Electronic structure of niobium oxides

Journal of Alloys and Compounds 347 (2002) 213–218

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Electronic structure of niobium oxides E.Z. Kurmaev a , A. Moewes b , O.G. Bureev c , I.A. Nekrasov c , V.M. Cherkashenko a , M.A. Korotin a , d, D.L. Ederer * a

Institute of Metal Physics, Russian Academy of Sciences–Ural Division, 620219 Yekaterinburg GSP-170, Russia Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada S7 N 5 E2 c Department of Theoretical Physics and Applied Mathematics, Ural State Technical University USTU-UPI, 19 Mira Str., Yekaterinburg 620002, Russia d Department of Physics, Tulane University, New Orleans, LA 70118, USA b

Received 3 April 2001; received in revised form 24 February 2002; accepted 4 March 2002

Abstract The results of X-ray fluorescence measurements of niobium metal and its oxides (NbO, NbO 2 and Nb 2 O 5 ) are presented. Non-resonant Nb M 4,5 and O Ka X-ray emission spectra (XES) have been measured at the Advanced Light Source (ALS, Berkeley). The experiments are compared with self-consistent band structure calculations of niobium metal and niobium oxides. We find the experimental spectra are consistent with band structure calculations of niobium metal and the oxides NbO 2 and Nb 2 O 5 . The difference between experimentally determined Nb M 4,5 XES of NbO and the calculated spectrum of Nb 1.0 O 1.0 is understood on the basis of vacancies. We have mimicked the natural structure of NbO, which contains vacancies amounting to 25% in both sublattices. The electronic DOS of Nb 0.75 O 75 is characterised by the formation of additional subbands connected with vacancy states. The calculated electronic structure based on this model is consistent with the XES measurements.  2002 Elsevier Science B.V. All rights reserved. Keywords: Electronic structure; Niobium oxides; X-ray fluorescence

1. Introduction Transition metal oxides exhibit a variety of interesting physical properties, such as excellent chemical stability, high melting temperature, catalytic activity, etc. Electrical properties of these compounds vary from insulator to semiconductor, metals and superconductors [1,2]. Some of these oxides (with rutile type structure) undergo a structural phase transition at high-pressure [3]. The physical properties of 3d- and 4d-transition metals have been intensively studied and show very interesting behaviour. For instance, NbO 2 exhibits a metal–semiconductor transition at 1070 K. NbO offers a particularly intriguing case, since it crystallises in the NaCl-type structure having 25% ordered vacancies on each sublattice [4], which is the highest number of point defects found among all transition-metal monoxides. The electronic structure of 4d-oxides, especially the Nb-oxides, has been studied less than that of 3d-oxides theoretically and experimentally. To our *Corresponding author. Tel.: 11-504-862-8268; fax: 11-504-8628207. E-mail address: [email protected] (D.L. Ederer).

knowledge only two papers have been published during the decade of the 1980s in which self-consistent band structure calculations have been performed for NbO [5] and NbO 2 [6]. The paucity of data prompted us to perform a new theoretical and experimental study of the electronic structure of Nb-oxides. In the present paper we present the results of X-ray fluorescence measurements of NbO, NbO 2 and Nb 2 O 5 and compare them with self-consistent band structure calculations. We find that Nb M 4,5 (5p→3d transition) XES is quite suitable for the study of the electronic structure of Nb oxides because the Nb 5p partial density of states (DOS) correlates quite well with the total DOS. Following Wimmer et al. [5], we generated the electronic structure of NbO with a hypothetical NaClstructure consisting of 25% structural vacancies using Nb 0.75 O 0.75 and found additional structure was introduced in the DOS compared with the DOS of Nb 1.0 O 1.0 . Vacancyinduced electron states are surmised to produce the differences. 2. Experimental and calculation details X-ray fluorescence measurements were performed at

