Structure And Thermoelectric Properties Of EuBSb(B=Ag, Cu) Base Zintl Phase Compounds

Thermoelectric?TE?material is a kind of functional materials with capability of realizing the direct conversion between thermal energy and electrical energy,which has been widely applicated in many areas,such as power generation and solid-state refrigeration.The efficiency of TE device is closely related to the figure-of-merit ZT of the TE materials.Therefore,it is of significance to enhance the ZT values.The Zintl phase compound is a typical"phonon glass-electron crystal".It was found recently that a new 2-1-2 type Zintl phase compound Eu₂ZnSb₂ has an ultra-low lattice thermal conductivity due to the effective phonon scattering by the intrinsic heavier Eu atomic layer and plenty of Zn vacancies.The lattice thermal conductivity is⁰.37 W m^-1K^-1 at 623 K.A state-of-the-art ZT?1.0 at 823 K was achieved in Eu₂Zn_0.98Sb₂,indicating a new promising TE material.In this paper,a new 1-1-1 type EuBSb?B=Ag,Cu?-based Zintl phase TE material with the same crystal structure as Eu₂ZnSb₂ is studied.The research results were presented as follows:A new 1-1-1 type EuAgSb and EuCuSb single-phase samples were prepared by directly high-energy ball milling combined with spark plasma sintering.Compared with Eu₂ZnSb₂,Ag⁺and Cu⁺in EuAgSb and EuCuSb would like to fill all the anionic vacancies respectively in order to balance the charge.So the concentration of the point defects is reduced and the phonon scattering is weakened,leading to an enhanced lattice thermal conductivity.The room temperature lattice thermal conductivity of EuAgSb is⁰.97 Wm^-1K^-1,and that of EuCuSb is¹.87 Wm^-1K^-1.However,the electrical conductivity of EuAgSb and EuCuSb is relatively high.The room temperature electrical conductivity of both compounds is²×10⁵ S m^-1.EuAgSb-based TE materials with Ag vacancies and EuCuSb-based TE materials with Cu vacancies were designed and prepared.The lattice thermal conductivity was reduced and the ZT value was increased.Since Eu₂ZnSb₂ and EuAgSb have the same crystal structure,we prepared EuAg_1-2x-2x Zn_x Sb by substitution of Ag with divalent Zn.Then some Ag vacancies will occur for the charge balance.With increasing content of Zn,the concentration of vacancy gradually increases,resulting in increased point defect for enhanced phonon scattering.The decreased lattice thermal conductivity contributes to the improved ZT value.Finally,the ZT value is⁰.76 at 673 K for EuAg_0.1Zn_0.45Sb,which is 105%higher than that of EuAgSb.EuAg_1-2x-2x Mg_x Sb compound is obtained by substitution of Ag with divalent Mg.Since the crystal structure of EuMg₂Sb₂ and EuAgSb is different,Mg cannot be heavily alloyed.When x is higher than 0.3,the impurity will appear.This small amount of Ag vacancies still offer effective point defects to increase phonon scattering and reduce lattice thermal conductivity.However,due to the small amount of the vacancy,the lattice thermal conductivity is not significantly reduced.Finally,the ZT value is⁰.54 at 673 K for EuAg_0.6Mg_0.2Sb,which is 46%higher than that of EuAgSb.We also prepared single-phase EuCu_1-2x-2x Zn_x Sb by using divalent Zn to substitute Cu.With increasing content of Zn,the concentration of Cu vacancy increases.The enhanced phonon scattering contributes to the reduced lattice thermal conductivity,leading to an improved ZT value⁰.76 at 673 K for EuCu_0.1Zn_0.45Sb,which is 153%higher than that of EuCuSb.The lattice thermal conductivity could be effectively reduced by isoelectronic substitution.Considering the same crystal structure of EuAgSb and EuCuSb,we prepared single-phase solid solution EuCu_1-x-x Ag_x Sb.Due to the different atomic radius of Ag and Cu,the addition of Ag will cause lattice distortion,scatter phonons and reduce the lattice thermal conductivity.The lattice thermal conductivity of EuCu_0.5Ag_0.5Sb is lower than that of EuAgSb and EuCuSb.The lattice thermal conductivity of EuCu_0.5Ag_0.5Sb is about 0.18 W m^-1K^-1 at 773 K,which is lower than that of EuAgSb(⁰.21 W m^-1K^-1)and EuCuSb(⁰.4 W m^-1K^-1).