Electronic structure of CuWO4: XPS, XES and NEXAFS studies

Journal of Alloys and Compounds 389 (2005) 14–20

Electronic structure of CuWO4 : XPS, XES and NEXAFS studies O.Yu. Khyzhuna,∗ , T. Strunskusb , S. Crammc , Yu.M. Solonina a

Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky str., UA-03142 Kiev, Ukraine b Lehrstuhl f¨ ur Physikalische Chemie I, Ruhr-Universit¨at Bochum, Universit¨atsstraße 150, D-44801 Bochum, Germany c IFF, Forschungszentrum J¨ ulich, D-52425 J¨ulich, Germany Received 30 July 2004; accepted 5 August 2004

Abstract X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and near-edge X-ray absorption fine structure (NEXAFS) methods were applied to study the electronic structure of copper tungstate, CuWO4 . For the compound, XP valence-band spectra with different energies of excitation were studied, as well as the XE O K␣ band and the NEXAFS O 1s spectrum were derived. The binding energies of the XP core-level electrons of the constituting elements of CuWO4 were measured. For comparison, some spectra of the hexagonal form of tungsten trioxide, h-WO3 , were investigated. It was found that the half-width of the O K␣ band decreases somewhat but that of the XP valence-band spectrum increases when going from h-WO3 to CuWO4 , however the energy positions of the maxima and of the centres of gravity of the O K␣ band remain constant for the above compounds. Measurements of the XP O 1s core-level binding energies and of the energy positions of the inflection point of the NEXAFS O 1s spectra reveal that the effective negative charge of oxygen atoms in CuWO4 is close to that in h-WO3 , while XPS W 4f core-level measurements reveal that the positive effective charge of tungsten atoms decreases somewhat in the sequence h-WO3 → CuWO4 . © 2004 Elsevier B.V. All rights reserved. Keywords: Copper tungstate; Tungsten oxide; Electronic structure; XPS; XES; NEXAFS

1. Introduction The hexagonal form of tungsten trioxide, h-WO3 , was first synthesized by Gerand et al. [1] due to dry heating of the WO3 ·1/3·H2 O hydrate. In the structure of h-WO3 , every six [W−O6 ] octahedra linked by corner sharing form hexagonal channels oriented along the c-axis [1–3] and the channels were found to be a very prospective intercalation host of lithium for obtaining Lix WO3 bronzes, a remarkable cathode material of rechargeable lithium batteries [4,5]. Therefore, a number of alternative routes for synthesis of hWO3 were developed since the beginning of 1980s. A review of the alternative routes (using as precursors the WO3 ·xH2 O (x = 0.8−1.1), (NH4 )0.30 ·WO3 , (NH4 )10 [H2 W12 O42 ]·10H2 O and (NH4 OH)x ·WO3 substances) was made comparatively

∗

Corresponding author. E-mail address: [email protected] (O.Yu. Khyzhun).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.013

recently by Han et al. [6]. As shown in Refs. [7,8], copper tungstate, CuWO4 , is also a very prospective precursor for synthesis of the hexagonal form of tungsten trioxide. A product derived during reduction of CuWO4 in flowing hydrogen at 300 ◦ C with following treatment by concentrated HNO3 upto the full desolving of copper leads to the formation of a brown-blue hexagonal hydrogen tungsten bronze, Hx WO3 , with x = 0.24 [9]. The oxidation of H0.24 WO3 in air at 400–450 ◦ C leads to the synthesis of pure h-WO3 with unitcell parameters a = 0.7276 nm and c = 0.7800 nm [7], which are very close to those (a = 0.7298 nm and c = 0.7798 nm) obtained by Gerand et al. [1]. The crystal structure of CuWO4 belongs to a triclinic ¯ distorted wolframite type, with unit-cell (space group P 1) parameters a = 0.47026 nm, b = 0.58389 nm, c = 0.48784 nm, α = 91.677◦ , β = 92.489◦ , γ = 82.805◦ [10]. As established by Kihlborg and Gebert [10], in the structure of CuWO4 every metal atom is surrounded by six oxygen atoms: the ranges of the six M–O distances are within 0.1961–0.2450 nm for

