Structural Changes in Electrochromic WO3 Thin Films ...

Journal of The Electrochemical Society, 151 共1兲 H21-H26 共2004兲

H21

0013-4651/2003/151共1兲/H21/6/$7.00 © The Electrochemical Society, Inc.

Structural Changes in Electrochromic WO3 Thin Films Induced by the First Electrochemical Cycles T. Pauporte´,a,z M. C. Bernard,b Y. Soldo-Olivier,c and R. Faurec Laboratoire d’E´lectrochimie et Chimie Analytique, UMR-CNRS 7575, Ecole Nationale Supe´rieure de Chimie, 75231 Paris cedex 05, France b Physique des Liquides et Electrochimie, UPR 15 CNRS, Universite´ Pierre et Marie Curie, 75252 Paris cedex 05, France c Laboratoire d’E´lectrochimie et Physico-Chimie des Mate´riaux et des Interfaces, UMR 5631 CNRS/INPG/UJF, 38402 Saint Martin d’He`res, France a

We have investigated the effect of the very first electrochemical cycles on the local structure and bonding for two different amorphous transparent electrochromic WO3 thin films, namely, a sputtered film and an electrodeposited film. The cycles are performed at a constant potential scan rate between 0.65 and ⫺0.55 V vs. NHE in a 0.3 M LiClO4 /propylene carbonate solution. We show that this treatment has a weak influence on the short range order in sputtered films. On the contrary, dramatic changes are found in electrodeposited ones. Both Raman spectroscopy and extended X-ray absorption fine structure 共EXAFS兲 data show that the initial electrodeposited oxide films are highly disordered with a highly asymmetric structure. The Fourier transforms of the EXAFS spectra of these films show four different W-O distances in the first oxygen shell with values ranging between 1.5 and 2.4 Å 共after phase correction兲. After the electrochemical treatment, the radial distribution of the oxygen atoms is changed. The structure becomes similar to that classically reported for amorphous WO3 films and can be fitted assuming two different main distances. Raman spectroscopy and EXAFS reveal a larger W-O distance distribution in the two different amorphous films compared to a monoclinic WO3 reference sample. They show a higher dispersion of the W-O distances in cycled electrodeposited films, compared to the cycled sputtered ones. © 2003 The Electrochemical Society. 关DOI: 10.1149/1.1631285兴 All rights reserved. Manuscript submitted February 3, 2003; revised manuscript received July 6, 2003. Available electronically December 9, 2003.

Amorphous tungsten trioxide thin films have been a topic of considerable interest for the last two decades, in particular due to their remarkable electrochromism properties.1 Evaporation and sputtering are the two main preparation methods generally used,1 but wet methods such as sol-gel2-4 or electrodeposition5-15 are also interesting routes as simple, low-cost processes. We have recently shown that films, with a high color change efficiency, can be prepared by a simplified electrodeposition method.14 The deposition mechanism involves peroxo-tungstate precursors and has been studied by electrochemical quartz crystal microbalance 共EQCM兲14 and X-ray absorption spectroscopy 共XAS兲.15 In spite of the very large literature devoted to electrochromism in WO3 , surprisingly no attention has been paid to the effect of the very first electrochemical cycles on the structure of the amorphous WO3 films. In a previous study,15 we have evidenced a surprising behavior of as-electrodeposited and as-sputtered electrochromic films when they are subjected to some electrochemical cycles between ⫹0.65 and ⫺0.55 V vs. NHE in lithium perchlorate/propylene carbonate 共PC兲 solution. During these cycles, lithium ion insertion/ extraction occurs16 WO3 共 bleach兲 ⫹ yLi⫹ ⫹ ye⫺ Liy WO3 共 blue兲

关1兴

By X-ray absorption near edge structure 共XANES兲 analysis, a marked fall of the white line height 共WLH兲 from 3.4 down to 3.0 is observed, this latter value being similar to that of crystallized monoclinic WO3 . In this work15 we have shown that, due to the high mean-free path of the photoelectrons at low energy, the XANES is sensitive to the middle-range environment of the absorber and that the decrease of the WLH is closely related to a condensation process of the WO6 octaedras, the basic structural units of WO3 . After that investigation of the structural middle range changes, we report here a further study of the effects of the first electrochemical cycles on the redox state of W in the film, on the bondings, and on the short-range structure around the W absorber. The techniques used are voltamperometry, Raman spectroscopy, and extended X-ray absorption fine structure 共EXAFS兲 analysis at the LIII edge of W.

