Electronic structure and optical, mechanical, and transport properties of the pure, electron-doped, and hole-doped Heusler compound CoTiSb

Abstract

The Heusler compound CoTiSb was synthesized and investigated theoretically and experimentally with respect to electronic structure and optical, mechanical, and vibrational properties. The optical properties were investigated in a wide spectral range from 10 meV to 6.5 eV and compared with ab initio calculations. The optical spectra confirm the semiconducting nature of CoTiSb, with a strong exciton absorption at 1.83 eV. The calculated phonon dispersion as well as elastic constants verify the mechanical stability of CoTiSb in the cubic C1 b system. Furthermore, solid solution series of CoTi 1−x M x Sb (M = Sc, V and 0 x 0.2) were synthesized and investigated. The transport properties were calculated by all-electron ab initio methods and compared to the measurements. The thermoelectric properties were investigated by measuring the temperature dependence of electrical resistivity, Seebeck coefficient, and thermal conductivity. The thermal conductivity of the substituted compounds was significantly reduced. Sc substitution resulted in a p-type behavior with a high Seebeck coefficient of + 177.8 μV/K (350 K) at 5% Sc substitution. This value is in good agreement with the calculations. Fully relativistic Korringa-Kohn-Rostoker calculations in combination with the coherent potential approximation clarify the different contribution of states in the (001) plane of the Fermi surface for Sc-or V-substituted compounds CoTi 0.95 M x Sb (M = Sc, V).

Miscibility Gap in the Phase Diagrams of Thermoelectric Half-Heusler Materials CoTi $$_{1-x}Y_x$$ 1 - x Y x Sb (Y = Sc, V, Mn, Fe)

“…This leads to an increased electrical conductivity and a reversed sign of the Seebeck coefficient, resulting in p-type behavior with a high Seebeck coefficient of +260 lV/ K at 700 C. Such behavior was previously reported for Sc-substituted CoTiSb. 32 In the case of substitution by 10% Mn (3d 5 4s 2 ) or 10% Fe (3d 6 4s 2 ), the alloys are overdoped and become metallic with reduced Seebeck coefficient and ZT.…”

Half‐Heusler materials as model systems for phase‐separated thermoelectrics

“…In addition to the methods used in Refs. , a quantum theory of atoms in molecules (QTAIM) () analysis of the Bader charges was performed based on the Wien2 k calculations using Critic 2 .…”

A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal <i>X<sub>2</sub>YZ</i> Compounds

“…The values estimated in this way are generally larger than Debye temperatures determined from phonon calculations or in experiments [59] because the optical phonon branches are neglected when the elastic constants are used for calculating the "acoustical" Debye temperature acc D θ . In a similar way, the average sound velocities can be used to estimate the "acoustical" Grüneisen parameter ac ζ [60].…”