O.Yu. Khyzhun et al. / Journal of Alloys and Compounds 389 (2005) 14–20

the [Cu–O6 ] octahedra and within 0.1760–0.2208 nm for the [W–O6 ] octahedra. The crystal structure of CuWO4 can be described within a framework of oxygen atoms in an approximately hexagonal close-packing with copper and tungsten atoms occupying half of the octahedral sites [10,11]. In the structure of CuWO4 , copper and tungsten atoms form alternating sequence of layers laying between the oxygen sheets: infinite zigzag chains are formed by edge-sharing alternating [W–O6 ] and [Cu–O6 ] octahedra [10]. As shown in Ref. [12], hexagonal close packing of oxygen atoms does not remain after termination of the CuWO4 → hWO3 transformation. While the value of average volume per oxygen atom is ca. 0.01659 nm3 for the structure of CuWO4 , the average volume per oxygen atom is ca. 0.0197 nm3 in the case of h-WO3 [12]. Therefore, the value of average volume per oxygen atom in the hexagonal form of tungsten trioxide is far away from the value of ca. 0.015 nm3 for the ideal anionic close packing [7,12]. Electron diffraction analysis [12] has revealed that an ideal correlation for reciprocal orientation of the initial lattices are characteristic of CuWO4 and h-WO3 : ¯ (0 0 0 1)h-WO3 (1 1¯ 1)CuWO 4 ¯ the normal to (1 2¯ 1 0)h-WO3 [0 1 1]CuWO 4. The fact that the packing of anions in h-WO3 significantly differs from that in CuWO4 indicates that reciprocal orientation of the CuWO4 and h-WO3 phases is defined by a heredity of the elements of their cationic sublattice. This phenomenon cannot be explained within the close-packing framework (within this framework cations should not play the main role [12]). The phenomenon seems to be similar to the effect of “stable cationic motives” suggested by Chichagov et al. [13] and by Borisov and Podberezskaya [14]. Analysis of the structures of a number of fluorides and oxides [13,14] has revealed an existence of a type of compounds with different arrangements of anionic sublattices but with very similar arrangements of atoms in the cationic sublattices. For example, comparison of the structures of CdWO4 (wolframite FeWO4 -type) and CdMoO4 (scheelite CaWO4 -type) reveals [13] that the stacking of anions significantly changes in the sequence CdWO4 → CdMoO4 (in the above sequence even the orientation of the close-packing layers changes dramatically), nevertheless the cationic motif remains unchanged for both compounds. Taking the same arguments, one can conclude that the main reason of formation of h-WO3 (instead of the usual monoclinic form of tungsten trioxide, m-WO3 ) during selective reduction of copper from CuWO4 is smaller distortion of the cationic motif of CuWO4 in the case of the CuWO4 → h-WO3 transformation as compared with that in the case of the CuWO4 → m-WO3 transformation. As shown very recently in Ref. [15], this was true in the case of formation of a new hexagonal WO2.8 phase with the structure of UO3 -type during reduction of h-WO3 . In addition to the ability to serve as a prospective precursor for synthesis of Lix WO3 bronzes, CuWO4 was found to be a remarkable substance for synthesis of almost porous-

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less W–Cu pseudoalloys containing 10–35 mol% of copper [16,17]. Since properties of solids can be well understood by considering their electronic structure, it is interesting to study the electronic structure of the mentioned copper tungstate. While the electronic structure of some relativevely close tungstates and molybdates (CuMoO4 , CaMoO4 , CdMoO4 , PbMoO4 , CaWO4 , CdWO4 , PbWO4 ) was studied comparatively recently in a series of works [18–20], to the best of our knowledge, the electronic structure of CuWO4 has not been studied yet either by theoretical band-structure calculations or by experimental methods. It is also of interest to compare the electronic structure of CuWO4 with that of h-WO3 . The band calculation of h-WO3 was made by Hjelm et al. [21] using the ab initio relativistic full-potential linear muffin-tin orbital (FP-LMTO) procedure. As it was suggested by Hjelm et al. [21], because 2p-like states originating from the oxygen atoms in the hexagonal planes of h-WO3 and those between the planes dominate in the low-energy part and the top of the O 2p-like band, respectively, one should expect broadening the valence band when going from the monoclinic (cubic) form of tungsten trioxide to h-WO3 . This conclusion was confirmed experimentally in Refs. [22,23]. The purpose of the present work was to carry out a complex investigation of the electronic structure of CuWO4 using the X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and near-edge X-ray absorption fine structure (NEXAFS) methods. In this paper a comparison will be given of changes of the electronic structure when going from CuWO4 to h-WO3 . Since the electronic structure of the hexagonal form of tungsten trioxide was investigated experimentally in [22,23], only new results for h-WO3 will be reported in the present paper.

