- Email: [email protected]
Interactions of uranium atom with tetraketone complexes
Chemical Physics Letters 415 (2005) 243–245 www.elsevier.com/locate/cplett
Interactions of uranium atom with tetraketone complexes Q. Sun a
a,*
, Q. Wang a, Y. Shiokawa b, Y. Kawazoe
b
Physics Department, Virginia Commonwealth University, 1020 West Main Street, Richmond, VA 23284, United States b Institute for Material Research, Tohoku University, Sendai 980-8577, Japan Received 18 July 2005; in final form 26 August 2005 Available online 26 September 2005
Abstract Using first principles theory based on density functional formulation we have studied the interactions of uranium atom with tetraketone complexes used for all-uranium redox flow battery. The geometry and electronic structures are studied in detail. We found that uranium atom interacts strongly with tetraketone complexes, the interaction energy is more than 7.0 eV, and uranium atom acts as an electron acceptor with about 0.6 electrons transferred from tetraketone complexes, while uranium atom carries the magnetic moment of 2 lB. 2005 Elsevier B.V. All rights reserved.
Recently, the research on new energy resources becomes a hotly pursued field in energy science and environmental science. This is motivated partly by the fact that conventional petroleum-based fuels like gasoline or diesel, as well as natural gas and coal, are limited and harmful to environment. Because these conventional fuels contain carbon, when burned, their carbon recombines with oxygen from the air to form carbon dioxide (CO2), the primary greenhouse gas that causes global warming. Being one of the new energy resources, battery has received much attention. In batteries, electrical energy is generated by conversion of chemical energy via redox reactions at the anode and cathode. To improve the performance, the researchers in ShiokawaÕs group have systematically synthesized and characterized uranium complexes for uranium redox flow battery [1–3]. Two tetraketone ligands, which possess two monomer acetylacetone moieties, were synthesized as shown in Fig. 1. From physics point of view, some basic questions are not yet answered by experiments, such as: (1) How strong is the interaction between U atom and tetraketone molecules? (2) How is the geometry changed by the interaction? (3) How many electrons are transferred between U atom *
Corresponding author. E-mail address: [email protected] (Q. Sun).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.006
and molecules? (4) What is the magnetic moment carried by U atom in the complex? In this Letter, we present the answers to these questions by density functional calculations. Spin-polarized calculations of total energies and forces, and optimizations of geometry were carried out using a plane-wave basis set with the projector augmented plane wave (PAW) method [4] as implemented in the Vienna ab initio simulation package (VASP) [5]. The particular advantage of the PAW method over the ultrasoft pseudopotentials is that it can improve accuracy especially in those cases (such as d- and f-element) where the overlap between the valence- and core-charge densities and hence the nonlinearity of the core-valence exchange are important. In the PAW approach, charge and spin densities are decomposed into pseudodensities and compensation densities which account for the difference between the pseudodensities and all-electron densities. The pseudodensities consist of a smooth part expressed in a plane-wave representation and localized augmentation charges that account for the violation of norm conservation. The structure optimization is symmetry unrestricted and uses conjugate-gradient algorithm. The exchange-correlation PW91 functional is used ˚ vacuum spaces [6]. We have used super-cells with 15 A along x, y, and z directions for all the calculated structures. The A point is used to represent the Brillouin zone due to
244
Q. Sun et al. / Chemical Physics Letters 415 (2005) 243–245
Fig. 1. Tetraketone-1 (a) and tetraketone-2 (b). Fig. 3. Optimized geometries without (a) and with (b) U atom in tetraketone-2. Table 1 Interaction energy E (eV), magnetic moment M (lB), charge transfer Q (in electrons), and the average bond length R between U–O, C–O, C–C and ˚) C–H (A E
M
RU–O
RC–O
RC–C
RC–H
Keto-1
7.86
2.0
0.561
2.298
Keto-2
7.38
2.0
0.571
2.202
1.392 (1.292) 1.360 (1.224)
1.511 (1.524) 1.476 (1.481)
1.114 (1.101) 1.102 (1.098)
Q
The numbers in parentheses are for the bond length before the insertion of U atom.
that U atom becomes negatively charged in tetraketone complexes, 0.561 electrons are transferred to U atom in tetraketone-1 and 0.578 electrons to U atom in tetraketone-2. In Table 1, we summarized these data.