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Beamline 8.0 at the ALS (Advanced Light Source of Lawrence Berkeley National Laboratory). Monochromatized photons with an energy of 238, and 540 eV, the former well above the Nb M edge and the latter well above the O K edge, were obtained from a 5.0-cm undulator and focused onto the sample. The resulting fluorescence emission spectra are recorded with a high efficiency X-ray spectrometer [7]. This soft X-ray fluorescence spectrometer is a grazing incidence instrument with a fixed entrance slit and a position sensitive area detector. Emission spectra are measured by positioning the detector along the Rowland circle to intercept the wavelength region of interest. Nb M 4,5 and O Ka XES were recorded using a 5- and 10-m gold-coated spherical grating (1500 600 l / mm) with an entrance slit of 100 mm which provided an energy resolution of 0.2 and 0.4 eV, respectively. Spectra were calibrated using reference samples of pure niobium and MgO. To calculate the band structure of Nb, NbO, NbO 2 , Nb 2 O 5 we implemented a self-consistent tight-binding (TB) linear muffin-tin orbital (LMTO) code in the atomic sphere approximation (ASA) [8]. Calculations were performed with version 47 of the TB-LMTO-ASA code developed in Professor Anderson’s group. Nb and NbO have cubic crystal structure that is bodycentred and face-centred, respectively. Both compounds have point group symmetry m-3m. Lattice parameters are ˚ for Nb and 4.211 A ˚ for NbO [9]. In both 3.3007 A compounds, the Brillouin zone was divided in reciprocal space in the ratio (8 8 8). Because of crystal symmetry there are 29 irreducible k-points. In the case of Nb 0.75 O 0.75 the unit cell was derived from the conventional cubic NaCl-unit cell (containing 4 formula units) by introducing one vacancy on a Nb-site and one vacancy on an oxygensite. NbO 2 has a tetragonal crystal structure of body-centred type. The point group symmetry is 4 1 / a. The following lattice parameters were used for the calculations: a5 ˚ c55.981 A ˚ [10]. 13.696 A, Nb 2 O 5 has a monoclinic crystal structure. The point group symmetry is 2 / c with lattice parameters: a512.73 ˚ b54.88 A, ˚ c55.56 A, ˚ b 5105.18 [11]. For both NbO 2 A, and Nb 2 O 5 the division of the Brillouin zone in reciprocal space was (4 4 4) for each direction and contains 14 and 24 irreducible k-points for NbO 2 and Nb 2 O 5 , respectively. Calculations were performed for stable room temperature phases. For each compound we included 5s, 5p and 4d orbitals in the basis set of Nb and the 2s, and 2p orbitals of oxygen in the basis set for oxygen.

ray emission resulting from the transition of valence electrons to core holes. This means the 3d core level holes can only be filled by valence electrons having p or f symmetry. Therefore, the intensity of M 4,5 (5p→3d transition) non-resonant X-ray emission spectra provides a measure of the contribution of the partial 5p-type density of states (DOS) at the niobium site. In the case of O Ka (2p→1s transition) XES the intensity distribution corresponds to the partial O 2p DOS. The technique provides a means of extracting chemical information in the form of a local density of states distribution with certain symmetries that can be directly compared with band structure calculations. Fig. 1 shows the results of measurements of Nb M 4,5 XES in niobium metal and Nb-oxides. The spectrum of the metal consists of the Nb M 5 -band (5p–3d 5 / 2 transition), a weaker Nb M 4 -band (5p–3d 3 / 2 transition) and a satellite of the Nb Mß-line (Mß9) in full agreement with results of Ref. [12]. For Nb-oxides an energy shift of Nb M 4,5 XES (maximal for NbO) with respect to the spectrum of the metal is observed, owing to the chemical shift of the Nb 3d-core level [13] and the appearance of an oxygen 2p-like subband. In the spectra shown in Fig. 1, the fine structure

3. Results and discussion

3.1. Experimental spectra The dipole selection rule (Dl561) governs normal X-

Fig. 1. Nb M 4,5 X-ray emission spectra (XES) in pure metal and binary oxides.