2. Experimental Copper tungstate, CuWO4 , studied in the present paper was obtained by solid state reaction of CuO and the usual monoclinic form of tungsten trioxide, m-WO3 , according to the relation CuO + WO3 = CuWO4 [7,8]. The solid-state reaction was carried out on air at 800 ◦ C [15]. After reduction of CuWO4 in flowing hydrogen at 300 ◦ C and following treatment by concentrated HNO3 upto the full desolving of copper, the hexagonal phase of hydrogen tungsten bronze H0.24 WO3 was obtained [22]. Pure h-WO3 was derived during oxidation of H0.24 WO3 in air at 400–450 ◦ C as reported in Ref. [22]. The conventional X-ray diffraction analysis carried out on a DRON-3 diffractometer using Cu K␣ radiation revealed [12,22] that unit-cell parameters of the CuWO4 and h-WO3 phases are in good agreement with the results of Kihlborg and Gebert [10] and Gerand et al. [1], respectively. The method of studies of the XP valence-band spectra in CuWO4 was analogous to that used in Ref. [24] for molybdenum oxides. Briefly, the experiments were made in a turbopumped ultrahigh vacuum (UHV) chamber (base pressure

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(2–3) × 10−10 mbar) equipped with a VG Clam2 hemispherical electron energy analyzer. The UHV system was connected to the HE-SGM beam-line at BESSY II, Berlin. The beamline is equipped with a spherical grating monochromator. The XP valence-band spectra were recorded with constant pass energies either 100 eV or 20 eV using 400 ␮m slits. There were no indications of reduction of the analyzing near-surface region of CuWO4 under the influence of synchrotron light because no changes of the XP spectra during data acquisition were observed. The binding energy scale was calibrated by assigning 284.6 eV to the C 1s line as suggested in Refs. [25,26] for transition-metal oxides. XPS measurements with Al K␣ excitation (E = 1486.6 eV) were made in the above conditions, but those with a Mg K␣ source (E = 1253.6 eV) were carried out in an ion-pumped chamber of an ES-2401 spectrometer (base pressure ∼1 × 10−9 mbar) using the technique reported in Ref. [22]. The method of investigation of the XE O K␣ band (K → LII,III transition), reflecting the energy distribution of the O 2p-states, was analogous to that used earlier for studies of the band in sub-stoichiometric tungsten oxides [27]. The spectrometer energy resolution in the range corresponding to the energy of the O K␣ band was found to be better than 0.5 eV. The base pressure in the spectrometer analyzing chamber was routinely less than 1 × 10−8 mbar. The operation conditions of the electron tube were: accelerating voltage, Ua = 7 kV; anode current, Ia = 7 mA. The dispersing element was a diffraction grating with 600 lines/mm and a radius of curvature of R = 6026 mm. The grating and a reflection mirror (R = 4000 mm) were covered by a layer of gold (thickness of about 30 nm). The detector was a secondary electron multiplier with a CsI photocathode. For each specimen studied, seven independently derived spectra of the O K␣ band were chosen for averaging the results obtained. The NEXAFS O 1s spectra were recorded using a method, which was analogous to that employed in Ref. [24]. The experiments were made in a turbopumped UHV chamber with base pressure typically (2–4) × 10−9 mbar mounted to synchrotron source (Beam-line 5, DELTA, Dortmund). The beam-line was equipped with a plane grating monochromator. The 75 ␮m slits were used to reach the energy resolution of ca. 0.6 eV.

3. Results and discussion A typical evolution of the XP valence-band spectra on the expanded scale (including also photoemission from the O 2s-like states) for the CuWO4 specimen under investigation is presented in Fig. 1. The spectra presented in the above figure were excited by photons with energies ranging from 100 to 500 eV and recorded with pass energy of 100 eV. The XP spectra in Fig. 1 are normalized so that intensities of their valence-band peaks are equal. As Fig. 1 reveals the XP valence-band of CuWO4 possesses a very simple structure, and only an increase of relative intensities of the O 2s-like

Fig. 1. XP valence-band spectra, excited by photons with energies ranging from 100 to 500 eV and recorded with pass energy of 100 eV, of CuWO4 .

