as the cathode materials of recharge lithium battery. In general, the poor electronic conductivity of BiF3 can be improved by adding conductive agent or introducing more covalent bonds into the structures. Recently, BiF3 carbon metal fluoride nanocomposites (CMFNC) have been fabricated by the high energy milling [12,13]. Besides, it has also reported that nanocomposites of BiF3 and MoS2 can form mixed conducting matrix (MCM) and represent better conductivity than the pure BiF3 [14]. The use of nanosized materials can reduce the electron path length and nanosized BiF3 easily embeds into conductive carbon matrix, which both contribute to improvement of conductivity of BiF3. However, it cannot sustain good cycling performance due to the large band gap. Since the ionic radii of oxygen and fluoride are very similar (1.32 Å vs. 1.33 Å), O atom can be substituted for the F atom in the BiF3 to reduce band gap between bismuth cation and fluoride anion. It has experimentally been found that bismuth oxyfluorides could be an appealing cathode material, combining the high voltage of fluorides with the high electrochemical activity of the oxides [15]. Since O doping in BiF3 will apparently influence its electrochemical properties, it will be a significant work to investigate theoretically the mechanism of O doping in BiF3. However, to the best of our knowledge, no theoretical study on O doping in BiF3 was reported. In this paper, the mechanism of O doping in BiF3 will be investigated by the first-principles calculations, which may offer some theoretical guidance for future experiments.

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Fig. 1. The prototype structures of oxygen doped BiF3 compounds Bi4OF11 and Bi3OF12: (a) pure BiF3; (b) Bi4OF11 (I), the site of oxygen is (0.5, 0.5, 0.5); (c) Bi4OF11 (II), the site of oxygen is (0.25, 0.25, 0.25); (d) Bi3OF12 (I), the site of oxygen is (0, 0, 0); (d) Bi3OF12 (II), the site of oxygen is (0, 0.5, 0.5). The biggest ball is Bi atom, the medium F and the smallest O.

2. Computational method A super-cell consists of 1  1  1 BiF3 was built. The calculations have been performed using the ab-initio total-energy and molecular-dynamics program Vienna ab initio simulation program (VASP) developed at the institut für Materialphysik of the Universität Wien [16–19]. The projector-augmented wave (PAW) was used. Perdew– Burke–Ernzerhof parameterization of the generalized gradient approximation (GGA) was employed for treating the exchange–correlation potential. In all calculations, the cut-off energy (Ecut) was set 550 eV. For the geometry optimization of the pure and doped BiF3, Brillouin zone integration was adopted with the k-point set 4  4  4 grids, and finer k-point set 12  12  12 grids were employed for calculating the electronic properties of pure and doped BiF3. The atomic geometries are not fully optimized using a Quasi Newton (QN) algorithm until the Hellmann–Feynman forces on each ion are converged to less than 0.01 eV/Å.

the cubic structure of BiF3, Bi atoms are closely packed and the F atoms occupy all octahedral and tetrahedral interstitial sites [20]. We first calculate pure BiF3 with a = 5.786 Å and the calculated value is in good agreement with the experimental data (a = 5.861 Å

3. Results and discussion 3.1. Structural properties and stability of doped BiF3 Pure BiF3 is a cubic structure with Fm-43 m space group (number 225). The structure model of cubic BiF3 is depicted in Fig. 1a. In Table 1 Summary of total energies including the nonmagnetic state (Enon total ) and magnetic state non mag (Emag total ) and corresponding calculated cohesive energies (Ecoh and Ecoh ). Compound

Enon total (eV)

(eV) Emag total

Enon coh (eV/atom)

(eV/atom) Emag coh

Bi4OF11 Bi4OF11 Bi3OF12 Bi3OF12

72.4176 73.3313 60.5401 60.5325

72.6763 73.4033 62.3706 62.3812

3.7091 3.7662 3.0304 3.0299

3.7252 3.7707 3.1448 3.1454

(I) (II) (I) (II)

Table 2 The calculated equilibrium constants a (Å), length of Bi–F bonds LBi–F (Å), length of Bi–O bonds (Å) and volume (Å3). Compound

a (Å)

LBi–F (Å)

LBi–O (Å)

Volume (Å3)

BiF3 Bi4OF11 (II)

5.786 5.811

2.505 2.752 2.478

– 2.392

193.712 196.211

Fig. 2. The TDOS and PDOS: (a) BiF3; (b) Bi4OF11 (II). The Fermi level is indicated by the dashed line.

