University of Groningen Lithium trapping by excess oxygen in WO3 Wijs, G.A. de; Groot, R.A. de Published in: Physical Review B DOI: 10.1103/PhysRevB.62.1508 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2000 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Wijs, G. A. D., & Groot, R. A. D. (2000). Lithium trapping by excess oxygen in WO3: A first-principles study. Physical Review B, 62(3). DOI: 10.1103/PhysRevB.62.1508 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 31-07-2017 PHYSICAL REVIEW B VOLUME 62, NUMBER 3 15 JULY 2000-I Lithium trapping by excess oxygen in WO3 : A first-principles study G. A. de Wijs1 and R. A. de Groot1,2 1 Electronic Structure of Materials, Research Institute of Materials, Faculty of Sciences, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands 2 Laboratory of Chemical Physics, Materials Science Center, Rijksuniversiteit Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands 共Received 9 March 2000兲 The process of lithium trapping by excess oxygen atoms in both crystalline and amorphous WO3 is studied by first-principles calculations. In both materials, the excess oxygen is incorporated in the bonding network by a peroxide-type bond. In both c-WO3 and a-WO3 , breaking of this bond makes oxygen states available for the accommodation of the Li electrons. However, only in a-WO3 is this mechanism accompanied by a strong reduction in total energy as this material has the flexibility to accommodate 共incorporate兲 the extra O2⫺ ion. I. INTRODUCTION In electrochromic devices, the optical properties are modulated by a reversible insertion of small ions and chargecompensating electrons. Ideally, a change in, e.g., the optical density, is apparent for 共and proportional to兲 any nonzero fraction x of ions inserted. However, nonideal behavior is known to occur frequently. An important aspect of this nonideal behavior is the irreversible uptake, i.e., trapping, of Li 共or H兲 atoms. Several experimental studies have reported on this phenomenon in amorphous films of the ‘‘archetypical’’ electrochromic material WO3 .1 In these studies 共repeated兲 cycles of insertion and extraction of lithium 共or H兲 were carried out. For sputter-deposited films, sizable amounts of lithium2–5 or hydrogen6 were found to intercalate irreversibly, without electrochromic coloration. The nature of the trapping center is still under debate, and depends on the film deposition method. For sputter-deposited films, a possible candidate has been strongly bonded water 共present in the film兲 that could possibly react with Li. However, this possibility has been ruled out on the basis of infrared measurements.2,4 Other suggestions are singly coordinated oxygen atoms,7 peroxotype links 共W-O-O-W兲,2 and the presence of gaseous oxygen.2 In a recent study, Vink et al. have determined experimentally that lithium trapping is associated with the excess oxygen content y of the film (WO3⫹y ).5 In their films, all irreversibly intercalated lithium was trapped in the first coloration/bleaching cycle. Moreover, they showed that the amount x of irreversibly inserted lithium is twice the excess oxygen content. In this paper, we study the potential of excess oxygen to act as a trapping site for lithium, in both crystalline (c-WO3 ) and amorphous (a-WO3 ) tungsten trioxide, using firstprinciples simulation.8 The next section gives the computational details. Sections III A and III B present the results for excess oxygen in c-WO3 and a-WO3 , respectively. Concluding remarks are in Sec. IV. II. COMPUTATIONAL DETAILS The calculations have been performed using the ab initio total-energy and molecular-dynamics program VASP 共Vi0163-1829/2000/62共3兲/1508共4兲/$15.00 PRB 62 enna ab initio simulation program兲 developed at the Institut für Theoretische Physik of the Technische Universität Wien.9–11 Electron-ion interactions were described using Vanderbilt ultrasoft pseudopotentials 共USPP兲 共Ref. 12兲 with a frozen 关 Xe兴 4 f 14 and 1s 2 core for W and O, respectively, and using a norm-conserving pseudopotential with 关He兴 core for Li,13 including nonlinear