AmericanChemicalSociety.com

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.  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.  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.  While for most CuMO₂ delafossites (M = Al, Ga, In etc) the predominant character of the valence band is Cu d_z2 orbitals, 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.  The determination of the orbital character of the valence band in the Rh oxides and the comparison with that of CuMO₂ delafossites (M = Al, Ga, In etc) constituted our immediate objective. 

We carried out experiments Elettra Sincrotrone Trieste radiation facility (Trieste, Italy) and Advanced Light Source, berkeley CA. The spectra collected in angular integrated mode at room temperature on CuRh_0.9Mg_0.1O₂ single crystals.  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 Resonant Photoemission (RESPES) and XES measurements. Determining unambiguously that the DOS at E_F is indeed dominated by Rh t_2g orbitals will 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 ametallic component.

In the upcoming fiscal year we will focus on carrying on systematic studies of this type in order to observe the impact of Mg substitution on the electronic structure. The available funding ($10K) in year 2009 have supported field work and the travel expenses for the PI and one postdoctoral fellow (Dr. Paolo Vilmercati) to Sincrotrone Trieste, Italy and Advanced Lihgt Source, Berkeley CA.  A graduate student, Mrs. Amal Al-Wahish started to work on the data analysis.  The budget ($40,000) for fiscal year 2010 will cover her salary and expenses for field work consisting of data collection at synchrotron radiation facilities.