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The Electronic Structure of the Ti 4 O 7 Magneli Phase

The Electronic Structure of the Ti 4 O 7 Magneli Phase. Leandro Liborio Giuseppe Mallia Nicholas Harrison Computational Materials Science Group. Magneli Phases. 1). 2). Metal nets in antiphase. (121) r Cristallographic shear plane. .

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The Electronic Structure of the Ti 4 O 7 Magneli Phase

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  1. The Electronic Structure of the Ti4O7 Magneli Phase Leandro Liborio Giuseppe Mallia Nicholas Harrison Computational Materials Science Group

  2. Magneli Phases 1) 2) Metal nets in antiphase. (121)r Cristallographic shear plane. TnO2n-1 composition. For n between 3 and 9 shear planes are the (121). Ti4O7 is a semiconductor at T<120K semicond. with 0.25eV band gap(1). T4O7 Metal-semicond. transition at ~150K, semicond-semicond. trans. at T~135K. (1) D. Kaplan et al., Philosophical Mag., Vol. 36, pp. 1275, 1977. (2) P. Waldner and G. Eriksson, Calphad Vol. 23, No. 2, pp. 189-218, 1999.

  3. Ti4O7 Magneli Phase: Electric and Magnetic properties 1) 2) Conductivity of Ti4O7 single crystals • Ti4O7 conductivity is higher than the graphite one. • 3 well-differentiated phases. • semicond-semicond and semicond-metal transitions. • Exp. Evidence suggests: Charge localization on the Ti atoms changes at every phase. Table (1) 1) J. R. Smith et al, J. Appl. Electroch., 28, pp 1021, (1998). 2) S. Lakkis et al, PRB., 14, pp 1429, (1976). 3) L. N. Mulay et al, J. of Appl. Phys., 41, pp 877 , (1970).

  4. 2) Ti4O7 Magneli Phase: The Bipolaron Model 1) 3) Charge distribution at low and intermediate T H.T.P. I.T.P. L.T.P. • Ti-Ti pairs: small on-site localised bipolarons, which are bound states of two Ti+3 ions stabilised by a lattice distortion. • In the low T phase the Ti 3d electrons forming the bipolarons were paired in non-magnetic bonds. The bipolarons were ordered. • In the intermediate temperature phase the bipolarons disordered. • In the high temperature phase the bipolarons dissociated and the 3d electrons delocalized. 1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973). 2) S. Lakkis et al, PRB., 14, pp 1429, (1976).

  5. 1) Intensity of the EPR signal as a function of T. Ti4O7 Magneli Phase: The Bipolaron Model drawbacks • No intrinsic EPR signal in the bipolaron model. • New model for the 140K phase. This structure shows long range order: the bipolarons are still present, but they are not disordered (Reference (1)). 1) Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984). 2) S. Lakkis et al, PRB., 14, pp 1429, (1976).

  6. Ab inito calculations CRYSTAL Hybrid density functional: B3LYP, GGA Exchange GGA Correlation 20% Exact Exchange Local basis functions: atom centred Gaussian type functions. Ti: 27 atomic orbitals, O: 18 atomic orbitals Supercell approach. Ti4O7 structures at the low, intermediate and high temperature phases taken from M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973). Ti4O7 structure at the different phases taken from Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).

  7. 298 K 140 K 120 K Ti atoms in Ti4O7. View of a (1x2x2) supercell along the tetragonal Ti4O7 unit cell a axis. Spin +/- 0.05e/bohr3 isodensity surfaces. • At 120K the spin localises in Ti+3 t2g-like orbitals which are antiferromagnetically coupled forming dimmers. • The 140K and 298K structures are essentially equivalent. • At 298K the electrons delocalise. Ti4O7 low, intermediate and high T structures from Marezio et al (1). (1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).

  8. 298 K 140 K 120 K Ti atom µ = nα-nβ 298 K 140 K 120 K 1 -0.4491 -0.4417 -0.8722 3 0.4199 0.6719 -0.0259 5 0.3311 -0.3796 0.8748 Ti t2g-like spin population in bohr magnetons 7 -0.3027 0.1263 0.0387 Ti4O7 low, intermediate and high T structures from Marezio et al (1).