core corrections for W and Li.14 For efficiency, the real-space projection scheme15 for the nonlocal part of the pseudopotentials was used. Calculations were done in the local-density approximation 共LDA兲 using the parametrization by Perdew and Zunger16 of the Ceperley and Alder functional.17 The preference for LDA over calculations in the generalized gradient approximation 共GGA兲 was motivated by the better agreement with the experimental structural parameters obtained with the former for c-WO3 .19 Structural optimizations were carried out with a 2⫻2⫻2 Monkhorst-Pack mesh,18 while total energies and densities of states were calculated with a 3⫻3⫻3 mesh. Further details concerning the electronic structure part of the calculations can be found in Refs. 19 and 20. As a starting point for the calculations on c-WO3 , the optimized LDA structure from Ref. 19 was used. For a-WO3 , the starting point was an amorphous LDA structure from Ref. 20. This stoichiometric, amorphous structure contains eight formula units in a cubic, periodically repeated, supercell. A sample with excess oxygen was made by adding an extra oxygen to the amorphous structure from Ref. 20 共fixed volume兲, followed by molecular dynamics at very high temperature, and an abrupt quench 共a procedure similar to that of Ref. 20兲. For further details, we refer to Ref. 20. III. RESULTS A. c-WO3 As a starting point, we take the optimized structure of the monoclinic room-temperature modification of c-WO3 at the LDA equilibrium volume.19 Its density of states 共DOS兲 is shown in Fig. 1共a兲. It is a semiconductor with a LDA gap of 1.1 eV. Monoclinic c-WO3 共Ref. 21兲 has a distorted perov1508 ©2000 The American Physical Society PRB 62 BRIEF REPORTS FIG. 1. Total density of states for c-WO3 and c-WO3 with extra atoms added. 共a兲 WO3 , 共b兲 W8 O25 , 共c兲 Li2 •W8 O25 , and 共d兲 Li2 O•W8 O24 . skite structure ABO3 with W at the A position inside the octahedron and the B site left empty. The unit cell contains eight oxygen octahedra, which are linked at the corners, in a 2⫻2⫻2 arrangement. The tungsten atoms are located slightly off center in the octahedra. The shape of the octahedra deviates a bit from the perfect octahedron, and they are somewhat tilted relative to each other. One unit cell (W8 O24) is taken as the periodically repeated unit. In all subsequent calculations, the positional parameters are allowed to relax, whereas the cell shape is fixed. Oxygen excess is mimicked by the addition of an extra oxygen atom (W8 O25). The extra atom was introduced in the center of one of the voids in between the octahedra 共a B site兲 and left to find a favorable binding site. The relaxed structure is shown in Fig. 2共a兲. The extra oxygen has moved close to one of the tungsten atoms (d W-O⫽1.93 Å兲. It has formed a bond with one of the oxygens from the surrounding octahedron (d O-O⫽1.44 Å兲. This situation is quite comparable with peroxide bonds as they occur in other materials 共e.g., in crystalline LiO, the O-O distance is 1.51 Å兲.22 The DOS pertaining to this structure is shown in Fig. 1共b兲. An occupied state of oxygen O-p character is found at the top of the valence band, the 共empty兲 antibonding state mixes with the conduction band. Thus, by forming the peroxidelike bond, the oxygen atom prevents hole formation in the valence band. Also note the 7 eV bonding–antibonding splitting of the oxygen 2s states 共for the LiO crystal, we obtain a comparable large splitting of 5.5 eV兲. FIG. 2. Crystalline structures with extra atoms added. Tungsten, oxygen, and lithium are white, light gray, and dark spheres, respectively. 共a兲 W8 O25 , 共b兲 Li2 •W8 O25 , the Li ions are approximately at two neighboring C sites, 共c兲 Li2 O•W8 O24 . 