z

E-mail: [email protected]

Experimental The amorphous electrodeposited films (ed-WO3 ) were synthesized at room temperature according to the method described elsewhere.14 The precursor deposition bath was prepared by mixing reagent grade 25 mM Na2 WO4 共Acros Organics, 99%兲 plus 25 mM H2 O2 共Prolabo, 30%兲 and acidifying at pH 1.3 by addition of perchloric acid. The deposition was generally carried out at ⫺0.25 V vs. NHE for 300 s with a freshly prepared solution. The substrates were F:SnO2 -coated glass 共Raman, cyclic voltametry兲, vitreous carbon 关Rutherford backscattering spectrometry 共RBS兲兴 or a high purity carbon substrate 共Toray Carbon Paper from E-TEK兲共EXAFS兲. During deposition the films were blue colored, but once the potential was switched off, the films underwent spontaneous reoxidation in the deposition bath and became transparent within a few seconds. The films were then carefully rinsed with Milli-Q quality water. The redox potentials of these films were measured between ⫹0.65 and ⫹0.75 V vs. NHE in a 1 M H2 SO4 electrolytic solution; they were thus fully reoxidized 共see Fig. 2a in Ref. 14兲, and free of W5⫹. For the sake of comparison, amorphous sputtered WO3 thin films (sp-WO3 ) deposited onto the same substrates have been studied in parallel. The target was made of metal tungsten, the argon partial pressure was 2 Pa, the O2 partial pressure was 2 or 3 Pa and the O2 flow 18 cm3 min⫺1. The as-prepared films were transparent and in the same redox state as the electrodeposited one since their rest potential, measured in 1 M H2 SO4 , was in the same range 共between ⫹0.65 and ⫹0.75 V vs. NHE兲. The electrochemical cycling treatment 共EC treatment兲 consisted of several potential scans at 7 or 5 mV s⫺1 between ⫹0.65 and ⫺0.55 V vs. NHE in a 0.3 M LiClO4 共Fluka 99%兲/propylene carbonate 共Fluka兲 solution (LiClO4 /PC). The first negative-going scan was started from the rest potential and the EC treatment was always stopped at ⫹0.65 V vs. NHE. Crystallized monoclinic WO3 powder (m-WO3 ), heated several hours in air at 500°C, has been used as a reference sample 共Raman and EXAFS兲. The crystalline structure of this compound was checked by X-ray diffraction 共XRD兲. For EXAFS measurements 1 wt% of m-WO3 powder was mixed and ground with BN for a long time, prior to being pressed in order to obtain a 1 mm thick pellet. All the experiments were performed in

H22


Table I. Determination by RBS and nuclear reaction of OÕW atomic ratio in different electrochromic WO3 thin films. O/W

Thicknessa 共nm兲

2.74-2.80 3.47 3.41 3.54 4.85

460 216 156 409 220

Deposition conditions Sputtering Electrodeposition at ⫺0 V vs. NHE Electrodeposition at ⫺0.25 V vs. NHE Electrodeposition at ⫺0.4 V vs. NHE by the method in Ref. 12 a

Ref. This study

13

Calculated from the number of W atoms per cm2, assuming massive films with a density of 7.2 g cm⫺3.

