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The synthesis of thermoelectric materials (i.e. materials which convert heat into electricity and vice versa) is of primary importance for energy-saving and environmental science.  Conventional thermoelectric semiconductors such as chalcogenides, antimonides and Si-Ge alloys exhibit a high figure of merit ZT ≈ 1 at high temperature, but the lack of chemical stability at high temperature in air has prevented these materials from exploitation in practical applications.  Co- and Rh-based transition metal oxides (TMO) have recently been identifies as possible compelling thermoelectric materials due to their unusually large thermoelectric power, stability in air at elevated temperatures, and low costs [[1]].  The spin and orbital degrees of freedom of the d⁵ and d⁶ ions in the low spin state are believed to be the essential ingredients for the large thermoelectric power [[2]].  A strong interplay of different degrees of freedom such as charge, lattice and spin is a general characteristic of TMO, and it determines their physical properties.  In the research conducted so far, we targeted a specific compound, Mg-doped CuRhO₂, with the purpose of isolating a model system for addressing one of the key issues surrounding the physics of thermoelectric oxides: The fundamental role of spin and orbital degrees of freedom of the d⁵ and d⁶ ions for the onset of large thermoelectric power. 

With an overall figure of merit ≈ 0.2 at temperature as high as 1000 K, Mg-doped Rh oxides (CuRh_1‑xMg_xO₂) and delafossite-type structure have been found to demonstrate high potential for thermoelectric applications [[3]].  First principle electronic structure calculations and preliminary photoelectron spectroscopy investigations suggest instead that the topmost valence band in CuRhO₂ consists predominantly of Rh t_2g orbitals in which holes are introduced by Mg²⁺ substitutions [[4]].  The determination of the orbital character of the valence band in the Rh oxides constituted our immediate objective. 

We carried out experiments at Elettra Sincrotrone Trieste (Italy) and Advanced Light Source, (Berkeley CA) synchrotron radiation facilities. After routine characterization measurements, the first target in this research project was identifying the nature of the electronic states at the Fermi level. We carried out photoemission (PES), x-ray absorption (XAS) and x-ray emission (XES) spectroscopy experiments. The valence band PES spectra collected in angular integrated mode at room temperature show a strong dependence on the photon energy. By a direct comparison with theoretical photoemission cross section, it was possible to determine that Rh 4d states dominate the density of states (DOS) at the Fermi level (E_F). These results have been further corroborated by additional XES measurements. Determining unambiguously that the DOS at E_F is indeed dominated by Rh t_2g orbitals is of extreme importance for establishing the role of spin and orbital degrees of freedom for d⁵ and d⁶ ions in the LS state as being one of the essential ingredients for the large thermoelectric power in oxides.

Furthermore, the presence of satellite peaks in the Cu 2p core level PES spectra is indicative of the occurrence of strong correlations. A direct comparison between Rh 3d core levels in CuRh_0.9Mg_0.1O₂ and pure Rh metal shows that upon doping the Rd atoms have a metallic component.  

During the second year of the project we exploited the possibility of performing polarization dependent measurements on single crystals for studying the orbital character. We measured the XAS spectrum at Cu L₂₃ edge and O K edge with the electrical field vector of the incoming synchrotron radiation both parallel (in plane) and perpendicular (out of plane) to the sample surface. The difference between spectra recorded with out-of-plane and in-plane polarization constitutes the dichroic signal.  The use of linear polarization of synchrotron radiation (i.e. linear dichroism) is a powerful tool for probing the orbital symmetry and the directions of chemical bonds.  Contrary to the case of Cu L₂₃ and O K edges, we detected no strong dichroism at the Rh M₂₃ edge, but just a small signal comparable with the noise level.

Also the valence band photoemission spectra of single crystals show strong dichroic effects in correspondence to the Cu 3d bands and the O 2p band. In particular, the electronic structure in proximity to the Fermi level shows a clear out of plane character.  Cu has a higher DOS with out of plane character close to Fermi level while both oxygen and rhodium show a mixed character.  In order to unveil the topology of the Rh 4d states close to the Fermi level, we carried out resonant photoemission (ResPES) at the Rh M₂ edge. A ResPES experiment consists in collecting VB photoemission spectra while the photon energy scans trough a ionization edge, in this case Rh M₂.  Only the spectral features belonging to the excited Rh atom can respond with a resonant signal. When the measurement is carried out with in-plane geometry, no resonant signal is detected.  On the contrary, when the measurement is carried out with out-of plane geometry, a weak resonant signal can be observed. Given that the Rd states constitutes the states at the Fermi level, this results suggests that the empty orbitals directed perpendicular to the sample surface are more localized than the orbital located in the surface plane. From a macroscopic point of view this is consistent with the layered structure of the material.

The available funding in year 2010 have supported field work for one postdoctoral fellow (Dr. Paolo Vilmercati) to Sincrotrone Trieste, Italy and Advanced Lihgt Source, Berkeley CA, and the salary of a graduate student, Mrs. Amal Al-Wahish. Since June 2010, the analysis of these data has been carried out by Mr. Eric Martin.  Mr. Martin made outstanding progress on the project by analyzing in deteails recent ResPES data on single crystals samples.  Mr. Martin is going to give a talk on this topic at the upcoming March Meeting 2011 in Dallas, TX.  Our immediate plan is to start writing a manuscript.  

[[1]]    Terasaki et al., Phys. Rev. B 1997, 56, R12685-R12887   

[[2]]    Koshibae et al.,  Phys. Rev. Lett. 2001, 87, 236603-236606

[[3]]    Kuriyama et al., Proceedings of International Conference on Thermoelectrics, 2006.

[[4]]    Sasagawa, T.; Mannella, N. private communications.