Proton Insertion in Polycrystalline WO 3 Studied with Electron Spectroscopy and Semiempirical...

Proton Insertion in Polycrystalline WO3

Studied with Electron Spectroscopy and

Semi-empirical Calculations

A. Henningsson,1 A. Stashans,2 A. Sandell,1 H. Rensmo,1 S. Sodergren,3

H. Lindstrom,4 L. Vayssieres,4 A. Hagfeldt,4 S. Lunell2 and H. Siegbahn1

1Department of Physics, Uppsala University, P.O. Box 530, S-751 21 Uppsala, Sweden2Department of Quantum Chemistry, Uppsala University, P.O. Box 518, S-751 20

Uppsala, Sweden3Gammadata Scienta AB, P.O. Box 15120, S-750 15 Uppsala, Sweden

4Department of Physical Chemistry, Uppsala University, P.O. Box 579, S-751 23

Uppsala, Sweden

Dedicated to Professor Osvaldo Goscinski on the occasion of his 65th birthday.

AbstractProton insertion in polycrystalline tungsten oxide has been studied with photoelectron

spectroscopy and semi-empirical calculations. The W 4f spectra show growth of W5þ and

W4þ oxidation states as a function of inserted proton content in the crystal lattice. A

concomitant growth of bandgap structure displays two separate peaks. Semi-empirical

calculations using the periodic large unit cell (LUC) method suggest the growth of W4þ states

based on the total energy variation with respect to the number of protons/unit cell. At this

stage the general picture of insertion is that of a multiphase system containing regions of

different x values.

Contents

1. Introduction 24

2. Experimental methods 25

3. Theoretical methods 26

4. Results 27

4.1. Electron spectroscopy 27

4.2. Calculations 30

4.2.1. Geometry of monoclinic WO3 with two H atoms 30

4.2.2. W5þ and W4þ centers in H-inserted WO3 31

5. Discussion 32

6. Conclusions 35

Acknowledgements 35

References 35

ADVANCES IN QUANTUM CHEMISTRY, VOLUME 47 q 2004 Elsevier Inc.ISSN: 0065-3276 DOI 10.1016/S0065-3276(04)47002-6 All rights reserved

1. INTRODUCTION

Tungsten oxide (WO3) was the first discovered electrochromic material [1]and has been by far the most extensively studied one. WO3 films are oftenprepared by evaporation or sputtering, but many electrochemical andchemical techniques are also utilized [2]. For example, depositions, usingsolutions of WCl6 dissolved in ethanol, N,N-dimethylformamide or2-propanol in the presence of air, have been reported [3–8]. The WO3

films were crystalline if the substrate temperature was higher than 300 8Cduring deposition. The crystal structure of WO3 is of perovskite type, butsome atomic displacements and rotations of WO6-octahedra normally occurso that, depending on the temperature, tetragonal, orthorhombic, monoclinic,or triclinic symmetries are found. In the present paper, we have preparedmonoclinic WO3 films by a simple deposition technique using WCl6solutions.

When ions are inserted into WO3, both X-ray photoelectron spectra andelectron spin resonance show that W5þ as well as W6þ are present, and thatthe W5þ density scales with the intensity of the electrochromic absorptionpeak [2,9]. The picture that emerges is that the inserted electrons arelocalized at tungsten sites, where they participate in processes responsiblefor the electrochromism. The insertion/extraction process can then berepresented, schematically, by

xHþ þ WO3 þ xe2 $ HxWO3 ð1Þ

Interestingly, cyclic voltammograms (CVs) of monoclinic films prepared bychemical vapour deposition (CVD) [10] showed clear peaks both in thecathodic and anodic sweep directions, indicating different electrochemicalprocesses in specific voltage ranges. It was discussed [10] that these featurescould be due to Hþ from two sources: adsorbed on the oxide surface and inthe bulk of the electrolyte. Alternatively, at least two redox waves in theCV can be interpreted by a two-step oxidation of WO3, from W6þ to W5þ

and then further to a W4þ state. The existence of W4þ states during protoninsertion in evaporated amorphous WO3 films for x . 0:3 has beendiscussed by Ottermann et al. [11].