air. No special attention was paid to control the atmospheric composition of the experiment. The W content of the films were determined by RBS of ␣( 4 He⫹) particles at 2 MeV and the O content by direct observation of the nuclear reaction 16O共d, p) 17O. The nuclear measurements were conducted using the 2 MeV Van de Graaff accelerator of the Universities Paris 6th and 7th. The laser Raman spectrometer was a LABRAM model from Jobin-Yvon. The excitation line was 514.5 nm. The samples were placed under the objective of a microscope and the Raman beam was focused on the spectrometer slit after a 180° scattering by the sample. The investigated area was in the order of 1 ␮m2. It was verified that the films did not undergo any structural change under the laser illumination. The measurements were carried out at 10 mW for 0.5 s for m-WO3 powder and 300 s for the thin films. The spectra were recorded between 200 and 1300 cm⫺1. The film spectra were obtained by subtracting the bare substrate signal 共baseline兲 from that of the sample. The XAS spectra were acquired at the ID26 beamline of the European synchrotron radiation facility 共ESRF, Grenoble, France兲. The electron energy in the storage ring was 6 GeV with an operating current ranging between 160 and 200 mA. The monochromator was a pair of parallel flat Si关111兴 single crystals. The photon flux at the sample was higher than 1013 ph s⫺1. The spectra were recorded in the fluorescence mode with a multielement silicon drift detector, at the L3 -edge of the W absorber that is between 10 and 11.3 keV. The data analysis was performed using the SEDEM software package written by Aberdam.17 Prior to spectrum comparison, the different curves were normalized. The background was removed by fitting the pre-edge by a straight line and the postedge baseline was fitted by a cubic spline curve. The extracted EXAFS signal, ␹共k兲, k being the wave vector, was multiplied by k2 and a standard Kaiser window with parameter ␶ ⫽ 2 was applied in order to minimize truncation effects. A Fourier transform 共FT兲 of the resulting spectrum was calculated. The theoretical backscattering phase and amplitude functions were calculated by the FEFF 6.01 code18 from the WO2 structure in which the oxygen octaedra contains two different W-O distances at 1.951 Å 共four atoms兲 and 2.062 Å 共two atoms兲.19 The first peak, isolated by back-FT, corresponding to oxygen neighbors around the W absorber has been fitted with two different W-O interatomic distances and a set of structural parameters has been extracted for each distance with the help of the EXAFS formulas.20 As the basic unit of tungsten oxide compounds is a WO6 octaedra, the total number of nearest oxygen neighbors was fixed at six. This approach is quite simplified since according to Loopstra and Rietveld,21 two nonequivalent types of WO6 octaedra would be present in m-WO3 and each W-O distance in each octaedra would be different, giving a total of twelve different distances in the first shell. The three parameters, ⌫, ␩, ␤ describing the electron mean-free path were calculated by the FEFF 6.01 code.18 They were found equal to 1.14, 3.56, and 0.032 respectively. The prefactor S 20 was fixed at 0.8.

Results and Discussion We have determined the O/W atomic ratio in different films, and the results are summarized in Table I. The films deposited by sputtering are slightly oxygen deficient with a ratio ranging between 2.74 and 2.8. The oxygen deficiency in sputtered transparent films,

Figure 1. First three cyclic voltamogramms of amorphous WO3 films in 0.3 M LiClO4 -PC at a sweep rate of 5 mV s⫺1. 共a兲 Film electrodeposited at ⫺0.25 V vs. NHE, 共b兲 sputtered film.


Figure 2. Raman spectra 共a兲 m-WO3 ; sputtered film before 共b兲 and after 共c兲 EC cycling. 共Full line, experimental curve; dashed lines, Gaussian fit curves; dotted line, total fit.兲

H23

water free, has been explained by the presence of both W6⫹ and W4⫹. 22 For the films electrodeposited at two different potentials, 0 and ⫺0.25 V/NHE, the ratio is close to 3.5 and independent of both deposition potential and film thickness. This value suggests the presence of structural water in the film and WO3•1/2 H2 O as a global chemical formula. In Table I, we also report the O/W atomic ratio measured by Bernard et al.13 in films prepared by another electrodeposition method, based on tungsten metal dissolution by concentrated hydrogen peroxide for peroxo-precursor synthesis. Surprisingly, this parameter is much higher in this case and attributed to the presence of two water molecules for each WO3 共WO3•2H2O兲. By RBS measurements we have also checked that the electrodeposited films are not contaminated by Na even if this species is present in the deposition bath and can be inserted into WO3 . The lithium ion insertion properties of films prepared by sputtering and electrodeposition have been characterized by cyclic voltametry in a LiClO4 /propylene carbonate solution. The first three cycles are presented in Fig. 1. Figure 1a has been obtained with a film, ca. 200 nm in thickness, electrodeposited at ⫺0.25 V/NHE. An extra cathodic charge is observed throughout the first negative-going scan whereas the subsequent scans are reproducible and overlap. The charge in excess is measured at 14%. The same behavior is observed with a ca. 150 nm thick sputtered film, but the extra charge appears at ⫹0.15 V/NHE. The charge in excess is similar at about 13%. A likely explanation of the first negative scan behavior is a slight dissolution of the film in the organic medium. Globally, no dramatic irreversible electrochemical changes are recorded during the very first potential scans in the insertion/deinsertion potential range. The literature indicates that Raman spectroscopy has been largely used to obtain fundamental information on the vibrational properties of the W-O molecular bondings in the different tungsten oxides 共e.g. Ref. 13, 23-28兲. Figure 2a shows a typical spectrum of crystallized monoclinic WO3 . Two main strong peaks are observed at 714 and 804 cm⫺1 in agreement with literature data as shown in Table II. They are both assigned to the vibrational stretching modes of the W-O-W bridging oxygens23 and the former peak, at lower Raman wavenumber, has been shown to be sensitive to the change in crystal symmetry and distortion.20 Figure 2b shows a Raman spectra of an as-deposited sputtered film. The two previous narrow peaks are replaced by a broad featureless band centered at 760 cm⫺1. A new peak appears at 946 cm⫺1. The first band has been deconvoluted into two Gaussian curves, noted G1 and G2 , whereas the second peak has been fitted with a Gaussian curve, noted G3 . The total fit matches very satisfactorily the experimental data. The two first Gaussian curves, assigned to the vibrational stretching mode of W-O-W bridging oxygens, are centered at 681 and 789 cm⫺1 respectively. The intensity peak ratio, G2 /G1 is preserved if compared to the reference sample 共Table II兲. The energy shift is attributed to a difference in structure:

Table II. Raman characteristic frequencies „cmÀ1… of WO3 massive and thin film compounds. W ⫽ O stretching mode

W-O-W stretching mode a

Sample

EC treatment

m-WO3

¯

sp-WO3

No Cycled No Cycled ¯

ed-WO3 WO3 -1/3H2 O a

Gi-Gaussian fit curve.

a

G 1 Raman wavelength 共cm⫺1兲

G 2 Raman wavelength 共cm⫺1兲

G 2 /G 1 intensity ratio

G 3 a Raman wavelength 共cm⫺1兲

714 715 681 685 680 688 680

804 807 789 788 793 779 805

1.90

¯ ¯ 947 955 960 965 945

1.88 2.17 0.42 1.88

Ref. This study 23 This study

23

H24


Figure 4. Fourier transforms of k 2 ␹(k) spectra in the 2-14 Å range of 共a兲 m-WO3 , 共b兲 initial sp-WO3 , 共c兲 initial ed-WO3 共⫺0.25 V vs. NHE兲 and 共d兲 same film as 共c兲 after EC treatment.

Figure 3. Raman spectra of electrodeposited films. Initial film deposited at ⫺0.25 V: 共a兲 rough Raman spectrum before baseline correction, 共b兲 after baseline correction. 共c兲 ed-WO3 film after EC treatment. 共Full line, experimental curve; dashed lines; Gaussian fit curves; dotted line; total fit.兲

m-WO3 is made of octaedra layers whereas, sp-WO3 is made of tridimensionally corner-sharing octaedra.23 The lower Raman wavenumbers suggest that the mean W-O distance is lengthened. The band at 947 cm⫺1 is assigned to the stretching mode of the short W ⫽ O terminal bond23 and reveals that the film is composed of nanocrystallites made of an assembly of WO6 octaedra.24 This band is absent in the massive compound 共Fig. 2a兲. After the EC treatment, the spectrum shape is globally unchanged 共Fig. 2c兲. The full width at half maximum 共fwhm兲 of G1 and G2 are close before and after cycling at 148/146 cm⫺1 (G1 ) and 159/167 cm⫺1 (G2 ), respectively. Figure 3a shows a Raman spectrum, before baseline correction, of an electrochemically deposited film. Two bands are also found,

but the first one presents a shape remarkably asymmetric with the presence of a tail at the highest Raman wavenumbers. The presence of the W ⫽ O line at 954 cm⫺1 suggests a nanocrystalline structure of the film.24 Daniel et al.23 have reported that the presence of W-OH2 bond in WO3 hydrates gives rise to a peak centered in the 320-380 cm⫺1 range. Amazingly, in spite of the high O/W ratio determined above, and that we have interpreted as due to the presence of structural water in the film, no peak is found in the Raman spectrum in this wavenumber interval 共Fig. 3a兲. The main broad band has been deconvoluted as two Gaussian curves 共Fig. 3b兲. If the positions of these curves are preserved and are in agreement with the structure described above for the sputtered film, the change in shape is mainly due to a G2 /G1 intensity ratio inferior to 1 共Table II兲. Lee et al.28 have shown that this ratio is highly sensitive to the symmetry of the structure and lattice distortion. The structure of as-prepared ed-WO3 is thus highly asymmetric and distorted. After the EC treatment, the shape of the spectrum changes markedly and becomes similar to that of the sputtered films, suggesting a strong reorganization process 共Fig. 3c兲. The classical G2 /G1 intensity ratio is recovered 共Table II兲. The fwhm of the Gaussian fits, 157 cm⫺1 (G1 ) and 210 cm⫺1 (G2 ), are larger than those of the sputtered films before and after cycling. This reveals a higher bond distance distri-