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of Nb M 4,5 XES is richest in NbO (features labelled 1, 2, 29) and becomes simpler in NbO 2 (features labelled 1 and 2) and the spectrum of Nb 2 O 5 has just a single peak identified as feature 2. A low-intensity subband (3) is observed in Nb M 4,5 XES for all oxides. This subband is absent in the spectrum of the pure metal. The energy difference between subbands (3-2) is found to be about 16 eV, which is close to the difference of 2s and 2p orbital energies for the oxygen atom [14]. Therefore, we suggest subbands 3 and 2 are connected with O 2s and O 2p-bands and occur in the Nb M 4,5 XES spectra because of O 2s–Nb 5p and O 2p–Nb 5p hybridisation, respectively. This conclusion was also reached by Mueller et al. in their study of Nb 2 O 5 [15]. The Nb M 4,5 XES of NbO and NbO 2 consists of the high-energy subband (1), which is nearly coincident in energy to subband (1) in the spectrum of pure Nb, and could be obscured by the width of the band in the spectrum of Nb 2 O 5 . We interpret subband (1) in the spectra of Nb oxides as features originating from transitions having dlike band character where the intensity is expected to be reduced as the oxidation state increases from NbO to NbO 2 to Nb 2 O 5 , respectively. In the O Ka XES of Nb-oxides shown in Fig. 2, the high-energy subband (1) is found only for NbO and NbO 2 . The origin of this subband is also connected with Nb 4d-states because this subband only occurs in NbO and NbO 2 , which have occupied 4d orbitals. The oxide Nb 2 O 5 has a d 0 configuration, thus the 4d-band is unoccupied, and subband (1) is not expected and indeed is not observed.

Fig. 2. O Ka XES of NbO, NbO 2 and Nb 2 O 5 .

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3.2. Band structure calculations and comparison with experiment The results of a self-consistent LMTO-TB band structure calculation for pure Nb and Nb-oxides are presented in Figs. 3–6. The XES measurements for niobium metal reduced to a binding energy scale is shown as dots in Fig. 3. The measurements are found to be in reasonably good agreement with the LMTO-TB calculation [16]. The features (a, b, c) of the total DOS are reproduced quite well in the Nb 5p partial DOS probably due to diffuse character of Nb 5p wave functions and their strong hybridization with Nb 4d functions. The good agreement between the calculated Nb 5p DOS and the observed Nb M 4,5 XES suggests that ignoring the perturbation of the core on the DOS is a reasonable approximation. A similar conclusion was arrived at in our recent publication [17], where the Nb M 4,5 XES of KNbO 3 was compared with the calculated DOS based on the Nb 5p DOS modulated by transition probabilities. Calculated total and partial density of states for NbO 2 and Nb 2 O 5 are presented in Figs. 4 and 5, respectively. The results from the M 4,5 XES and the O K XES (shown as the dotted curves) are reduced to a binding energy scale and compared with the calculations in the lower two panels of the figures. In the energy spectrum of both compounds one observes atomic-like oxygen 2s-bands with binding

Fig. 3. Total and partial calculated DOS of Nb. The bottom panel compares the calculated DOS with the experimental results shown as dots.

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Fig. 4. Total and partial DOS of NbO 2 . The third panel from the top compares the calculated DOS with the experimental results shown as dots. The solid curve in the lower panel is the 2s and 2p PDOS of oxygen.

Fig. 5. Total and partial DOS of Nb 2 O 5 . The third panel from the top compares the calculated DOS with the experimental results shown as dots. The solid curve in the lower panel is the 2s and 2p PDOS of oxygen.

energies of 19 and 15 eV for NbO 2 and Nb 2 O 5 , respectively. The next band has O 2p-character, and has binding energies of 4 and 0 eV for NbO 2 and Nb 2 O 5 , respectively. In the case of NbO 2 , where Nb atoms have d 1 configuration, the Fermi energy crosses the Nb 4d-band. This accounts for the metallic properties of this compound. In Nb 2 O 5 the niobium atoms have a d 0 configuration because all of the d-electrons have been transferred to the O 2p-band and the d-band is empty. The Fermi energy of this compound is located near the top of the valence band, which is typical for insulating transition metal oxides where the highest oxidation state of the metal ion yields unoccupied d-orbitals. This situation also occurs in TiO 2 , and V2 O 5 [18]. The values of the calculated and experimental band gap for Nb 2 O 5 are 3.53 and 3.9 eV, respectively [11]. Nb M 4,5 and O Ka XES are in reasonable agreement with calculated Nb 5p and O 2p partial DOS for both oxides. The energy separation of O 2s and O