In the present chapter, we have studied monoclinic polycrystalline WO3

films during proton insertion using electron spectroscopy. Several workers[9,11–15] have used electron spectroscopy to investigate both electro-chromic tungsten oxides and tungsten bronzes. Features in the W 4f andvalence level photoemission spectra show pronounced similarities for thedifferent systems, but the detailed interpretations are still not clearlyresolved. In particular, the relation between the observed spectral changesdue to ion insertion and the electrochemical conditions has not been fullyinvestigated. The purpose of the present study is, firstly, to connect observed

A. Henningsson et al.24

peaks in the CVs of these films with the electron spectra and, secondly, basedon theoretical models using semi-empirical methods, interpret the results.

2. EXPERIMENTAL METHODS

Photoelectron spectroscopy (PES) was performed at the Swedish NationalSynchrotron Radiation Laboratory, MAX lab. The beamline and thespectrometer are unique in construction since all three phases of matter(gas, liquid, and solid) can be studied ([16] and references therein). This ismade possible by means of efficient differential pumping of the analysischamber of the instrument. To the existing spectrometer we have developedan electrochemical preparation technique where the electrochemistry isperformed in a specially designed preparation chamber attached to theanalysis chamber of the spectrometer. Thus, all electrochemistry isperformed inside the vacuum system of the spectrometer. There are severaladvantages with this technique. First, the electrochemistry is performed in acontrolled atmosphere without any exposure to air. Second, the surface isanalyzed within minutes after the electrochemical reaction. Third, the sameelectrode is analyzed at the same spot for the different electrochemicaltreatments. The device used for the electrochemical preparations is describedin detail elsewhere [17].

The electrodes were prepared from solutions of 0.6 M WCl6 in ethanol.Conducting SnO2 glass substrates were spin coated with the WCl6 solution,and the film turns dark blue within a few seconds. WCl6 decomposes to WO3

at 60 8C in the presence of air. Heating the electrode with a heat gun in air at400 8C for 30 min forms a polycrystalline monoclinic WO3 film, asdetermined by X-ray diffraction. The films were visually transparent afterthe heat treatment. The WCl6 solution was prepared with dry ethanol (3 Amolecular sieves) at 220 8C. Under these conditions a clear, yellowishsolution is obtained. If the solvent was not dry or the preparation of thesolution was made at room temperature, a dark blue solution, with someprecipitation, was obtained. From blue solutions it was not possible to formreproducible films (cf. Ref. [5]).

All electrochemical preparations were made in argon atmosphere at roomtemperature in the preparation chamber of the spectrometer using an EcoChemie PGSTAT10. The electrolyte was 50 mM H2SO4. The electrode wasrinsed in water after each electrochemical modification (before each surfaceanalysis). During surface analysis the electrolyte and the rinse bath werestored in a closed compartment in argon atmosphere.

Spectra were recorded with photon energies of 157 eV in the W 4f regionand with 80 eV in the valence region. The experimental (photon width andspectrometer resolution) contribution to the energy resolution in the W 4fspectra was 150 meV and in the valence region 100 meV.

Proton Insertion in Polycrystalline WO3 25

3. THEORETICAL METHODS

A quantum-chemical intermediate neglect of differential overlap (INDO)method [18], modified for crystals calculations [19,20], has been used tostudy theoretically a H-inserted WO3 crystal. The space group of WO3 isknown to be C5

2hðP21=aÞ; and the symmetry of WO3, considered at roomtemperature, is monoclinic, pseudocubic. The W atoms are distorted fromtheir crystallographic sites in the following way: W atoms arranged parallelto the a and b axes are displaced along the c axis in an alternate sense, but Watoms arranged parallel to the c axis show only small displacements. As aresult, the W–O bonds form zigzag chains along the three crystallographicaxes. The bond lengths are between 1.86 and 1.98 A (along the a direction),1.76 and 2.17 A (along the b direction) and 1.83 and 2.12 A (along the cdirection). In the b and c directions the bonds are alternately long and shortwhile they are similar in length in the a direction. To reproduce such acomplex structure, we have to include 32 atoms in the unit cell of WO3 anduse a semi-empirical approach, in which some numerical parameters areused to simplify the ab initio Hartree–Fock formalism, thus reducingconsiderably the computational time. The parametrization scheme for WO3

is described in detail in Ref. [21]. Here, we shall only note that thecomputations are carried out using the CLUSTERD computer code [20] in a(non-relativistic) valence basis set of tungsten 6s, 6p, and 5d and oxygen 2sand 2p atomic orbitals (AOs). By fitting our results to the main properties ofthe electronic band structure of the perfect WO3 as well as to the structuralparameters of the crystalline lattice of monoclinic WO3, the parameters areanticipated to partially incorporate relativistic effects, which are sizable intungsten (see Ref. [21] for more details).