sub-band is detected with increased energy of photons employed in the present work for excitation of the spectra. A little shift of the maximum of the XP valence-band spectra towards the Fermi energy, EF , can be observed with increasing photon energy. This shift could be caused by the existence of two fine-structure features positioned very close to each other and not resolvable in our experiments on the XP valenceband spectra of CuWO4 . Therefore, in order to investigate the structure of the valence-band of copper tungstate more carefully (with higher energy resolution), we have recorded also the XP valence-band spectra of CuWO4 employing pass energy of 20 eV. As can be seen from Fig. 2, it is apparent that the valence bands of CuWO4 excited with photons of energies ranging from 140 to 400 eV and recorded with pass energy of 20 eV reveal no new features as compared with those of the XP valence bands presented in Fig. 1. The simple structure of the valence band of CuWO4 is confirmed also by XPS measurements using Mg K␣ excitation. For the compounds under investigation, Fig. 3 displays the XP valence band spectra taken with Mg K␣ source and corrected for Mg K␣ , ␣3–6 satellite excitation using the technique previously employed for tungsten oxides in [22,23]. As can be observed from Fig. 3, the XP valence-band spectrum of CuWO4 excited by Mg K␣ radiation does not contain any pronounced fine-structure features, while that of h-WO3 shows existence of a small shoulder “b” in the energy region of ∼4 eV below EF . It has been established previously [23] that appearance of the feature “b” of the XP valence-band spectrum of h-WO3 is due to the bonding O 2p␲ -like states, which are not hybridized with the W 5d-like states. The XP

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Table 1 Some characteristics (in eV) of the XP valence-band and core-level spectra of the compounds studieda Compound

CuWO4 h-WO3 Uncertainty

Binding energies of some XP core-level electrons O 1s

W 4f7/2

530.6 530.8 ±0.1

35.5 36.0 ±0.1

Position of the maximum of the XP valence band

Half-width of the XP valence-band spectrum

5.4 7.5 ±0.2

6.8 6.5 ±0.1

a XP valence-band spectra were excited by Mg K␣ radiation, but those of the core-level electrons by Al K␣ radiation; the XP Cu 2p 3/2 core-level spectrum represents mainly that of Cu3+ ions (see the text).

Fig. 2. XP valence-band spectra, excited by photons with energies ranging from 140 to 400 eV and recorded with pass energy of 20 eV, of CuWO4 .

valence-band spectra presented in Fig. 3 were normalized to the same integral intensity of the XP W 4f core-level spectra of the corresponding compound. And it is apparent from Fig. 3 that the peak intensity of the XP valence-band spectra does not change when going from h-WO3 to CuWO4 . Further,

Fig. 3 shows that the XP valence-band spectrum of copper tungstate under investigation has its maximum at 5.4 eV below EF , but that of the hexagonal form of WO3 at 7.5 eV (Table 1). As can be seen from the data listed in Table 1, the half-width of the XP valence-band spectrum increases by about 0.3 eV when going from h-WO3 to CuWO4 . Bandstructure calculations [21,28–32] indicate that the contribution of the O 2p-like states is dominant in the valence band but the W 5d-like states dominate the conduction band of WO3 . Due to this reason, the changes of the XP valenceband spectra (reflecting the energy distribution of the total density of states (DOS) in investigating substances) and the O K␣ bands (reflecting the energy distribution of the O 2plike states) were found to go by similar way due to changing the crystal structure [22] or due to sub-stoichiometry of tungsten oxides [33]. We may also expect that changes of the half-width of the XP valence-band spectra when going from h-WO3 to CuWO4 should be similar to that of the O K␣ bands. However, as could be seen from the data listed in Table 2, the half-width of the O K␣ band decreases by about 0.4 eV in the sequence h-WO3 → CuWO4 . Therefore, in the valence band of copper tungstate under investigation, in addition to a contribution of the O 2p-like states, a comparable contribution of the W 5d-like states (and also some contribution of partial DOS of copper) should exist. The same statement is suggested by density-functional calculations using the linearized-augmented-plane-wave (LAPW) method that were made by Zhang et al. [18] for scheelite materials AMoO4 and AWO4 , where A = Ca and Pb, as well as by Abraham et al. [20] for CdWO4 with the wolframite-type and for CdMoO4 with the scheelite-type structures. The results of Refs. [18,20] indicate that the upper portion of the valence band of the above materials is dominated by the O 2p␲ -like states which have also significant contributions throughout the main portion of the band, while the main contributors to Table 2 Some characteristics (in eV) of the O K␣ emission bands of the compounds studied

Fig. 3. XP valence-band spectra, excited by Mg K␣ radiation, of CuWO4 and h-WO3 .