2p-like bands is found to be smaller in theory than in the XES experiment, which is typical for transition metal oxides. This observation is related to the fact that the hole relaxation, effectively increases the binding energy of an electron having a comparatively localised state such as is found with O 2s. Our band structure calculation describes the ground state accurately, but does not include the relaxation produced by the hole [19]. The total and partial DOS for Nb 1.0 O 1.0 and Nb 0.75 O 0.75 are given in Fig. 6. All energies in these curves of the densities of states are referred to the Fermi energy (Ef1 ) of Nb 1.0 O 1.0 . In Nb 0.75 O 0.75 the Fermi energy (Ef2 ) is lowered slightly, and new features appear in the occupied part of the Nb 4d-band, in accord with the calculations reported in Ref. [4]. Some significant features can be observed in the DOS of Nb 0.75 O 0.75 . Band (c), having a binding energy of about 22.5 eV, has mainly O 2s-character. The O 2p-like band (b–b9) has a binding energy of about 9.5 eV and is

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Nb 4d-functions. The vacancy-induced Nb 4d-states and the splitting of the O 2p-band are observed in the Nb 5p DOS for Nb 0.75 O 0.75 and are less pronounced in Nb 1.0 O 1.0 . The measured Nb M 4,5 XES of NbO show a pronounced three-peak structure (features labelled 1, 2, 29 in Fig. 1) which we suggest can be associated with the calculated structure (a, b9, b) of Nb 5p DOS for Nb 0.75 O 0.75 and not Nb 1.0 O 1.0 . Finally we compare the computed O 2s DOS with the O Ka XES spectrum. The energy scale of the XES spectrum has been adjusted so that the small subband associated with hybridisation of 4d valence electrons with O 2p electrons is aligned with the calculated 4d states at a binding energy of 3.5 eV. The observed spectrum is in accord with the calculated O 2p DOS for Nb 0.75 O 0.75 , which is consistent with the conclusions obtained in our discussion of the M 4,5 spectrum in the previous paragraph.

4. Conclusions

Fig. 6. Total and partial DOS of Nb 1.0 O 1.0 and Nb 0.75 O 0.75 . The lower two panels are a comparison of the calculated DOS with the experimental results shown as dots. The dashed curve represents the partial DOS of Nb 0.75 O 0.75 , and the solid curve is the partial DOS of Nb 1.00 O 1.00 .

To conclude, X-ray emission spectra (XES) of the Nb valence–M 4,5 (5p→3d transition) are especially useful for the study of the electronic structure of Nb oxides because the Nb 5p partial density of states (DOS) is a good approximation of the total DOS in the valence band. The results of X-ray fluorescence measurements of NbO 2 , and Nb 2 O 5 compare well with self-consistent LMTO-TB band structure calculations. Natural NbO consists of 25% structural vacancies. We found that calculations, using Nb 0.75 O 0.75 with a hypothetical NaCl-structure, as the foundation for the structure of NbO adds structure to the DOS and provides a better match to the experiment than that of Nb 1.0 O 1.0 .

