Electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2determined by polarized x-ray spectroscopy and Seebeck measurements

Abstract

Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, with large contemporary scientific and technological interest. Here we report on the anisotropic Seebeck coefficient in nanolaminated Ti 3 SiC 2 single-crystal thin films and trace the origin to anisotropies in element-specific electronic states. In bulk polycrystalline form, Ti 3 SiC 2 has a virtually zero Seebeck coefficient over a wide temperature range. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of Ti 3 SiC 2 , measured on single-crystal films, has a substantial and positive value of 4-6 μV/K. Employing a combination of polarized angle-dependent x-ray spectroscopy and density functional theory we directly show electronic structure anisotropy in inherently nanolaminated Ti 3 SiC 2 single-crystal thin films as a model system. The density of Ti 3d and C 2p states at the Fermi level in the basal ab plane is about 40% higher than along the c axis. The Seebeck coefficient is related to electron and hole-like bands close to the Fermi level, but in contrast to ground state density functional theory 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 plane is compensated by 73% enhanced Si 3d electronic states across the laminate plane that give rise to a negative Seebeck coefficient in that direction. Strong phonon vibration modes with three to four times higher frequency along the c axis than along the basal ab plane also influence the electronic population and the measured spectra by the asymmetric average displacements of the Si atoms. These results constitute experimental evidence explaining why the average Seebeck coefficient of Ti 3 SiC 2 in polycrystals is negligible over a wide temperature range. This allows the origin of anisotropy in physical properties of nanolaminated materials to be traced to anisotropies in element-specific electronic states.

Electronic structure, transport, and phonons ofSrAgChF(Ch = S, Se, Te): Bulk superlattice thermoelectrics

“…Alternately two dimensionality of the electronic structure, leading to open Fermi surfaces when doped can lead to this type of behavior. 37,38 This is the case here. Thus both the conductivity and thermopower indicate practically two dimensional transport for p-type samples.…”

Anisotropy of the resistivity and charge-carrier sign in nanolaminated Ti2AlC: Experiment andab initiocalculations

“…This explanation is coherent with Ti 3 SiC 2 Seebeck coefficient data which were recently published. 21 The resistivity measurements also clearly evidence a strong anisotropy of Ti 2 AlC electronic properties. Comparing the data recorded on the thin film to those obtained on the bulk sample, which are interpreted in terms of an effective medium approach, it is shown that the room temperature resistivity ρ zz is more than one order of magnitude larger than ρ xx .…”

“…This was demonstrated by Chaput et al who showed that the negligible Seebeck coefficient of a polycrystalline Ti 3 SiC 2 sample could be understood from the compensation of electronlike states along the c axis by holelike states within the basal plane orientation at the Fermi level. 5,20,21 An accurate description of both the electronic structure and the transport properties is all the more desirable that MAX phases exhibit complex Fermi surfaces with generally more than two bands [see Fig. 1(b) for Ti 2 AlC], 22 and that the chemical bonding in these materials is based on complex charge transfers involving metallic, ionic, and covalent bonds.…”

Tailoring of the thermal expansion of Cr2(Alx,Ge1−x)C phases

“…The MAX phases have been extensively studied not only in order to shed light on their unique combination of properties but also to explore their potential for numerous industrial applications (see review articles [1,2,3,4] and recent examples [5,6,7,8,9,10,11,12,13]). It has long been appreciated that a promising strategy to tailor properties is to form solid solutions on the M, A and/or X sites [1,2].…”