We use a periodic supercell model based on the large unit cell (LUC)method [22] which is free from the limitations of different cluster modelsapplicable mainly to ionic solids, e.g., alkali halides. The main computationalequations for calculating the total energy of the crystal within the frameworkof the LUC have been given in Refs. [22–24]. Here, we shall outline somekey elements of the method. The basic idea of the LUC is in computing theelectronic structure of the unit cell extended in a special manner at k ¼ 0 inthe reduced Brillouin zone (BZ), which is equivalent to a band structurecalculation at those BZ k points, which transform to the reduced BZ center onextending the unit cell [22]. The total energy of the crystal is

ELUC ¼ 1

2

XA–B

ZAZB

R00AB

þ 1

2

XA[LUC

EAC þ

Xocc

j

1jð0Þ

þ 1

2

Xm;n[LUC

rm;nð0ÞQmn ð2Þ


where 1jðkÞ are the eigenvalues of Fock’s matrix, occ is the number ofoccupied electronic states in the system, and rm;nðkÞ are the density-matrixelements on the basis of Bloch’s combinations of AOs,

EAC ¼ ZA

Xl–0

XB[LUC

ZB

R0lAB

2Xm[B

rmmð0ÞðnA0Þllmm

$ %ð3Þ

Qmn ¼ T00mm 2

XA[LUC

ZAðyA0Þ00mm þ

Xl–0

H0lmm; if m ¼ n ð4Þ

Qmn ¼X

l

H0lmn; if m – n ð5Þ

where T00mm are the matrix elements of the kinetic energy operator, ðyA0Þll

mm arethose of the electron–core interaction operator, R0l

AB is the distance betweenthe cores of atoms A and B, and ZA and ZB are charges of these cores.

The total energy equation (2) is obtained after the introduction of the‘cut-off function’,

vðrmnÞ ¼P0lmn

rmnð0Þð6Þ

which is possible due to the fast decrease of the overlap integral S0lmn with the

interatomic distance rmn: We can also note that the Coulomb interaction istreated quite well in the LUC model due to the theory of special k-points [25].

The theoretical method described above has been used successfully ininvestigations of various perfect and defective oxide crystals (see Refs. [24,26–29] and references therein) as well as a number of ionic crystals anddifferent semiconductors.

4. RESULTS

4.1. Electron spectroscopy

Figure 1 shows a CV of a polycrystalline monoclinic WO3 electrode in50 mM H2SO4, at a scan rate of 1 mV/s, obtained in the electrochemicalpreparation stage described above. There are two reduction peaks, at 20.10and 20.34 V vs. saturated calomel electrode (SCE), respectively. The twopeaks are characteristic of polycrystalline WO3 [30–32]. The W 4f lines andthe bandgap states for different electrochemical treatments are shown inFig. 2a,b. Depending on electrochemical treatment the W 4f core levelspectrum undergoes substantial changes. For a freshly prepared electrode,the spin–orbit split W 4f5/2, W 4f7/2 doublet of WO3 completely dominates


the spectral appearance. These peaks are characteristic of WO3 with formalvalence W6þ. The asymmetry at lower binding energy of the W 4f core leveland the peaks appearing in the bandgap indicate that the electrode is notcompletely oxidized.

Inserting protons at 20.10 V for 3 min (the position of the first redox peakin Fig. 1) turns the electrode green. The effect of proton insertion is clearlyseen in the photoelectron spectrum, a low binding energy shoulder appears inthe W 4f core level region (Fig. 2a). The new state, referred to as W5þ, isshifted 21.27 eV relative to W6þ, and has a relative intensity of 0.22 witha full width half maximum (FWHM) of 0.91 eV. We also observe that theW6þ state increases in FWHM to 1.27 eV. The intensity of the state in thebandgap region has also increased.