1509 To investigate the possibility of Li trapping, first one, and then a second Li were introduced at B-type positions near the extra O. The relaxed structure is shown in Fig. 2共b兲. The peroxide-type bond has not been broken (d O-O⫽1.45 Å兲. The Li⫹ have moved closer to several of the oxygen ions, but have failed to establish a strong bond: The distances to the extra oxygen are 1.95 and 2.37 Å , to be compared with 1.64 Å for the calculated Li-O distance in the gas-phase Li2 O molecule. Li-O bond formation is also not apparent from the electronic structure: The DOS 关Fig. 1共c兲兴 shows that the Li electrons just serve to fill the lowest conduction band 共tungsten兲 states, i.e., no oxygen states have been made available as the peroxide-type bond has remained intact. From this information, the excess O does not seem a suitable Li trapping center. This is confirmed by energetic considerations. If we compare a situation with two Li atoms at large distance and far from the excess oxygen 共modeled with two Li•W8 O24 and W8 O25) with the present cluster with all three extra atoms 共modeled by Li2 •W8 O25 and two W8 O24), the present cluster is unstable by 0.7 eV. The above estimate is a bit crude, as it comes from 共a兲 a comparison of rather small periodically repeated configurations, that 共b兲 are all infinite crystals with bandlike electronic states. Point 共a兲 might cause an underestimation of lattice relaxation effects. As the configurations are neutral themselves, we do not expect large contributions from artificial electrostatic effects. Point 共b兲 is particularly troublesome for the occupancy of the tungsten-derived conduction band states. Indeed, taking a Li2 •W8 O24 共and W8 O25) as a starting point reduces the destabilization to about 0.2 eV. Part of this problem is overcome considering the Li trapping relative to an external reservoir 共res兲 with chemical potential Li2 .23 For the trapping reaction: Li2 共 res兲 ⫹O•W8 O24→Li2 •O•W8 O24 , 共1兲 we find a formation energy of ⫺9.9 eV⫺ Li2 . The addition of Li did not cause a breaking of the oxygenoxygen bond, and therefore no holes were present in the valence band that could be filled by the extra electrons donated by Li. This mechanism, which might lead to Li trapping, could possibly have been prevented by a barrier for breaking the oxygen-oxygen bond. To test for this possibility, we carried out a minimization starting from a situation without an O-O bond where a Li2 O molecule was placed inside the WO3 crystal. The final geometry 关Fig. 2共c兲兴 does not contain an O-O bond, but the extra oxygen ion has formed a bond with one of the tungsten atoms (d W-O⫽1.75 Å兲. The Li2 O has partly decomposed and the oxygen remains close to only one Li (d Li-O⫽1.75 Å兲. The excess electrons from the Li are trapped in a dispersionless, i.e., localized, in-gap state 关Fig. 1共d兲兴. Thus, the extra oxygen has formed an O2⫺ by trapping the excess Li electrons in the in-gap state. Energetically, this configuration is about 0.2 eV more favorable, i.e., the formation energy of the final configuration from O•W8 O24 and a reservoir of Li2 is ⫺10.1 eV⫺ Li2 . It is still unstable against decomposition in the solid by ⬃0.5 eV. BRIEF REPORTS 1510 FIG. 3. Amorphous structures with extra atoms added. Atom symbols as in Fig. 2. 共a兲 W8 O25 , the distance from O2 to the tungsten atom with the dot is 2.31 Å , 共b兲 Li2 •W8 O25 , and 共c兲 Li2 O•W8 O24 . B. a-WO3 In the model of the amorphous structure, the extra oxygen also gives rise to a peroxide bond 关 d O-O⫽1.43 Å, see Fig. 3共a兲兴. The two oxygen atoms involved share the same, single, tungsten neighbor (d O1⫺W⫽1.89 Å, d O2⫺W⫽2.00 Å兲 and one of them comes close to another tungsten ion 关 d O2⫺W⫽2.31 Å, see Fig. 3共a兲兴. Thus, in a fashion similar to c-WO3 , hole formation in the valence band is again prevented 关see Fig. 4共a兲兴. Li intercalation was modeled, like for c-WO3 , by addition of one 共followed by a second兲 Li atom close to the peroxide species, and a relaxation of the atomic coordinates. The bonding network was not much affected, the major change being the breaking of an O-W bond and formation of another 共with the same W兲. However, none of these atoms are participating in 共or very close to兲 the peroxidelike species. The peroxide-type bond itself was not broken 关Fig. 3共b兲兴 and the Li valence electrons were just donated into a conduction band state 关Fig. 4共b兲兴. The energy released by formation of the complex 共with lithium taken from a Li2 reservoir兲 is E form⫽⫺9.5 eV⫺ Li2 . A rough estimate gives Li2 ⫽⫺10.1 eV,24 and thus a positive formation energy E form⫽0.6 eV. Just as for the crystal, the possible presence of a barrier for peroxide decomposition, or a possible unfavorable start- FIG. 4. Total density of states for a-WO3 with extra atoms added. 