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Electronic Structure Determination of CuRh 1-x Mg x O 2 using Soft X-Ray Spectroscopies

This research paper discusses the electronic structure determination of CuRh1-xMgxO2 using soft X-ray spectroscopies. The study focuses on the thermoelectric properties of the material and the importance of d5-d6 states in describing the thermopower in oxides.

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Electronic Structure Determination of CuRh 1-x Mg x O 2 using Soft X-Ray Spectroscopies

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  1. Electronic Structure Determination of CuRh1-xMgxO2 using Soft X-Ray Spectroscopies Eric Martin1, Paolo Vilmercati1, Christine Cheney1, Takao Sasagawa2, Federica Bondino3, Elena Magnano3, Norman Mannella1 1Dept. Of Physics and Astronomy, University of Tennessee 2Materials and Structures Laboratory, Tokyo Institute of Technology 3Elettra Sincrotrone Trieste

  2. Thermoelectricity • Seebeck Effect • Thermoelectrics are optimized by three parameters: • Minimization of electric resistivity (ρ) • Minimization of thermal conductivity (κ) • Maximization of thermopower (STEP): Figure of merit:

  3. Thermoelectric - Cobalt Oxides Terasaki: IEEE Intl. Conf. on Thermoelectrics, 289 (2005) • Use at elevated temperatures (up to 1000 K) • Stability in air

  4. CuRh1-xMgx O2 Delafossites • Copper based delafossite-type structure • Hexagonal RhO2 layer isomorphic to the hexagonal CoO2 layer in NaxCoO2 • Insulator-Metal transition upon Mg doping Shibasaki et al: PRB 74, 1 (2006) Maignan et al: PRB 80, 6 (2009)

  5. CuRh1-xMgx O2 Delafossites • Similar thermoelectric properties at high temperatures to Cobaltates • At T > 1000K, ZT ≈ 0.15 • Power Factor (S2/ρ) almost twice that of NaCoO2 Maignan et al: PRB 80, 6 (2009)

  6. States at EF: are they Rh-derived? • Establish the importance of the d5-d6 • states for the microscopic description • of thermopower Role of d5-d6 states in High Thermopower • d6 ion: Co3+ • d5 ion: Co4+ • Different states due to the interplay between Hund’s rule and crystal field • These degenerate states are believed to be responsible for thermopower in oxides • Details of the electronic structure are crucial Koshibae et al: PRB 62, 6870 (2000)

  7. Initial Calculations Suggest Rh at the Fermi edge Maignan et al: PRB 80, 4 (2009)

  8. Photoemission (PES) • Excite electron with photon (energy: hυ) • Measure Kinetic Energy of photo emitted electrons • KE = hυ – BE – ϕsp

  9. Polarization Dep. Photoemission

  10. Resonant PES

  11. Photon dependence of photoemission cross section hυ 60 65 70 75 80 90 100 105 110 120 130 150 170 200 Yeh, Lindau: Atomic Data and Nuclear Data Tables 32, 7-8 (1985) Spectral weight at EF decreases in proximity of Rh Cooper minimum Suggests states at EF are Rh derived

  12. Valence Band Fitting

  13. X-Ray Emission (XES) Partial Density of States with Elemental Sensitivity 4.8 eV O 2p-1s transition Excitation hυ = 545eV Intensity (arb. units) 1.83 eV

  14. hυ 60 65 70 75 80 90 100 105 110 120 130 150 170 200 Intensity (arb. units)

  15. XPS Quantitative Analysis Both Rh and Cu are in valence band: ERh ≈ Ecu By stoichiometry: ρRh ≈ ρCu = differential photoelectron cross section for peak k ρk = atomic volume density of peak k Ω = acceptance angle D0 = instrumental detection efficiency a = thickness of layer Λe = inelastic mean free path

  16. Rh/Cu intensity ratio determined via peak fitting normalized wrt Cu/Rh atomic densities compared with photoemission cross sections. Emphasizes that the states found at the Fermi level are Rhodium derived.

  17. Summary • Determination of the states at EF for • CuRh1-xMgxO2 with PES, RESPES and XES • Spectral weight at EF decreases in proximity of Rh Cooper minimum + NRh/NCu Rh/Cu Cross Section ratio • States at the Fermi level (those involved in transport) are Rhodium derived • Establish the importance of d5/d6 ionic states for a correct microscopic theoretical description of thermopower in oxides

  18. Backup Slides

  19. 6-fold Degeneracy Theory of High Thermopower CuRh0.9Mg0.1 O2 eg t2g Co4+ Co3+ Co3+ Co3+ Co3+ Co3+ Co3+ eg t2g Degeneracy 6Entropy kBln6 Degeneracy 1Entropy 0 NaxCoO2 x ~ 0.5 Co3+:Co4+=1:1 Charge of e flows with an entropy of kBln6 Koshibae et al.PRB 62, 6869 (2000)

  20. 1997: Thermoelectic Sodium Cobalt Oxides CuRh0.9Mg0.1 O2 Resistivity: In-plane 200 cm at 300 K Out-of-plane 8 mcm at 300 K Themopower (Entropy per carrier): In-plane 100 V/K at 300 K (Terasaki : PRB 56 (1997) R12685) Thermal conductivity:(Data are scattered from sample to sample) In-plane 40 mW/cmK at 300 K (Satake: JAP 96 (2004) 931)

  21. X-Ray Emission (XES) • Excite core hole electron • Dipole Transition: Valence electrons fill unoccupied states • Photon energies can be measured to determine the density of states of the valence band • Partial density of states (DOS)with elemental sensitivity

  22. Partial DOS over Valence Band

  23. XPS Quantitative Analysis Both Rh and Cu are in valence band: ERh ≈ ECu = differential photoelectron cross section for peak k ρk = atomic volume density of peak k Ω = acceptance angle D0 = instrumental detection efficiency a = thickness of layer Λe = inelastic mean free path

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