Compound

Energy position of the maximum “b” of the band

Energy position of the centre of gravity of the band

Half-width of the band

CuWO4 h-WO3 Uncertainty

526.3 526.4 ±0.2

525.4 525.2 ±0.2

4.2 4.6 ±0.1

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Fig. 4. O K␣ emission bands of h-WO3 and CuWO4 .

the bottom of the valence band are O 2p␴ - and W(Mo) d-like states. The O K␣ emission bands for the compounds under investigation are shown in Fig. 4. The spectra presented in Fig. 4 have been normalized so that intensities of their main peaks “b” are equal. As Fig. 4 shows, the formation of only one low-energy fine-structure feature “a” is characteristic for the bands of both compounds studied. The relative intensity of the feature “a” of the O K␣ bands increases somewhat when going from CuWO4 to h-WO3 (from the value of Ia /Ib =0.53 ± 0.02 for CuWO4 to the value of Ia /Ib = 0.57 ± 0.02 for h-WO3 ); perhaps this is the main reason of increasing halfwidth of the band in the above sequence of compounds (cf. Table 2). Taking into account the results of LAPW calculations of tungstates and molybdates of Ca, Cd and Pb [18,20], we can conclude that the feature “a” of the O K␣ band in CuWO4 is created due to almost equal contributions of the O 2p␴ - and O 2p␲ -like states, while the O 2p␲ -like states are the main contributors in the energy region corresponding to the position of the maximum “b” of the band. As the data listed in Table 2 reveal, the energy positions of the maximum “b” of the O K␣ band and of its centre of gravity within accuracy of the measurement (±0.2 eV) remain constant for both compounds under investigation. Fig. 5 shows XP spectra excited by Al K␣ radiation in the regions corresponding to the positions of the W 4f and Cu 2p3/2 core-level electrons of CuWO4 . Taking into account the reference data [34,35], the charge state of copper atoms in CuWO4 corresponds mainly to that of Cu3+ ions with little admixture of Cu2+ ions. According to H¨ufner [35], the difference between maxima of the XP Cu 2p3/2 signals of metallic copper (Cu0 ) and of CuO (Cu2+ ) is 0.80 eV, while that between the signals of metallic copper and of NaCuO2 (Cu3+ ) is 2.10 eV. As Fig. 5 depicts, positions of the features of the XP Cu 2p3/2 core-level spectrum of CuWO4 are in good coincidence with the positions of the XP Cu 2p3/2 peaks listed

Fig. 5. XP spectra, excited by Al K␣ radiation, in the region corresponding to (a) Cu 2p3/2 and (b) W 4f7/2,5/2 core-level electrons of CuWO4 . Positions of the Cu 2p3/2 peaks in formal oxidation states +3 and +2 determined in Ref. [35] are also presented in the upper panel of the figure.

by H¨ufner [35] for Cu2+ and Cu3+ ions. The XP W 4f7/2 core-level binding energy in copper tungstate is by 0.5 ± 0.1 eV smaller as compared with that in h-WO3 (Table 1). This indicates that the positive effective charge on the tungsten atoms decreases somewhat when going from h-WO3 to CuWO4 . Nevertheless, as the data of Table 1 indicate, the XP O 1s core-level binding energies are coincidence for both compounds studied within the accuracy of the present experiment. Therefore, the negative effective charge on oxygen atoms does not change when going from h-WO3 to CuWO4 . The NEXAFS O 1s spectra measured for compounds under investigation are presented in Fig. 6. As one could see from the figure, the NEXAFS O 1s spectra in the energy region from 520 eV to 550 eV reveal four resonance features, labeled “a–d”, for CuWO4 and three features for h-WO3 . Energy positions of the resonance features for the compounds investigated are listed in Table 3. The curves presented in Fig. 6 were normalized by the method previously used in Ref. [21], however the spectrum of h-WO3 was shifted somewhat up, for clarity, along the Y-axis. As Fig. 6 shows, a strong resonance “a” positioned in the energy region (530.1–530.2) ± 0.2 eV (Table 3) dominates the spectra of both compounds studied. From Fig. 6 it is apparent that the formation of an additional resonance (fine-structure peculiarity “c” of the NEXAFS O 1s spectrum in the energy region of 540.1 ± 0.3 eV, Table 3),