15
Total DOS (arb.units)
a
EF
spin-up spin-down
10
5
0 -12 15
-10
-8
-6
b Total DOS(arb.units)
the large supercell. The energy cutoff was set to 400 eV and the convergence in energy and force were 10 4 eV and ˚ , respectively. 1 · 10 3 eV/A The optimized geometries are shown in Figs. 2 and 3 for tetraketone-1 and teraketone-2, respectively. For comparison, the structures before the insertion of U are also studied. We found that in both cases strong interactions exist between U atom and the molecules, and the structures become more compact. If we define the interaction energy E as the difference in total energy between U–molecule complex and the separate ones, i.e. E = E(U–mol) E(mol) E(U), we found that the values of E are, respectively, 7.38 and 7.86 eV for tetraketone-1 and tetraketone-2. The strong interactions change the bond length in molecules: the aver˚ (tetrakage C–O bond length changes from 1.292 to 1.392 A ˚ etone-1) and from 1.224 to 1.360 A (tetraketone-2). The average C–C bond length shrinks, changing from 1.524 to ˚ (tetraketone-1) and from 1.481 to 1.476 A ˚ (tetrake1.511 A tone-2). On the other hand, because H atoms are far from U atom, the C–H bond length changes very little. In tetraketone-1, the average C–H bond length changes from 1.101 ˚ , the corresponding values in tetraketone-2 are to 1.114 A ˚ . While the average U–O bond length in 1.098 and 1.102 A ˚, tetraketone-1 and tetraketone-2 is 2.298 and 2.202 A respectively. It is interesting to note that for an isolate U atom, the electronic configurations are [Rn]5f36d17s2, hence an U atom carries the magnetic moment of 4 lB. However, when U atom is inserted into molecules, due to the strong interactions with O atoms, the magnetic moment is quenched to 2 lB both in tetraketone-1 and tetraketone-2. Compared to the electronic configurations of isolate U atom, it is found
-4
-2
0
2
EF
spin-up spin-down
10
5
0 -12
Fig. 2. Optimized geometries without (a) and with (b) U atom in tetraketone-1.
-10
-8
-6
-4
-2
0
2
Energy (eV) Fig. 4. Total DOS for U–tetraketone-1 (a) and U–tetraketone-2 (b).
Q. Sun et al. / Chemical Physics Letters 415 (2005) 243–245
245
result in the differences in bonding and the interaction energy. In summary, detailed density functional calculations have been carried out for U–tetraketone complexes. The geometry, electronic structure and magnetic properties are studied. We found that U atom strongly interacts with tetraketone molecules, resulting in the C–O bond expansion and the C–C bond shrink. The U–tetraketone complex is magnetic and chemically active for further growth. These results provide some basic understandings for the physics involved in active materials for all-uranium redox flow battery. Fig. 5. Charge distributions in tetraketone-1 (a) and tetraketone-2 (b).
Fig. 4 shows the total density of states (DOS). The Fermi level is shifted to 0.0 as indicated with the dotted lines. We can see that due to the spin-polarization in the complexes, the spin-down DOS is shifted up in energy resulting in asymmetric distribution between spin-up and spin-down. This gives rise to the net magnetic moments in the complexes. Moreover, these two complexes show high density of states at Fermi level, suggesting that these complexes are chemically active, more molecules can be added to these complexes being able to form super-structure. The charge density distributions are given in Fig. 5, from which we can see some difference of bonding between U–tetraketone-1 and U–tetraketone-2. U atom interacts with the molecules mainly through O atoms. There are five O atoms in tetraketone-1 and four in tetraketone-2 with the average ˚ , respectively. These U–O bond length of 2.298 and 2.202 A
Acknowledgments The authors thank the crew of the Center for Computational Materials Science, the Institute for Materials Research, and Tohoku University, Japan for their continuous support of the HITAC SR8000 supercomputing facility. References [1] Y. Shiokawa, H. Yamana, H. Moriyama, J. Nucl. Sci. Technol. 37 (2000) 253. [2] T. Yamamura, Y. Shiokawa, H. Yamana, H. Moriyama, Electrochim. Acta 48 (2002) 43. [3] T. Yamamura, K. Shirasaki, Y. Shiokawa, Y. Nakamura, S.-Y. Kim, J. Alloys Compd. 374 (2004) 349. [4] P.E. Bloechl, Phys. Rev. B 50 (1994) 17953. [5] G. Kresse, J. Heffner, Phys. Rev. B 54 (1996) 11169. [6] Y. Wang, J.P. Perdew, Phys. Rev. B 44 (1991) 13298.