  9. Ti4O7 new intermediate T structure from Le Page et al (2) • At 140 K the spin is localised in Ti+3 t2g-like orbitals. Only a subset of these Ti+3 ions form antiferromagnetically bonded pairs. • The 140 K electronic structure is not a bipolaronic state: there is a mixture of polarons and bipolarons. (2) Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).

  10. 1) Intensity of the EPR signal as a function of T. Interpretation of magnetic measurements Susceptibility measurements • Ferromagetic. • Flip the spin of half of the elctrons forming bipolarons. • Flip the spin of half of the electron forming polarons. • Lowest energy for change: 0.1 eV per Ti4O7 unit. EPR measurements

  11. Conclusions • We propose an alternative interpretation of the Ti4O7 electronic structure. • The Ti4O7 120K phase is an antiferromagnetic charge-ordered semiconducting state. • The Ti4O7 120K phase is a bipolaronic state, but the bipolarons are NOT covalently bonded: the spin localises in t2g-like orbitals belonging to Ti+3 ions, and these ions are antiferromagnetically coupled. • According to our calculations, in the new ordered structure for the 140 K phase, spin localises in Ti+3 t2g-like orbitals. But the 140K state is not a bipolaronic state: there is a mixture of polarons and bipolarons. • In the 298K phase electrons delocalise and spin moments decrease their value. • Our results provide a sensible explanation for the behaviour of the magnetic susceptibility and EPR measurements with temperature.

  12. Ti4O7 low, intermediate and high T DOS. 298 K 120 K 140K (1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).

  13. 1) 2) 3) DOS for T<120K Optical Properties Is the theoretical band gap reasonable? • Some proposed absorption mechanisms: • Singlet-Triplet mechanism by Lakkis. • Spin flipping: Antiferro-Ferro • E(120K)Ferro-E(120K)Antiferro = 0.3 eV • Infrared active phonon modes 1) D. Kaplan et al, Phil. Mag., 36, pp 1275, (1977). 2) S. Lakkis et al, PRB., 14, pp 1429, (1976).

  14. 2) Ti atoms in Ti4O7. View of a (1x2x2) supercell along the tetragonal Ti4O7 unit cell a axis. Temperature T<120K. 3) DOS for T<120K Ti4O7 ground state • Charge-ordered semiconductor • 1.25 eV band gap • Antiferromagnetic state. • Ti+3 pairs with opposite spin on each Ti+3: bipolaron. • Bipolaronic ground state.

  15. 140 K 298 K Ti4O7 high and intermediate temperature phase • Semiconductor with charge distributed homogeneously • 0.4 eV band gap • Antiferromagnetic state (smaller spin values). • Bipolarons are gone M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973). Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).

  16. Charge distribution at low and high T View of Ti4O7 alongit’s aaxis. Ti are yellow and oxygen are red. Only some oxygen octahedra are plotted. Ti4O7 Magneli Phase: Electric and Magnetic properties

  17. Cp/T. The Debye temp is 493. Inset shows Debye temp as a function od T. Ti4O7 Magneli Phase: Alternative Magnetic measurements 2)

  18. Ti4O7 Magneli Phase: On the controversial semiconductor-metal transition A. D. Inglis et al, J. Phys. C: Solid State Phys., 16, pp 317, (317).

  19. Ti4O7 Magneli Phase: Optical Properties

  20. STM image of TiO2(110) surface. Ubias=2V, IT=0.2nA Magneli Phases at surfaces 1) 2) • TiO2(110) wafers were annealed in UHV for 24 h at 800ºC. • Sample from yellowish to blue. • STM at room temperature. 3) 4) Step to step distance is 17.6 Å along [-111] Periodicity along [-11-1] is 8.8 Å H. Norenberg et al, Surf. Sci., 402-404, pp 738, (1998).

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