The impact of site selectivity and disorder on the thermoelectric properties of Yb21Mn4Sb18 solid solutions: Yb21Mn4−xCdxSb18 and Yb21−yCayMn4Sb18

Abstract

Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb₂₁Mn₄Sb₁₈ phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K⁻¹) and low thermal conductivity (0.4 W m⁻¹ K⁻¹). These characteristics stem respectively from the unique [Mn₄Sb₁₀]²²⁻ subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb₂₁Mn_4−xCd_xSb₁₈ (x = 0, 0.5, 1.0, 1.5) and Yb_21−yCa_yMn₄Sb₁₈ (y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m⁻¹ K⁻¹) and that the Seebeck coefficients remain high (>200 μV K⁻¹). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T⁻¹ to T^−0.5, suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZT_avg ∼ 0.6 from 300 K to 800 K.