Further insertion, at 20.30 V (the beginning of the second reduction peakin the CV), turns the electrode dark blue. At this potential the formation of asecond low binding energy shoulder may be observed in W 4f spectrum. Thisstate is shifted 22.51 eV relative to W6þ, with a relative intensity of 0.05,and a FWHM of only 0.68 eV. At the same time there is an increase in thewidth of the W5þ state to 1.27 eV, whereas the W6þ state is unaffected,1.28 eV. The relative intensity of the W5þ state has increased to 0.32. Withincreasing insertion the intensities of the bandgap states also increase.Furthermore, at this potential there is a change in the structure of the bandgapstates, with an additional peak appearing at 0.15 eV.

Insertion at 20.45 V, beyond the second reduction peak, results in anelectrode with a metallic appearance. As can be seen in Fig. 2, the W 4f corelevel spectrum at this stage of insertion is very complicated. In contrast, thebandgap state features (Fig. 2b) only increase in intensity, while the structureremains similar to that observed at 20.30 V. It was not possible to fit the

–80

–60

–40

–20

0

20

40

Cur

rent

[nA

/cm

2 ]

–0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3Potential vs SCE [V]

Fig. 1. Cyclic voltammogram of polycrystalline monoclinic WO3.


core level spectrum with the same set of parameters used for the otherspectra.

Extraction at þ0.30 V turns the electrode transparent. As seen in Fig. 2,the electrode returns to the original W6þ state as expected from a reversibleinsertion reaction. The cycled electrode seems to be more oxidized sincethe low binding energy states in the core level spectrum and the bandgapstates are now almost absent. The width of the W 4f core levels of anelectrochemically reoxidized electrode is now 0.95 eV, considerably lessthan for an ion-inserted electrode.

For all curve fittings we have constrained the line profiles to Gaussianswith an intensity ratio of 0.74 and a spin–orbit splitting of 2.14 eV betweenthe W 4f5/2 and W 4f7/2 components. With increasing insertion there is also abuild up of a tail in the higher binding energy region. To obtain reasonablecurve fits, an additional satellite doublet has been added to include this tail.The results of the curve fits are given in Table 1.

In general, the main component W 4f core level shifts slightly to higherbinding energy with increasing insertion. This is probably to be associatedwith a corresponding shift of the Fermi level due to the filling of the bandgapstates associated with the insertion.

Fig. 2. (a) W 4f core levels for different electrochemical treatments. (b) Bandgapstates for different electrochemical treatments.


4.2. Calculations

4.2.1. Geometry of monoclinic WO3 with two H atoms

Periodic supercells with 32, 64 (2 £ 1 £ 1 extended), and 128 (2 £ 1 £ 1extended) atoms were used to study proton insertion in the monoclinicWO3 crystal. It was shown previously [21] that, when the hydrogen isintroduced into the material, it binds to one of the oxygens forming ahydroxyl unit. In the present study, we inserted two H atoms and investigatedtwo different spatial configurations (Fig. 3): (i) two hydrogens, H(1) andH(2), located in different structural voids and bound to two differentoxygens, and (ii) two hydrogens, H(2) and H(3), located in the samestructural void and bound to two oxygens situated at opposite sites of thisvoid.

The automated geometry optimization procedure, based on the downhillsimplex method [33] incorporated into the computer code, was applied tofind the total energy minimum in both configurations. As a result, theenergetically most favorable configuration was found to be the H(1)–H(2)pair. The 32-atom supercell with two hydrogens at positions 1 and 2 has1.5 eV lower energy compared to the case of the H(2)–H(3) pair. Theinteratomic distance between the H and O atoms in the hydroxyl units is0.99 A, which is close to the corresponding distance in the free molecule,0.97 A.

Table 1. Results from the curve fitting of the W 4f levels. The potentials given arefor different electrochemical treatments. At þ0.30 V the electrode is electro-chemically oxidized to WO3

Potential þ0.30 V 20.10 V 20.30 V

W6þ Relative intensity 1 0.73 0.54FWHM (eV) 0.95 1.23 1.24

W5þ Shifta (eV) 21.23 21.28Relative intensity 0.23 0.34FWHM (eV) 0.93 1.31

W4þ Shift (eV) 22.52Relative intensity 0.05FWHM (eV) 0.66

Satellite Shift (eV) 1.09 1.15Relative intensity 0.04 0.07FWHM (eV) 0.85 1.09

FWHM, full width half maximum.a Relative to W6þ.


