Hébert, C.

Schattschneider, P.

Schlögl, R.

Su, D.

Willinger, M.

English

MPG.PuRe

Site Specific Ab-Initio Calculations of the Oxygen 1s Ionisation Edge in Vanadium Pentoxide  M. Willinger1,2, D.S. Su1, C. Hebert2, P. Schattschneider2, R. Schlögl1 1Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin, Germany 2Inst. für Angew. u. Techn. Physik, TU Wien, Wiedner Hauptstr. 8-10, A-1040 Wien  1. Introduction Vanadium oxides are of considerable importance in many catalytic processes. The special chemistry of vanadium oxides results from the narrow d-states and their hybridization with the ligand p-orbitals. To gain detailed information about the overlap of the orbitals in vanadium pentoxide, the atom projected density of states (DOS) was calculated using the FP-LAPW (Full Potential Linearized Augmented Plane Wave) method [1]. EELS spectra from V2O5 single crystals were recorded and compared with the simulated ELNES (Energy Loss Near Edge Structure) spectra in order to support the interpretation of the different features. 2. Structure of V2O5 V2O5 forms an orthorhombic (Pmmn) layer crystal. The physical layers with normal direction along [001] are composed of distorted VO6 octahedra sharing edges and corners, see Fig. 1. The elementary cell contains two formula units. There are three different types of oxygens: terminal (vanadyl), O(1), oxygen bridging two vanadium centers, (bridge) O(2), and oxygen coordinated to three vanadium centers, (chain) O(3) [2]. The differently coordinated oxygen atoms exhibit quite different reaction activity and therefore, their electronic structure is the subject of the present work. 3. Ab-initio calculation and experimental: The program package WIEN97 (a FPLAPW package for calculating crystal properties) was used to calculate the atom projected density of states. The ELNES at the oxygen K-edge was simulated for the different oxygens (O1,O2,O3) via an extension to the WIEN97 program which permits to separate different contributions to the inelastic scattering cross section according to the character of the final state and an orientation dependent simulation of ELNES spectra [3]. A Philips CM200 FEG transmission electron microscope equipped with a Gatan GIF 200 spectrometer was used to acquire EELS spectra in image coupled mode. The microscope was operated at 200kV.  4. Results and discussions In Fig.2 the atom projected DOS of the vanadium 3d and vanadyl oxygen 2p orbitals are displayed. Due to the substantial hybridisation of the V 3d with the O 2p states, the distinct structure of the partial V-DOS is reflected by the partial oxygen density of states. For example, the oxygen 2pz overlaps with vanadium 3d3z2-r2 and the oxygen 2px with the 3dxz orbital, respectively. Within a molecular orbital picture, the overlap of the particular V 3d with O 2p orbitals can be summarised for the three kinds of oxygen as in Table I which is in good agreement with the results of [2]. Crystal field splitting leads to formation of t2g states at the lower edge of the conduction band and eg bands at higher energies. The t2g levels (lower band) correspond to V 3d orbitals which point in between the ligands. These states have a smaller overlap with ligand O 2p sites and form weaker p antibonding combinations. The smaller overlap with the ligand 2p states via p bonds is reflected by a reduced dispersion of the conduction bands below 4.5 eV and a higher density of states of the t2g states (see Fig.2). On the other hand, the eg levels (higher band) point towards the ligand and appear at higher energies because they form relatively stronger s antibonding combinations with the O 2p orbitals. The simulation shown in Fig. 3 reveals the contribution of the different oxygens to the total oxygen edge as well as their contributions to the t2g and eg levels and is in good agreement with the EELS measurements (Fig. 4).  Fig.2: DOS: Conduction bands of vanadium           d-orbitals and vanadyl-oxygen p-orbitals. 0 1 2 3 4 5 6 7 8O - pp zp xp y0 1 2 3 4 5 6 7 8DOSVdVdxx-yyVd3zz-rrVdxzVdyzVdxyFig.1: Crystal structure of V2O5  O(1) O(2) O(3) O(3) O(3) O(1) Oxygen Vanadium O(1): vanadyl oxygen O(2): bridge oxygen O(3): chain oxygen Energy [eV] t2g eg Table I:V3d-O2p orbital overlaps   O(1)      O(2)        O(3)  V 3dxy    2px, 2py     2py     3dxz  2px      2pz        2pz     3dyz  2py      2pz     3dx2-y2   2px, 2py     2px     3d3z2-r2 2pz  Fig.3: Contributions of the different oxygens to the total simulated oxygen K-edge  O-K edge of  V2O5525 530 535 540 545 550 555 560 565Energyloss [eV]Intensity [arb. units] ExperimentSimulationFig.4: Comparison between simulation and EELS experiment. V DOS O(1) DOS References [1] P. Blaha, K. Schwarz, J. Luitz, A full potential      linearized augmented plane wave package for       calculating crystal properties, WIEN97, K. Schwarz,      Techn. Univ. Wien, Austria 1999 [2] V.Eyert, K.H. Höck, Phys. Rev. B 57,12727 (1998) [3] C. Hébert-Souche, P.H. Louf, P. Blaha, M.Nelhiebel,       J. Luitz, P. Schattschneider, K. Schwarz, B. Jouffrey,        Ultramicroscopy, 83 (2000) 9-16 Acknowledgements The work is supported by SFB 546 of Deutsche Forschungsgemeinschaft (DFG) and by the Austrian FWF, project P14038-PHY. O-K edge of V2O50 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30Energy above FermiIntensity [arb. units]O(1)O(2)O(3)O "total"t2g eg 

Site Specific Ab-initio Calculations of the Oxygen 1s Ionisation Edge in Vanadium Pentoxide

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