Hébert, C.

Schattschneider, P.

Schlögl, R.

Su, D.

Willinger, M.

English

MPG.PuRe

Ab-Initio Simulation of the Oxygen 1s Ionisation Near Edge Structurein Vanadium PentoxideMark Willinger1,2, Dagnsheng Su1, Cecile Hebert2, Peter Schattschneider2, Robert Schlögl11Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin, Germany2Inst. für Angew. u. Techn. Physik, TU Wien, Wiedner Hauptstr. 8-10, A-1040 WienIntroductionVanadium oxides are of considerable importance in many catalytic processes. The specialchemistry of vanadium oxides results from the narrow d-states and their hybridization withthe ligand p-orbitals. To gain detailed information about the overlap of the orbitals invanadium pentoxide, the atom projected density of states (DOS) was calculated using the FP-LAPW (Full Potential Linearized Augmented Plane Wave) method [1]. EELS spectra fromV2O5 single crystals were recorded and compared with the simulated ELNES (Energy LossNear Edge Structure) spectra in order to support the interpretation of the different features.Ab-initio calculation and experimental:The program package WIEN97 (a FPLAPW package for calculating crystal properties) wasused to calculate the atom projected density of states.The ELNES at the oxygen K-edge wassimulated for the different oxygens (O1,O2,O3, Fig.1) via an extension to the WIEN97program which permits to separate different contributions to the inelastic scattering crosssection according to the character of the final state and an orientation dependent simulation ofELNES spectra [3]. A Philips CM200 FEG transmission electron microscope equipped with aGatan GIF 200 spectrometer was used to acquire EELS spectra in image coupled mode. Themicroscope was operated at 200kV.In Fig.2 the atom projected DOS of the vanadium 3d and vanadyl oxygen 2p orbitals aredisplayed. Due to the substantial hybridisation of the V 3d with the O 2p states, the distinctstructure of the partial V-DOS is reflected by the partial oxygen density of states. Within amolecular orbital picture, the overlap of the particular V 3d with O 2p orbitals can besummarised for the three kinds of oxygen as in Table I which is in good agreement with theresults of [2]. The smaller overlap with the ligand 2p states via p bonds is reflected by areduced dispersion of the conduction bands below 4.5 eV and a higher density of states of thet2g states (see Fig.2). On the other hand, the eg levels (higher band) point towards the ligand andappear at higher energies because they form relatively stronger s antibonding combinationswith the O 2p orbitals. The simulation shown in Fig. 3 reveals the contribution of the differentoxygens to the total oxygen edge as well as their contributions to the t2g and eg levels and is ingood 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)OxygenVanadiumO(1): vanadyl oxygenO(2): bridge oxygenO(3): chain oxygenEnergy [eV]t2g egTable I:V3d-O2p orbital overlapsO(1)      O(2)        O(3)V 3dxy  2px, 2py     2py    3dxz 2px      2pz        2pz    3dyz 2py      2pz    3dx2-y2  2px, 2py     2px    3d3z2-r2 2pzFig.3: Contributions of the different oxygens to thetotal simulated oxygen K-edgeO-K edge of  V2O5525 530 535 540 545 550 555 560 565Energyloss [eV]Intensity [arb. units] ExperimentSimulationFig.4: Comparison between simulation and EELSexperiment.V DOSO(1) DOSReferences[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) 9-16AcknowledgementsThe 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"t2geg

Ab-Initio Simulation of the Oxygen 1s Ionisation Near Edge Structure in Vanadium Pentoxide

Abstract

Similar works

Full text

Available Versions

MPG.PuRe

MPG.PuRe