4.2.2. W5þ and W4þ centers in H-inserted WO3

When the hydrogens are inserted into the supercell of WO3 host atoms, theirvalence electrons transfer to one or two different W atoms. In the first case, aW4þ center is formed and the multiplicity of the system is equal to 1 (singlet)while in the second case, two W5þ centers are formed and the multiplicity ofthe system is equal to 3 (triplet). This pattern is well illustrated by the netatomic charges given in Table 2 for both cases. In calculations of both thesinglet and triplet cases, for different size supercells containing two H atoms, itwas found that in the larger supercells the triplet state has a lower total energy,while in the smaller supercell the singlet is energetically more favorable.

Our computer simulations of different size supercells correspond to themodeling of different impurity concentrations in the material due to theperiodicity of our model. Thus, this implies that W5þ centers, whichcorrespond to our triplet state, are formed in H-inserted WO3 at lowconcentrations of hydrogen. When the hydrogen concentration exceeds acertain value (see Fig. 4) one should expect the formation of W4þ centers.From Fig. 4, the formation of W4þ centers begins when the hydrogen totungsten ratio is about 1/10.

W O H

1

1 1 2

2

2

33

44

3

Fig. 3. 32-atom supercell of monoclinic WO3 crystal with two H atoms placed atdifferent positions. The supercell with H(1)–H(2) pair was found to be theenergetically most favorable one, it has lower total energy by 1.5 eV compared tothe supercell with the H(2)–H(3) pair.


5. DISCUSSION

The present experimental data show similarities with those of previousworkers [12,34] but new features appear, which we will discuss in the light ofthe results from the calculations. Considering first the valence level spectra(Fig. 2b), the growth of bandgap peaks with respect to inserted protonconcentration involves two distinct peaks instead of the singular featurespreviously observed. Moreover, the appearance of both peaks may be

Table 2. The charges on atoms (in elementary charges) in H-inserted WO3,32-atom supercell. Negligible changes of the atomic charges were found in the caseswhen the two H atoms are inserted into 64-atom and 128-atom supercells. Thenumeration of the atoms corresponds to that in Fig. 4. Bold numbers refer to atomiccharges for W4 þ (singlet) and WS þ (triplet) states

Perfect WO3 Singlet Triplet

H(1) – 0.63 0.58H(2) – 0.67 0.68W(1) 4.84 2.90 3.86W(2) 4.79 4.78 4.76W(3) 4.78 4.57 4.69W(4) 4.81 4.78 3.83O(1) 21.59 21.47 21.45O(2) 21.61 21.50 21.51O(3) 21.64 21.60 21.63O(4) 21.53 21.47 21.48

0.05 0.10 0.15

x

0.00

E, eV

0.0

1.0

–1.0

Fig. 4. The pattern of formation of W5þ and W4þ centers in H-inserted WO3 crystal.Solid and broken curves denote the energetical preference of W4þ and W5þ centersin different size supercells, respectively. E denotes the difference of supercell’s totalenergies for singlet (W4þ center) and triplet (W5þ center) cases for the equivalenthydrogen concentration x: The ‘zero’ level of the energy corresponds to thehypothetical case when the formation of W5þ and W4þ centers is energeticallyequivalent.


directly associated with the onset of maxima in the CV (Fig. 1). We interpretthis behavior in terms of the existence of localized states within the bandgap.The localized character of the states is reasonable in a non-conductive regionof low x values for the ion-inserted material. The calculations imply thatlocalization occurs to the W sites of the lattice creating either W5þ or (athigher x values) W4þ states. The detailed nature of the localization should,however, be regarded with some caution since the structure of the materialallows at least two possible W sites, with high probability for each insertedproton. Such a proton, according to the calculation, is bound to an oxygenatom which in turn connects to two tungsten atoms. Thus, it is reasonable toexpect a rather substantial sensitivity of the localization of the bandgapelectron with respect to the bonding geometry of the proton to the oxygen.Inspection of the intensity at the Fermi level (Fig. 2b) shows that atan electrode potential of 20.3 V the density of states (DOS) at this energyis still very small, while at 20.45 V it has become significant. This reflectsthe general dominance of localized states below the former electrode poten-tials and the growing importance of itinerant, delocalized electronsabove 20.3 V.

Concomitant with the spectral development in the bandgap, the W 4flevels undergo substantial changes as detailed above. At 20.1 V, the extralow-binding energy component appears, which we interpret as due to W5þ

states and which we associate with the bandgap state at a binding energyof 0.5 eV.