References separated from the atomic-like O 2s-band (c) by about 13.5 eV independent of the crystal formula. The O 2p-band has a more pronounced structure with distinct subbands for Nb 0.75 O 0.75 (b and b9) compared with a single broad band for Nb 1.0 O 1.0 (b). This difference could arise because of the reduction of the octahedral co-ordination in Nb 1.0 O 1.0 to the square-planar co-ordination in Nb 0.75 O 0.75 and / or because the real-space unit cell of Nb 1.0 O 1.0 contains one formula unit, whereas that of Nb 0.75 O 0.75 contains three. In the case of Nb 0.75 O 0.75 , for an energy range around the Nb 4d-like band (a), additional states are formed which undoubtedly are induced by the presence of vacancies (as well as possibly by a difference of crystal co-ordination). These vacancy-induced states are located below the Fermi level and are occupied with electrons having mostly Nb 4d-like character. We note that the value of the density of Nb 4d-states at the Fermi level for Nb 1.0 O 1.0 is less than that for Nb 0.75 O 0.75 . According to the calculation, the Nb 5p-wave functions are strongly hybridised with O 2p and

[1] J.B. Goodenough, in: H. Reiss (Ed.), Progress in Solid State Chemistry, Vol. 5, Pergamon, Oxford, 1971. [2] D. Adler, Rev. Mod. Phys. 40 (1968) 714. [3] J. Haines, J.M. Leger, A.S. Pereira, D. Hausermann, M. Hanfland, Phys. Rev. B 59 (2000) 13650. [4] H.J. Goldschmidt, in: Interstitial Alloys, Butterworths, London, 1967. [5] E. Wimmer, K. Schwarz, R. Podloucky, P. Herzig, A. Neckel, J. Phys. Chem. Sol. 43 (1982) 439. [6] J.H. Xu, T. Jarlborg, A.J. Freeman, Phys. Rev. B 40 (1989) 7939. [7] J.J. Jia, T.A. Callcott, J. Yurkas, A.W. Ellis, F.J. Himpsel, M.G. ¨ Samant, J. Stohr, D.L. Ederer, J.A. Carlisle, E.A. Hudson, L.J. Terminello, D.K. Shuh, R.C.C. Perera, Rev. Sci. Instrum. 66 (1995) 1394. [8] O.K. Andersen, Phys. Rev. B 12 (1975) 3060. [9] M. Hansen, K. Anderko, Constitution of Binary Alloys, Moscow, 1962. [10] A.K. Cheetham, C.N.R. Rao, Acta Crystallogr. B 32 (1976) 1579. ¨ [11] Landolt-Bornstein, in: Numerical Data and Functional Relationships in Science and Technology, New Series, Group III: Crystal and Solid State Physics, Vol. 17, Subvolume g, Springer, Berlin, 1984.

218

E.Z. Kurmaev et al. / Journal of Alloys and Compounds 347 (2002) 213–218

[12] V.E. Dolgikh, E.Z. Kurmaev, V.M. Cherkashenko, G.P. Shveikin, Phys. Metal. Metallogr. (USSR) 40 (1975) 664. [13] A. Dacca, G. Gemme, L. Mattera, R. Parodi, Appl. Surf. Sci. 126 (1998) 219. [14] F. Herman, S. Skillman, in: Atomic Structure Calculations, PrenticeHall, Englewood Cliffs, NJ, 1963. [15] D.R. Mueller, D.L. Ederer, J. van Ek, W.L. O’Brien, Q.-Y. Dong, J.J. Jia, T.A. Callcott, Phys. Rev. B54 (1996) 15034. [16] L.L. Boyer, D.A. Papaconstantopoulos, B.M. Klein, Phys. Rev. B 15 (1977) 3685.

[17] A. Moewes, A.V. Postnikov, B. Schneider, E.Z. Kurmaev, M. Matteucci, V.M. Cherkashenko, D. Hartmann, H. Hesse, M. Neumann, Phys. Rev. B 60 (1999) 4422. [18] L.D. Finkelstein, E.Z. Kurmaev, M.A. Korotin, A. Moewes, B. Schneider, S.M. Butorin, J.-H. Guo, J. Nordgren, D. Hartmann, M. Neumann, D.L. Ederer, Phys. Rev. B 60 (1999) 2212. [19] E.Z. Kurmaev, V.M. Cherkashenko, Y.u.M. Yarmoshenko, S.t. Bartkowski, A.V. Postnikov, M. Neumann, L.-C. Duda, J.-H. Guo, J. Nordgren, V.A. Perelyaev, W. Reichelt, J. Phys: Condens. Matter 10 (1998) 4081.