H25

Figure 5. Comparison of the EXAFS oscillations 关 k 2 ␹(k) 兴 of an aselectrodeposited 共a兲 and an as-sputtered 共b兲 film.

bution in the cycled electrodeposited films compared to the sputtered ones. By comparing the white line heights 共WLH兲 of XANES of a large number of samples related to thin film preparation and containing W in an oxidation state of ca. ⫹VI, we have shown that the WLH parameter is closely related to the condensation degree of the W-containing species.15 Accordingly, the WLH is a good indicator of the middle range order in the film. As mentioned in the introduction, we have shown that the first electrochemical cycles produce a marked decrease of the W-LIII edge WLH15 of either sputtered or electrodeposited films. This was attributed to the condensation of the WO6 clusters. EXAFS analysis gives information on the short range order in the films, that is, the radial arrangement of the first shells of atoms around the W absorber. This analysis has been carried out on the two kinds of films before and after EC treatment. In a first step, we have computed a Fourier transform 共FT兲 of the EXAFS spectra in order to visualize a pseudo-radial distributions of the nearest neighbors. Figure 4a shows the spectrum of the monoclinic reference sample. A first peak, split into two subpeaks is found in the 0.8-2 Å distance range and is assigned to the first oxygen neighbors. With help of the literature data,26,29-32 we have also indexed the subsequent peaks 共Fig. 4a兲. Those between 2.2 and 3 Å are attributed to the multiple scatterings in the first oxygen shell, and those at 3.1-4 and 5-5.4 Å to the first and second W-W shells, respectively. Figure 4b shows the FT of the EXAFS spectrum of the asprepared sp-WO3 film. A main peak at 1.3 Å is present, due to the first W-O coordination shells contribution. This spectrum is strikingly different from that of an as-electrodeposited film 共Fig. 4c兲. In the latter, four different peaks are found in the 0.8-2 Å range. The main one, split into two parts, is flanked by two additional peaks at 1 Å 共noted A兲 and 1.9 Å 共noted B兲, respectively. Figure 5 compares the k 2 ␹(k) oscillations of the two initial films before FT filtration. Extra oscillations are clearly observed on ed-WO3 共some of them are pointed out by arrows on the figure兲. If the phase correction is considered 共evaluated at 0.5 Å from the reference compound data兲 the two extreme distances are estimated at about 1.5 Å 共peak A兲 and 2.4 Å 共peak B兲. The as-electrodeposited compound would be highly disordered with W-O distances varying in a large range. The longest distance cannot be attributed to the long W-OH2 bond observed in WO3 hydrates,23 since we have shown above that the corresponding band is not observed by Raman spectroscopy. More generally, the presence of these two additional distances are not accompanied by the appearance of new Raman bands in Fig. 3a. After the EC treat-

Figure 6. Back-FT of the spectra after filtration in the 0.86-2.02 Å interval. The experimental curves 共dashed lines兲 are fitted with two different W-O distances and assuming six to be the total number of oxygen atoms 共solid line兲. 共a兲 As-sputtered WO3 film 共k range between 3 and 11.5 Å⫺1兲; 共b兲 cycled sp-WO3 共k range between 3 and 10 Å⫺1兲 and 共c兲 cycled ed-WO3 共k range between 3 and 10 Å⫺1兲.

ment, the FT shape changes dramatically within this distance range 共Fig. 4d兲. The two additional distances disappear, and the spectrum becomes similar to the one reported in the literature for amorphous WO3 . 31 The fitting of the reference sample is analyzed in detail elsewhere,15 and we focus here on the modeling of the amorphous films. We could not extract the structural information for the aselectrodeposited film. The analysis of the large hump in the FT signal, containing four different distances, would require a large number of parameters for the fitting. Unfortunately, the maximum of the adjustable parameters as defined by Stern33 is limited to seven in the present conditions, a too-low value to describe satisfactorily the complex structure of the six nearest oxygen atoms. For the other films, the first oxygen shell contribution has been isolated in the