Interactions of uranium atom with tetraketone complexes Q. Sun a
a,*
, Q. Wang a, Y. Shiokawa b, Y. Kawazoe
b
Physics Department, Virginia Commonwealth University, 1020 West Main Street, Richmond, VA 23284, United States b Institute for Material Research, Tohoku University, Sendai 980-8577, Japan Received 18 July 2005; in final form 26 August 2005 Available online 26 September 2005
Abstract Using first principles theory based on density functional formulation we have studied the interactions of uranium atom with tetraketone complexes used for all-uranium redox flow battery. The geometry and electronic structures are studied in detail. We found that uranium atom interacts strongly with tetraketone complexes, the interaction energy is more than 7.0 eV, and uranium atom acts as an electron acceptor with about 0.6 electrons transferred from tetraketone complexes, while uranium atom carries the magnetic moment of 2 lB. 2005 Elsevier B.V. All rights reserved.
Recently, the research on new energy resources becomes a hotly pursued field in energy science and environmental science. This is motivated partly by the fact that conventional petroleum-based fuels like gasoline or diesel, as well as natural gas and coal, are limited and harmful to environment. Because these conventional fuels contain carbon, when burned, their carbon recombines with oxygen from the air to form carbon dioxide (CO2), the primary greenhouse gas that causes global warming. Being one of the new energy resources, battery has received much attention. In batteries, electrical energy is generated by conversion of chemical energy via redox reactions at the anode and cathode. To improve the performance, the researchers in ShiokawaÕs group have systematically synthesized and characterized uranium complexes for uranium redox flow battery [1–3]. Two tetraketone ligands, which possess two monomer acetylacetone moieties, were synthesized as shown in Fig. 1. From physics point of view, some basic questions are not yet answered by experiments, such as: (1) How strong is the interaction between U atom and tetraketone molecules? (2) How is the geometry changed by the interaction? (3) How many electrons are transferred between U atom *
Corresponding author. E-mail address: [email protected] (Q. Sun).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.006
and molecules? (4) What is the magnetic moment carried by U atom in the complex? In this Letter, we present the answers to these questions by density functional calculations. Spin-polarized calculations of total energies and forces, and optimizations of geometry were carried out using a plane-wave basis set with the projector augmented plane wave (PAW) method [4] as implemented in the Vienna ab initio simulation package (VASP) [5]. The particular advantage of the PAW method over the ultrasoft pseudopotentials is that it can improve accuracy especially in those cases (such as d- and f-element) where the overlap between the valence- and core-charge densities and hence the nonlinearity of the core-valence exchange are important. In the PAW approach, charge and spin densities are decomposed into pseudodensities and compensation densities which account for the difference between the pseudodensities and all-electron densities. The pseudodensities consist of a smooth part expressed in a plane-wave representation and localized augmentation charges that account for the violation of norm conservation. The structure optimization is symmetry unrestricted and uses conjugate-gradient algorithm. The exchange-correlation PW91 functional is used ˚ vacuum spaces [6]. We have used super-cells with 15 A along x, y, and z directions for all the calculated structures. The A point is used to represent the Brillouin zone due to
244
Q. Sun et al. / Chemical Physics Letters 415 (2005) 243–245
Fig. 1. Tetraketone-1 (a) and tetraketone-2 (b). Fig. 3. Optimized geometries without (a) and with (b) U atom in tetraketone-2. Table 1 Interaction energy E (eV), magnetic moment M (lB), charge transfer Q (in electrons), and the average bond length R between U–O, C–O, C–C and ˚) C–H (A E
M
RU–O
RC–O
RC–C
RC–H
Keto-1
7.86
2.0
0.561
2.298
Keto-2
7.38
2.0
0.571
2.202
1.392 (1.292) 1.360 (1.224)
1.511 (1.524) 1.476 (1.481)
1.114 (1.101) 1.102 (1.098)
Q
The numbers in parentheses are for the bond length before the insertion of U atom.
that U atom becomes negatively charged in tetraketone complexes, 0.561 electrons are transferred to U atom in tetraketone-1 and 0.578 electrons to U atom in tetraketone-2. In Table 1, we summarized these data.