O.Yu. Khyzhun et al. / Journal of Alloys and Compounds 389 (2005) 14–20

Fig. 6. NEXAFS O 1s spectra of CuWO4 and h-WO3 .

which is absent on the spectrum of the hexagonal form of tungsten trioxide under investigation, is characteristic of the spectrum of CuWO4 . Recent density functional calculations using the LAPW method applied by Zhang et al. and Abraham et al. [18,20] for investigation of the electronic structure of a number of AWO4 tungstates and of AMoO4 molybdates, where A = Ca, Cd and Pb, have revealed that the bottom of the conduction band of the compounds are dominated by contributions of the W(Mo) de -like states. Concerning the energy distribution of the O 2p-like states in the above tungstates and molybdates, the LAPW calculations [18,20] indicate that the O 2p␲ -like states form a single peak strictly at the bottom of the conduction band, while the upper portion of the conduction band is occupied by almost equal contributions of the hybridized O 2p␴ - and O 2p␲ -like states which form a broad sub-band just above the O 2p␲ -like peak. Thus, in the case of the NEXAFS O 1s spectrum presented in Fig. 6, the resonance “a” is due to contributions of the antibonding O 2p␲ -like states, while other fine-structure features are due to hybridized combination of the antibonding O 2p␴ - and O 2p␲ -like states. Hjelm et al. [21] have not presented the contributions of the O 2p␴ and O 2p␲ -like states to the curve showing partial O 2p-like DOS in the hexagonal form of tungsten trioxide. However, the FP-LMTO calculation [21] reveals a rather strong peak at the bottom of the conduction band on the curve of partial O 2p-like DOS of h-WO3 , and two rather pronounced fineTable 3 Energy positions (in eV) of some features of the NEXAFS O 1s spectra of the compounds studied Compound

CuWO4 h-WO3 Uncertainty a

Inflection point of the O K-edge 529.2 529.2 ±0.2

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structure peculiarities just above the peak. These results seem to be in good agreement with our experimental NEXAFS O 1s study of the hexagonal form of tungsten trioxide (cf. resonances “a”, “b” and “d” of the spectrum of h-WO3 presented in Fig. 6). As Fig. 6 depicts, the relative intensity of the resonance “a” is somewhat smaller in CuWO4 as compared with that in h-WO3 . Whether this is either due to a smaller portion of antibonding O 2p␲ -like states in the bottom of the conduction band of CuWO4 as compared with that of h-WO3 or this is only a partial property of the NEXAFS O 1s spectra in the compounds under investigation, it is not clear from our experiment, therefore we have left this fact for future research. It should be also mentioned, as the data listed in Table 3 reveal, the energy positions of the inflection point of the NEXAFS O 1s spectra coincide within accuracy of the measurements in the both compounds studied. The latter fact indicates that the effective negative charge of oxygen atoms does not change when going from h-WO3 to CuWO4 . This statement is in agreement with the results of measurements of the XP O 1s core-level binding energies as it was discussed above.

4. Conclusions The present XPS measurements employing both Mg K␣ excitation and photons with different energies (ranging from 100 to 500 eV) at the synchrotron facility of BESSY II (Berlin) reveal a simple structure of the valence band of copper tungstate, CuWO4 . Results of the present measurements of the XP W 4f core-level binding energies indicate that the effective positive charge of tungsten atoms decreases somewhat when going from h-WO3 to CuWO4 , while both the XP O 1s core-level and NEXAFS O 1s data reveal that the effective negative charges of oxygen atoms are very similar in both compounds studied. The half-width of the XE O K␣ band decreases by about 0.4 eV, while that of the XP valence-band increases by about the same value when going from h-WO3 to CuWO4 . The energy positions of the maxima and of the centres of gravity of the XE O K␣ bands remain constant within experimental error for the both compounds under investigation.

Acknowledgements The authors thank Professor Christof W¨oll for his support of carrying out the present XPS measurements on synchrotron facility of BESSY II in Berlin.

Resonances a 530.2 530.1 ±0.2

b 537.3 538.2 ±0.3

c

d

540.1 – ±0.3

545.8a

Position of the centre of gravity of the broad structure.

546.0 ±0.4

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