Electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2 determinedby polarized x-ray spectroscopy and Seebeck measurements [Elektronisk resurs]

• Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, withlarge contemporary scientific and technological interest. Here we report on the anisotropic Seebeck coefficient innanolaminated Ti3SiC2 single-crystal thin films and trace the origin to anisotropies in element-specific electronicstates. In bulk polycrystalline form, Ti3SiC2 has a virtually zero Seebeck coefficient over a wide temperaturerange. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of Ti3SiC2, measured on single-crystalfilms, has a substantial and positive value of 4–6 μV/K. Employing a combination of polarized angle-dependentx-ray spectroscopy and density functional theory we directly show electronic structure anisotropy in inherentlynanolaminated Ti3SiC2 single-crystal thin films as a model system. The density of Ti 3d and C 2p states atthe Fermi level in the basal ab plane is about 40% higher than along the c axis. The Seebeck coefficient isrelated to electron and hole-like bands close to the Fermi level, but in contrast to ground state density functionaltheory modeling, the electronic structure is also influenced by phonons that need to be taken into account.Positive contribution to the Seebeck coefficient of the element-specific electronic occupations in the basal planeis compensated by 73% enhanced Si 3d electronic states across the laminate plane that give rise to a negativeSeebeck coefficient in that direction. Strong phonon vibration modes with three to four times higher frequencyalong the c axis than along the basal ab plane also influence the electronic population and themeasured spectra bythe asymmetric average displacements of the Si atoms. These results constitute experimental evidence explainingwhy the average Seebeck coefficient of Ti3SiC2 in polycrystals is negligible over a wide temperature range. Thisallows the origin of anisotropy in physical properties of nanolaminated materials to be traced to anisotropies inelement-specific electronic states. 

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Natural Sciences  (hsv)

Naturvetenskap  (hsv)

Indexterm och SAB-rubrik

MAX-phases

nanolaminates

Seebeck coefficients

x-ray absorption

emission spectroscopy