Z. Yang et al. / Computational Materials Science 50 (2011) 3131–3135

[20]) with the error of 1.28%, indicating that the calculation model and parameters are reasonable. Then, in order to figure out the favorable site of O doping in BiF3, the different models of O-doped BiF3 were considered, in which one Bi atom or one F atom was replaced by one O atom. And the corresponding models are Bi4OF11 (I), Bi4OF11 (II), Bi3OF12 (I), and Bi3OF12 (II), respectively (seen in Fig. 1b–e). For Bi4OF11 (I) and Bi4OF11 (II), one F atom was substituted by one O atom located at (0.5, 0.5, 0.5) and (0.25, 0.25, 0.25), respectively. While for Bi3OF12 (I) and Bi3OF12 (II), one Bi atom was replaced by one O atom located at (0, 0, 0) or (0, 0.5, 0.5), respectively. We calculated the total energy and cohesive energy arisen from oxygen substituting for the F site or Bi site. The cohesive energy of O-doped BiF3 can be defined as follows:

Ecoh ¼

1 F ðEtotal  xEOatom  yEBi atom  zEatom Þ xþyþz

F where Etotal is the energy of O-doped BiF3. EOatom ; EBi atom , and Eatom are the energies of O, Bi and F atoms in freedom states, respectively. Integers x, y and z are the number of atoms of M (O, Bi and F). If the cohesive energy is negative, it means that the doped system is in a stable phase. The total energies and cohesive energies of different doping conditions in both magnetic and nonmagnetic states were

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obtained and listed in Table 1. It is interesting to note that the total energies of doped BiF3 in magnetic state are lower than those in nonmagnetic state, which suggests that BiF3 in magnetic state is more stable than that in nonmagnetic state. Moreover, the total energies of Bi3OF12 (I) and Bi3OF12 (II) are 62.3706 eV and 62.3812 eV, respectively. It reveals that O substituting site (0, 0.5, 0.5) can lead to a more stable case. On the other hand, the total energies of Bi4OF11 (I) and Bi4OF11 (II) are 72.6763 eV and 73.4033 eV, respectively. It means that O substituting the F atom located at the tetrahedral site (0.25, 0.25, 0.25) is more favorable than that at the octahedral site (0.5, 0.5, 0.5). Besides, it can be observed that the cohesive energy of system with O substituting for F-site is much smaller than that for Bi-site, from the viewpoint of cohesive energy, the lower cohesive energy of the O substitution at the F site is more energetically favorable. To further study the effect of O doping on the structural properties of BiF3, we compared the structural parameters of Bi4OF11 (II) with pure BiF3 (seen in Table 2). It can be found that the equilibrium constants and volume of BiF3 slightly increase. It can be explained as follows. Ionic radius of O2 and covalent radius of O are 1.32 Å and 0.73 Å, respectively, while ionic radius of F and covalent radius of F are 1.33 Å and 0.72 Å, respectively. And the component of covalent bond between Bi–O is larger than that between Bi–F. In general, because ionic radius and covalent radius of

Fig. 3. Contour of charge density of (a) BiF3 and (b) Bi4OF11(II) (contour line interval is 0.05 e/Å3).