H26

Table III. Structural parameters of the first two W-O distances in the first shell determined by EXAFS. r is the W-O interatomic distances, n is the coordination number with n 1 ⫹ n 2 ⫽ 6 and ␴ 2 is the Debye-Waller factor. In the present analysis, the k range was 3-11.5 ÅÀ1 for m-WO3 and sp-WO3 before cycling and 3-10 for the two cycled samples. The first coordination shell was isolated between 0.86 and 2.02 Å from the FT†k 2 ␹„k…‡ spectra. Sample

Subshell

r 共Å兲

n

m-WO3

1 2 1 2 1

1.80 2.09 1.79 2.07 1.80

3.2 2.8 4.3 1.7 4.2

2 1

2.08 1.79

1.8 3.6

2

1.98

2.4

sp-WO3 sp-WO3 cycleda ed-WO3 cycleda a b

Acknowledgments

␴ 2 (⫻103 ) 共Å2兲

dE0 共eV兲

1.88

1.1 6.3 8.4 12.0 9.4

9.7 9.5 6.7 6.3 6.9

1.87

13.0 7.8

3.1 7.9

26.1

0.8

r 共Å兲

¯b

1.93 1.87

not be fitted with the two W-O distances model successfully used with the monoclinic WO3 reference sample. This former compound is observed to completely reorganize during the first electrochemical cyclings in the LiClO4 /PC medium. The structure characterized by EXAFS becomes globally similar to that of the sputtered films, even if the second W-O distance is more dispersed.

Potential scan stopped at ⫹0.65 V vs. NHE. Means W-O distance is defined as (r 1 n 1 ⫹ r 2 n 2 )/6.

0.86-2.02 Å range of the FT spectra and back-FTs have been computed. In the fitting procedure, we consider two different distances as-described in the experimental section. Figure 6 shows back-FTs signals and their relative fits for the as-deposited sp-WO3 film 共Fig. 6a兲, the EC-treated sp-WO3 共Fig. 6b兲 and the EC-treated ed-WO3 films 共Fig. 6c兲. The modeling parameters are reported in Table III. The mean W-O interatomic distance is similar for these three different samples. The Debye-Waller 共D-W兲 factor is related to the interatomic distance distribution, and it reflects the disorder in the compound. As expected, the values of this parameter are higher in amorphous films compared to monoclinic WO3 . 15,31 The cycling treatment does not change the DebyeWaller factor of sp-WO3 . The D-W factor of the second W-O distance is higher in cycled ed-WO3 compared to cycled sp-WO3 . This shows a larger distance dispersion in good agreement with the higher fwhm of the first two Gaussian fit curves observed by Raman spectroscopy and reported above. Conclusions We have studied the effect of the first electrochemical cyclings in LiClO4 /PC medium on the short range structure and bonding in amorphous transparent films of WO3 . The behavior of sputtered and electrodeposited films is very different. Raman and EXAFS analysis show that the sputtered WO3 films are poorly affected by this treatment. On the contrary, the as-prepared electrodeposited films present singular Raman and EXAFS spectra revealing a highly disordered and asymmetric structure. This latter property is seen by the inversion of the Raman peak intensity of the two W-O-W stretching modes. EXAFS spectrum of the as-electrodeposited WO3 film presents extra oscillations compared to the sp-WO3 one. They have been assigned to the presence of two additional W-O distances, a short one at ca. 1.5 Å and a long one at ca. 2.4 Å. Due to the complicated structure, the spectrum of the initial ed-WO3 film could

P. E. Petit 共ESRF, Grenoble, France兲 and the staff of the ID26 beamline are acknowledged for XAS experiments. A. Billard 共LSGS, E´cole des Mines, Nancy, France兲 is acknowledged for thinfilm preparation by sputtering and P. Mandin 共LECA, ENSCP, Paris兲 for his help in data file processing. UMR-CNRS 7575 assisted in meeting the publication costs of this article.