15
Total DOS (arb.units)
a
EF
spin-up spin-down
10
5
0 -12 15
-10
-8
-6
b Total DOS(arb.units)
the large supercell. The energy cutoff was set to 400 eV and the convergence in energy and force were 10 4 eV and ˚ , respectively. 1 · 10 3 eV/A The optimized geometries are shown in Figs. 2 and 3 for tetraketone-1 and teraketone-2, respectively. For comparison, the structures before the insertion of U are also studied. We found that in both cases strong interactions exist between U atom and the molecules, and the structures become more compact. If we define the interaction energy E as the difference in total energy between U–molecule complex and the separate ones, i.e. E = E(U–mol) E(mol) E(U), we found that the values of E are, respectively, 7.38 and 7.86 eV for tetraketone-1 and tetraketone-2. The strong interactions change the bond length in molecules: the aver˚ (tetrakage C–O bond length changes from 1.292 to 1.392 A ˚ etone-1) and from 1.224 to 1.360 A (tetraketone-2). The average C–C bond length shrinks, changing from 1.524 to ˚ (tetraketone-1) and from 1.481 to 1.476 A ˚ (tetrake1.511 A tone-2). On the other hand, because H atoms are far from U atom, the C–H bond length changes very little. In tetraketone-1, the average C–H bond length changes from 1.101 ˚ , the corresponding values in tetraketone-2 are to 1.114 A ˚ . While the average U–O bond length in 1.098 and 1.102 A ˚, tetraketone-1 and tetraketone-2 is 2.298 and 2.202 A respectively. It is interesting to note that for an isolate U atom, the electronic configurations are [Rn]5f36d17s2, hence an U atom carries the magnetic moment of 4 lB. However, when U atom is inserted into molecules, due to the strong interactions with O atoms, the magnetic moment is quenched to 2 lB both in tetraketone-1 and tetraketone-2. Compared to the electronic configurations of isolate U atom, it is found
-4
-2
0
2
EF
spin-up spin-down
10
5
0 -12
Fig. 2. Optimized geometries without (a) and with (b) U atom in tetraketone-1.
-10
-8
-6
-4
-2
0
2
Energy (eV) Fig. 4. Total DOS for U–tetraketone-1 (a) and U–tetraketone-2 (b).
Q. Sun et al. / Chemical Physics Letters 415 (2005) 243–245
245
result in the differences in bonding and the interaction energy. In summary, detailed density functional calculations have been carried out for U–tetraketone complexes. The geometry, electronic structure and magnetic properties are studied. We found that U atom strongly interacts with tetraketone molecules, resulting in the C–O bond expansion and the C–C bond shrink. The U–tetraketone complex is magnetic and chemically active for further growth. These results provide some basic understandings for the physics involved in active materials for all-uranium redox flow battery. Fig. 5. Charge distributions in tetraketone-1 (a) and tetraketone-2 (b).
Fig. 4 shows the total density of states (DOS). The Fermi level is shifted to 0.0 as indicated with the dotted lines. We can see that due to the spin-polarization in the complexes, the spin-down DOS is shifted up in energy resulting in asymmetric distribution between spin-up and spin-down. This gives rise to the net magnetic moments in the complexes. Moreover, these two complexes show high density of states at Fermi level, suggesting that these complexes are chemically active, more molecules can be added to these complexes being able to form super-structure. The charge density distributions are given in Fig. 5, from which we can see some difference of bonding between U–tetraketone-1 and U–tetraketone-2. U atom interacts with the molecules mainly through O atoms. There are five O atoms in tetraketone-1 and four in tetraketone-2 with the average ˚ , respectively. These U–O bond length of 2.298 and 2.202 A
Acknowledgments The authors thank the crew of the Center for Computational Materials Science, the Institute for Materials Research, and Tohoku University, Japan for their continuous support of the HITAC SR8000 supercomputing facility. References [1] Y. Shiokawa, H. Yamana, H. Moriyama, J. Nucl. Sci. Technol. 37 (2000) 253. [2] T. Yamamura, Y. Shiokawa, H. Yamana, H. Moriyama, Electrochim. Acta 48 (2002) 43. [3] T. Yamamura, K. Shirasaki, Y. Shiokawa, Y. Nakamura, S.-Y. Kim, J. Alloys Compd. 374 (2004) 349. [4] P.E. Bloechl, Phys. Rev. B 50 (1994) 17953. [5] G. Kresse, J. Heffner, Phys. Rev. B 54 (1996) 11169. [6] Y. Wang, J.P. Perdew, Phys. Rev. B 44 (1991) 13298.