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O and F are close to each other, the volume of super-cell changes little. 3.2. Electronic structure and magnetic properties The total and partial densities of states (TDOS and PDOS) of pure BiF3 and Bi4OF11 (II) have been analyzed to investigate the electronic structure. Fig. 2 displays the spin-polarized TDOS and PDOS of pure BiF3 and Bi4OF11 (II). The spin direction (" and ;) is expressed as the O spin (spin up " and spin down ;). Generally speaking, it can be clearly observed that the spin up and spin down bands are symmetric in the TDOS and PDOS of pure BiF3, while nonsymmetrical spin up and spin down bands appear in the TDOS and PDOS of Bi4OF11 (II). The results indicate that BiF3 exhibits no magnetic character while Bi4OF11 (II) magnetic character, which is well consistent with structural properties of BiF3 and Bi4OF11 (II) mentioned above. The calculated total magnetic moments of the Bi4OF11 (II) is 0.904 lB. And the corresponding magnetic moments of the O and its nearest neighboring Bi and F atoms are 0.384, 0.006 and 0.001 lB, respectively. Hence, it can be clearly seen that the total magnetic moment of Bi4OF11 (II) is mainly contributed by the doped O atom. Besides, the electronic structure can be obtained. From Fig. 2a, the peaks between 6 eV and Fermi level mainly originate from the contribution from the electrons of F 2p orbits and from a small quantity of Bi 6p orbits. The conduction bands consist of Bi-6p and F-2p orbits and the calculated band gap is 3.94 eV, which is close to previous report that the band gap of BiF3 is 3.81 eV [21]. For the Bi4OF11 (II) (seen in Fig. 2b), the peaks between 6 eV and Fermi level also mainly originate from the contribution of the electrons of Bi-6p and F-2p orbitals, but the O 2p orbital appears, which induces the spin splitting impurity states and F-2p above the valence band. Thus, Fermi level passes through the spin down impurity states of Bi4OF11 (II). On the other hand, in the spin up state, it is also observed that the Fermi level is located in the band gap (the top of the valence band and the bottom of the conduction band are 0.22 eV and 3.71 eV, respectively). These results indicate that Bi4OF11 (II) exhibits half-metallic state and the band gap is 0.2 eV, suggesting that the conductivity of BiF3 can be remarkably improved by O doping. Moreover, in comparison to the pure BiF3, the density of states of O-doped BiF3 becomes dispersed, indicating local character of electron in BiF3 is weakened after O doping. To further study the electronic structure, Fig. 3 depicts the contour of the charge density of the pure BiF3 and Bi4OF11 (II). For the pure BiF3, the lower part of charge density around F atom is polarized slightly toward the nearest Bi atoms. Bi atoms lose almost all their valence electrons and are almost fully ionized as Bi3+. And thus, stronger ionic bond between F and Bi atom can be observed. After BiF3 is doped by O, the charge density at the midpoint of Bi–O bond is slightly more than that of the Bi–F bond, that is to say, O doping in BiF3 weakens the ionic bond, which is the result of lower electro-negativity of O than that of F. Besides, in order to investigate the quantities of the charge transfer between the Bi, F and O

atoms, we also calculated effective (Bader) charges [22–25] of these two compounds and the calculated results were given in Table 3. From Table 3, it can be seen that about 2.25 electrons transfer from each Bi atom to its neighboring F atoms in BiF3 and small difference in charge appears in the F basins depending on their position in the lattice, which indicates that the bonds are mainly ionic type. With the doping of O, each Bi atom transfers 2.20 electrons and each F atom obtains 0.72 or 0.73 electrons, which is slightly smaller than the value of electrons transferring Bi to F atom (0.74 or 0.78), indicating that O doping weakens the ionic bond between Bi and F atom. Besides, each O atom obtains 0.91 electrons from Bi atom, which suggests that the O–Bi bonds are the mixed ionic and covalent bond character. 4. Conclusions In conclusion, we have carried out first-principles calculations to investigate the structural, magnetic, and electronic properties of oxygen doped BiF3. The results reveal that O substitution at the tetrahedral site of F is energetically more favorable in the case of O-doped BiF3. The cell volume of Bi4OF11 (II) expands 1.29% relative to BiF3. Bi4OF11 (II) exhibits magnetic character due to O doping and its total magnetic moment is 0.904 lB. The calculated magnetic moment in Bi4OF11 (II) mainly arises from O atom with a little of contribution from the nearest-neighboring O and Bi atoms. Furthermore, BiF3 exhibits the character of insulator, and its calculated band gap is 3.94 eV. Besides, Bi4OF11 (II) reveals half-metallic ground states and the band gap is 0.22 eV. Therefore, it can be conclusionally found that O doping can effectively improve the conductivity of BiF3. At the same time, comparing with pure BiF3, the component of covalent bond in Bi4OF11 (II) increases, thus O doping will weaken the ionic bond between Bi and F atoms in BiF3. Acknowledgements This work is financially supported by National Natural Science Foundation of China (Grant No. 20871101) and Scientific Research Fund of Hunan Provincial Education Department (Grant No. 09C945). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Table 3 Bader effective atomic charges of BiF3 and Bi4OF11 (II).

[11]

Compound

Atoms

Atomic coordination

Bader charges (e)

[12]

BiF3

Bi F(1) F(2)

4a (0, 0, 0) 4b (0.5, 0.5, 0.5) 8c (0.25, 0.25, 0.25)

+2.25 0.78 0.74

[13]

Bi4OF11(II)

Bi F(1) F(2) F(3) O

4a (0, 0, 0) 4b (0.5, 0.5, 0.5) 8c (0.75, 0.75, 0.75) 8c (0.75, 0.25, 0.75) 8c (0.25, 0.25, 0.25)

+2.20 0.69 0.72 0.73 0.91

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