Structures and thermoelectric properties of co-doped quaternary chalcogenides of Cu 2 FeGeSe 4 Alaa Aldowiesh, Anthony V. Powell, Paz Vaqueiro University of Reading, UK Thermoelectric (TE) devices are capable of directly and reversibly converting waste heat into electricity. To improve the energy efficiency of a process, particularly those involving burning of fossil fuels and potentially making a contribution to address the energy crisis. In this study, TE materials comprised of earth abundant elements were synthesized and their TE performance was evaluated. The materials investigated are cation- ordered derivatives of zinc-blende, in which metal-centred tetrahedra are linked through their vertices. Series of copper-based p -type chalcogenide semiconductors, Cu 2 Fe 1- x Co x GeSe 4 (0 ≤ x ≤ 1), that exhibit promising thermoelectric properties, have been prepared by high temperature methods. Characterization by powder X-ray diffraction in conjunction with Rietveld refinement reveals that the materials adopt the stannite structure (space group I -42m) and that single phase behaviour is observed to x = 0.6 while at higher levels of Co 2+ reflections arising from an orthorhombic phase (space group F 222) begin to appear, which has an impact on the electrical properties. Measurement of electrical and thermal transport properties allowed determination of the TE figure of merit, ZT , while Hall effect measurements provide information on the mobility and carrier concentration. The TE performance is enhanced by the isovalent substitution of Fe 2+ by Co 2+ and the p -type behaviour is retained in all the substituted phases. The electrical resistivity, ρ, (Fig.1) is decreasing with substitution, which is primarily due to an increase in the hole mobility ,μ, while maintained the Seebeck coeffecnit even at small levels of substitution. In addition, substitution suppresses a discontinuity observed in r ( T ) of the iron end-member phase. The highest figure of merit reaches ZT = 0.5 at 825K for Cu 2 Fe 0.925 Co 0.075 GeSe 4 . This represents a 25% improvement in performance over that of the iron end-member phase, Cu 2 FeGeSe 4 (Fig.2).
Fig.1 The resistivity as a function of temperature or all samples in the series Cu 2 Fe 1- xCo x GeSe 4 (0 ≤ x ≤ 1).
Fig.2 Figure of merit as a function of temperature or all samples in the series Cu 2 Fe 1- xCo x GeSe 4 (0 ≤ x ≤ 1)
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© The Author(s), 2022
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