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

Thermoelectricity Seebeck Effect Thermoelectrics are optimized by three parameters: 1.Minimization of electric resistivity (ρ) 2.Minimization of thermal conductivity (κ) 3.Maximization of thermopower (S TEP ): Figure of merit:

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

Copper based delafossite-type structure Hexagonal RhO 2 layer isomorphic to the hexagonal CoO 2 layer in Na x CoO 2 Insulator-Metal transition upon Mg doping Shibasaki et al: PRB 74, 1 (2006) CuRh 1-x Mg x O 2 Delafossites Maignan et al: PRB 80, 6 (2009)

Similar thermoelectric properties at high temperatures to Cobaltates At T > 1000K, ZT ≈ 0.15 Power Factor (S 2 /ρ) almost twice that of NaCoO 2 CuRh 1-x Mg x O 2 Delafossites Maignan et al: PRB 80, 6 (2009)

Role of d 5 -d 6 states in High Thermopower d 6 ion: Co 3+ d 5 ion: Co 4+ 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) States at E F : are they Rh-derived? Establish the importance of the d 5 -d 6 states for the microscopic description of thermopower

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

Photoemission (PES) Excite electron with photon (energy: hυ) Measure Kinetic Energy of photo emitted electrons KE = hυ – BE – ϕ sp

Polarization Dep. Photoemission

Resonant PES

hυ Spectral weight at E F decreases in proximity of Rh Cooper minimum Suggests states at E F are Rh derived Yeh, Lindau: Atomic Data and Nuclear Data Tables 32, 7-8 (1985) Photon dependence of photoemission cross section

Valence Band Fitting

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

hυ Intensity (arb. units)

XPS Quantitative Analysis = differential photoelectron cross section for peak k ρ k = atomic volume density of peak k Ω = acceptance angle D 0 = instrumental detection efficiency a = thickness of layer Λ e = inelastic mean free path Both Rh and Cu are in valence band: E Rh ≈ E cu By stoichiometry: ρ Rh ≈ ρ Cu

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.

Summary Determination of the states at E F for CuRh 1-x Mg x O 2 with PES, RESPES and XES Spectral weight at E F decreases in proximity of Rh Cooper minimum + N Rh /N Cu  Rh/Cu Cross Section ratio States at the Fermi level (those involved in transport) are Rhodium derived Establish the importance of d 5 /d 6 ionic states for a correct microscopic theoretical description of thermopower in oxides

Backup Slides

Co 3+ Co 4+ Co 3+ t 2g egeg egeg Degeneracy 6 Entropy k B ln6 Charge of e flows with an entropy of k B ln6 Degeneracy 1 Entropy 0 Koshibae et al. PRB 62, 6869 (2000) Na x CoO 2 x ~ 0.5 Co 3+ :Co 4+ =1:1 CuRh 0.9 Mg 0.1 O 2 6-fold Degeneracy Theory of High Thermopower

CuRh 0.9 Mg 0.1 O 2 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) 1997: Thermoelectic Sodium Cobalt Oxides

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

Partial DOS over Valence Band

XPS Quantitative Analysis = differential photoelectron cross section for peak k ρ k = atomic volume density of peak k Ω = acceptance angle D 0 = instrumental detection efficiency a = thickness of layer Λ e = inelastic mean free path Both Rh and Cu are in valence band: E Rh ≈ E Cu