References 1. C. G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam 共1995兲. 2. K. Yamanaka, H. Oakamoto, H. Kidou, and T. Kudo, Jpn. J. Appl. Phys., Part 1, 25, 1420 共1986兲. 3. J. Livage and G. Guzman, Solid State Ionics, 84, 205 共1996兲. 4. K. Galatsis, Y. X. Li, W. Wlodarski, and K. Kalantar-Zadeh, Sens. Actuators B, 77, 478 共2001兲. 5. K. Yamanaka, Jpn. J. Appl. Phys., Part 1, 26, 1884 共1987兲. 6. L. H. Dao, A. Guerfi, and M. T. Nguyen, in Electrochromic Materials, M. C. Carpenter and D. A. Corrigan, Editors, PV 90-2, p. 30, The Electrochemical Society Proceedings Series, Pennington, NJ 共1990兲. 7. P. K. Shen, J. Syed-Bokhari, and A. C. C. Tseung, J. Electrochem. Soc., 138, 2778 共1991兲. 8. P. K. Shen and A. C. C. Tseung, J. Mater. Chem., 2, 1141 共1992兲. 9. A. Pennisi, F. Simone, and C. M. Lampert, Sol. Energy Mater. Sol. Cells, 28, 233 共1992兲. 10. P. M. S. Monk and S. L. Chester, Electrochim. Acta, 38, 1521 共1993兲. 11. E. A. Meulenkamp, J. Electrochem. Soc., 144, 1664 共1997兲. 12. E. A. Meulenkamp, R. J. J. De Groot, and J. M. L. de Vries, Mater. Res. Soc. Symp. Proc., 451, 321 共1997兲. 13. M. C. Bernard, A. Hugot-Le Goff, and W. Zeng, Electrochim. Acta, 44, 781 共1998兲. 14. T. Pauporte´, J. Electrochem. Soc., 149, C539 共2002兲. 15. T. Pauporte´, Y. Soldo-Olivier, and R. Faure, J. Phys. Chem. B, 107, 8861 共2003兲. 16. B. W. Faughnans, R. S. Crandall, and P. M. Hyman, RCA Rev., 36, 177 共1975兲. 17. D. Aberdam, J. Synchrotron Radiat., 5, 1287 共1998兲. The package can be downloaded from the ESRF website www.esrf.fr 18. J. J. Rehr, J. Mustre de Leon, S. I. Zabinsky, and R. C. Albers, J. Am. Chem. Soc., 113, 5135 共1991兲. 19. R. W. G. Wyckoff, Crystal Structure, 2nd ed., Vol. 1, Chap. 4, p. 250, Interscience Publisher, New York 共1964兲. 20. D. C. Koeningsberger and R. Prins, X-Ray Absorption: Principles, Application, Technique of EXAFS, SEXAFS, and XANES, p. 45, Wiley, New York 共1988兲. 21. B. O. Loopstra and H. M. Rietveld, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 25, 1420 共1969兲. 22. S. H. Lee, H. M. Cheong, J. G. Zhang, A. Mascarenhas, D. K. Benson, and S. K. Deb, Appl. Phys. Lett., 74, 242 共1999兲. 23. M. F. Daniel, B. Desbat, J. C. Lassegues, B. Gerand, and M. Figlarz, J. Solid State Chem., 67, 235 共1987兲. 24. T. Kubo and Y. Nishikitani, J. Electrochem. Soc., 145, 1729 共1998兲. 25. M. Gotic´, M. Ivanda, S. Popovic´, and S. Misic´, Mater. Sci. Eng., B, 77, 193 共2000兲. 26. Y. A. Yang and J. N. Yao, J. Phys. Chem. Solids, 61, 647 共2000兲. 27. E. Cazzanelli, L. Papalino, A. Pennisi, and F. Simone, Electrochim. Acta, 46, 1937 共2001兲. 28. S. H. Lee, M. J. Seong, H. M. Cheong, E. Ozkan, E. C. Tracy, and S. K. Deb, Solid State Ionics, 156, 447 共2003兲. 29. A. Balerna, E. Bernieri, E. Burattini, A. Kuzmin, A. Lusis, J. Purans, and P. Cikmach, Nucl. Instrum. Methods Phys. Res. A, 308, 234 共1991兲. 30. A. Balerna, E. Bernieri, E. Burattini, A. Kuzmin, A. Lusis, J. Purans, and P. Cikmach, Nucl. Instrum. Methods Phys. Res. A, 308, 239 共1991兲. 31. A. Kuzmin and J. Purans, J. Phys.: Condens. Matter, 5, 2333 共1993兲. 32. J. Purans, A. Kuzmin, and C. Gue´ry, Sov. Phys. Crystallogr., 968, 174 共1997兲. 33. E. A. Stern, Phys. Rev. B, 48, 9825 共1993兲.