Thermoelectric properties of the misfit cobaltate Ca3Co4O9

Abstract

The layered misfit cobaltate Ca 3 Co 4 O 9 , also known as Ca 2 CoO 3 [CoO 2 ] 1.62 , is a promising p-type thermoelectric oxide. Employing density functional theory, we study its electronic structure and determine, on the basis of Boltzmann theory within the constant-relaxation-time approximation, the thermoelectric transport coefficients. The dependence on strain and temperature is determined.In particular, we find that the xx-component of the thermopower is strongly enhanced, while the yy-component is strongly reduced, when applying 2% tensile strain. A similar anisotropy is also found in the power factor. The temperature dependence of the conductivity in the a-b plane is found to be rather weak above 200 K, which clearly indicates that the experimentally observed transport properties are dominated by inhomogeneities arising during sample growth, i.e., are not intrinsic.

Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO3/LaAlO3(001) superlattices

“…Another promising materials class for thermoelectric applications is cobaltates such as the mentioned Pd/Pt-based delafossites 10 or, in particular, the misfit-layered oxide Ca 3 Co 4 O 9 (CCO). [43][44][45] Remarkably, the 1/1 SLs can compete with this material, regardless of the underlying substrate (Table I). For a STO and a DSO , both the in-and cross-plane Seebeck coefficients of the 1/1 SLs are close to (or even exceed) the in-plane DFT results for pristine CCO, which are already much higher than DFT results for CCO strained to a STO .…”

“…The room-temperature values for the Seebeck coefficient and the power factor are compared to other topical oxide systems like La-doped STO 9,19,23 or layered cobaltates. [43][44][45] By contrasting the ultrathin SLs to the metallic longer-period (LNO) 3 /(LAO) 3 (001) case we point out the relevance of the MIT in obtaining a high thermoelectric response.…”

Crossover from itinerant to localized states in the thermoelectric oxide [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

“…Note that the band structure is also proposed theoretically but there is a difficulty originating from the misfit structure of this system, in which an approximate formula is required for calculations [32,[46][47][48][49]. In fact, the calculated thermopower strongly depends on the calculation methods [32,49]. Thus, the present results may offer an experimental clue for the challenging issue to theoretically obtain the transport properties precisely in correlated electron systems with an incommensurate structure like this material.…”

“…To clarify the origin of the anisotropic transport at low temperatures, we discuss the electronic band structure measured by ARPES at T = 40 K [31]. Note that the band structure is also proposed theoretically but there is a difficulty originating from the misfit structure of this system, in which an approximate formula is required for calculations [32,[46][47][48][49]. In fact, the calculated thermopower strongly depends on the calculation methods [32,49].…”

Tuning the Thermoelectric Properties of Transition Metal Oxide Thin Films and Superlattices on the Quantum Scale

“…Another prominent class of transition metal oxides is layered cobaltates such as the record‐value material , featuring a PF of , [ 14 ] or the misfit‐layered (CCO), [ 15,16 ] where values of and have been measured around room temperature for CCO films on STO(001). [ 17 ] Moreover, out‐of‐plane Seebeck coefficients of were predicted in delafossites such as and .…”