At an insertion potential of 20.3 V, an additional peak appears, shiftedeven further towards lower binding energy (cf. Fig. 2a). A peak at thisposition has been observed also by previous workers in the tetragonalhydrogen bronzes [12]. Several interpretations have been suggested [12]involving the coexistence of different crystal structures, the formation ofOH2 states or the creation of W4þ. The calculations performed here providestrong evidence for the last interpretation in terms of the growth of W4þ

states above a certain x value. This growth might be intermixed with theincreasing importance of itinerant states, leading finally to a conducting stateof the material. These are most likely the conditions leading to thecomplicated spectrum observed for insertion potentials exceeding20.45 V. Here, the final state (dynamic) effects are probably increasinglyimportant and will significantly distort the distribution among differentvalence states from expectations based on initial state considerations. Atthese high x values, it is also highly likely that significant structural changesoccur.

The general picture that emerges from the observations in the core andvalence spectra for varying degrees of proton insertion is that of a multiphasesystem, containing regions of different x values. Thus, three different valencestates coexist at high average x values. The observed broadening of the W 4fpeaks upon insertion corroborates this view.


These mixed-valence systems have been discussed by several authors withrespect to final state effects in their core level spectra [12,35]. A distinction isgenerally made between the cases where the extra electrons occur as itinerantconduction band electrons (metallic case) or whether they are completelylocalized to single sites. In the former case, the core ionization of one sitewill in itself lead to the creation of localized levels (by pulling down from theconduction band), whose occupancy in the final state of the ionizationprocess will depend statistically on the conduction electron density. Thus,the final state localized level may be either filled or empty. The net result is aloss of direct correlation between observed relative peak intensities and thenumber of inserted electrons per formula unit (x value), since the populationof the two possible core-hole states is considered to be entirely a final statephenomenon. On the other hand, for the case of complete single-sitelocalization (non-metallic case) the relative intensities are expected to trulyrepresent the relative number of the two possible valence states beforeionization and will not be affected by effects due to the final state of the coreionization process.

In the present spectra, we cannot exclude an intermediate case between thetwo extremes discussed above. On the one hand, there is evidence oflocalized character of the bandgap states in our spectra. However, at highinsertion potentials the core level spectra display final state effects in that therelative intensities of the core level peaks differ from those expected fromestimated x values. It should be realized that localization per se does notexclude final state effects in the core level spectrum. In fact, we have notedabove that localization in this case may well involve two tungsten centers,which provides sufficient grounds to expect significant final state effects.This character of the initial state of the core ionization process leads to asituation akin to those encountered in molecular cases, such as for exampleadsorbed molecules on metallic substrates. The general behavior may betreated within a common framework, involving the pulling down of a localatomic level on the core-ionized site. Model treatments [36–40] show that,depending on the interaction strength between the participating atomicorbitals, the core ionization may lead to concomitant interatomic chargetransfer resulting in a fully relaxed final state. In this case, core ionization ofa (W6þ–OH–W5þ) insertion site may lead to a W 4f peak at the low bindingenergy position corresponding to the W5þ peak in either of two ways: bydirect ionization of the W5þ site or by ionization of the W6þ site plus acharge transfer from the W5þ site. The conditions for the latter process tooccur are favorable in this case in view of the near degeneracy of thetwo local states on each of the tungsten sites. The general effect on theW 4f spectrum will thus be a net transfer of intensity from the W6þ peakover to the W5þ peak leading to a deviation from the ratio expected from thex value.


6. CONCLUSIONS

We have investigated monoclinic films of WO3 under different conditionsof electrochemical insertion of protons using electron spectroscopy.The valence and core level data are interpreted in terms of the existenceat low x values of localized W5þ states. At intermediate overall x values, theelectron spectra coupled with semi-empirical calculations using the periodicLUC method suggest the additional formation of W4þ states.

The general picture at this stage of insertion is that of a multiphase systemcontaining regions of different x values. Still higher x values lead to ametallic state of the material, where structural changes and final state effectsbecome important for the appearance of the core level spectrum.

ACKNOWLEDGEMENTS

The present work was supported by the Swedish Research Council (VR) andthe Foundation for Strategic Research.

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Proton Insertion in Polycrystalline WO 3 Studied with Electron Spectroscopy and Semiempirical...

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Transcript of Proton Insertion in Polycrystalline WO 3 Studied with Electron Spectroscopy and Semiempirical...