共a兲 W8 O25 , 共b兲 Li2 •W8 O25 , and 共c兲 Li2 O•W8 O24 . PRB 62 ing configuration for the lithium atoms, may have prevented trapping of the extra 共Li兲 electrons by the oxygens. To test for this possibility, we 共a兲 removed one of the peroxide oxygens 共O1兲, 共b兲 relaxed the resulting geometry, 共c兲 added a Li2 O species in the void near the original peroxide location, and 共d兲 relaxed this structure. The last step was carried out in two small steps. First only the positions of the added Li2 O were relaxed, later all atoms were allowed to relax. The fully relaxed result is shown in Fig. 3共c兲. Relaxation in step 共b兲 did not much affect the geometry. However, in step 共d兲 the peroxide bond had not restored again. The oxygen of the added Li2 O is accommodated as a terminal oxygen 共single W-O bond, to another W as initially, d O2⫺W⫽1.78 Å兲 and its distance to the Li atoms is increased (d O-Li⫽1.92, 1.95 Å, d Li-Li⫽3.09 Å兲. The other oxygen from the peroxide has remained in almost the same bonding configuration (d W⫺O1 ⫽1.80, 1.99 Å兲. Energetically, a much different picture emerges 共as for c-WO3 ). At the intermediate stage of only relaxing the Li2 O in the frozen host material, the formation energy is still quite high: E form⫽⫺8.6 eV⫺ Li2 . However, here the extra oxygen 共O2兲 has not found its way yet to a tungsten neighbor. Its 2p states lie above and in the gap and its 2s state more than 2 eV above the other oxygens’ 2s states. Full relaxation 关Fig. 3共c兲兴, including the host material, brings the formation energy down dramatically: E form⫽ ⫺14.1 eV⫺ Li2 , i.e., relative to the Li2 reservoir the complex is stable by 4 eV. Compared to the initial configuration W8 O25 , several W-O bonds have been broken and others formed. The electronic structure 关Fig. 4共c兲兴 shows no special features related to any special oxygen atom, contrary to the behavior of the crystal. IV. CONCLUSIONS The study of an extra oxygen atom in c-WO3 did not provide a plausible mechanism of lithium trapping. Our calculations provide a strong indication that a Li2 •O complex is energetically not favored inside the crystalline host material. Breaking of the O-O bond leads to a donation of the lithium electrons in oxygen-derived states. However, this mechanism does not provide a sufficiently strong driving force for trapping as the total energy reduction is too small. Moreover, the electrons are not trapped in a conduction band state, but remain in an in-gap state. In a-WO3 lithium trapping at the extra oxygen atom seems very well possible. If the peroxide-type bond is broken, the system falls into a well of ⬃4 eV, i.e., this price has to be paid in order to move the Li⫹ ions and their valence electrons away from the extra oxygen into a stoichiometric part of a-WO3 . We found that relaxations of the host material are very important: if only the added Li2 O is allowed to relax inside the frozen host material, the well seems absent. Moreover, the relaxation of the host material allows for a complete incorporation of the extra oxygen 共dressed with the Li valence electrons兲 into the amorphous host: it cannot be distinguished from the other oxygen ions. In c-WO3 , on the contrary, the features of the extra oxygen still stand out clearly in, e.g., the DOS. In conclusion, comparing the behavior of the extra oxy- PRB 62 BRIEF REPORTS gen atom in c-WO3 and a-WO3 , we think that the key element of the Li trapping mechanism is the adaptability of the amorphous host, which allows for a ‘‘natural’’ incorporation of the extra oxygen ion. As a result, we expect that Li trapping in crystals occurs at defect structures, such as, e.g., shear planes 共than a different mechanism applies, see, e.g., Ref. 25兲. 1 C. G. 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