Crystal Structures And Thermoelectric Properties Of Yb-Mn-Sb Based Zintl Phases

Thermoelectric materials can achieve low-temperature refrigeration and waste heat power generation in extreme and harsh environments,thus having high research and application value.The complex crystal structure of antimony based Zintl phase compounds often corresponds to lower thermal conductivity,while a smaller bandgap leads to good electrical transport performance,thus meeting the "electron-crystal phonon-glass" characteristics required for thermoelectric materials.Yb-Mn-Sb based compounds are typical representatives among them,for example,Yb14MnSb11 is currently the most promising p-type alternative of Si-Ge based alloys for high-temperature thermoelectrics,in which the performance at high temperature is three times that of the latter;Yb9Mn4+xSb9 compound bears glass-like thermal conductivity as well as the intrinsic optimum carrier concentration,which shows great thermoelectric application potential at the medium temperatures.Herein,a series of Yb-Mn-Sb based Zintl compounds were prepared through metal flux reactions,conventional solid-state syntheses,induction melting and high-energy ball milling methods,and their crystal structures and thermoelectric properties were systematically explored.The main contents are as follows:1.Two novel Zintl compounds with the ideal "2-1-2" composition,A2MnSb2(A=Ca,Yb),were synthesized by induction melting method for the first time,and their crystal structures,thermal stability and thermoelectric properties were systematically explored.They are LiGaGetype layered structures with transition metal sites highly vacant.The substantial defects resulted in the poor stability of the materials,and they underwent decomposition after annealing at high temperatures.Besides,phase diagram explorations implied the pivotal impact of the transition metal concentration on the phase distribution and phase stability.A series of A2(MnxAg2(1-x))Sb2(A = Ca,Yb)solid solutions were prepared by substituting Ag on Mn sites,which illustrated the quite broad homogeneity ranges in the structures.The introduction of Ag increased the occupancy of the transition metal sites,resulting in a significant improvement in the thermal stability of the crystal structures.Owing to the complex and highly disordered structures,A2(MnxAg2(1-x))Sb2(A=Ca,Yb)featured extremely low thermal conductivity.The thermal conductivity of Yb2(Mn0.9Ag0.2)Sb2 was～0.5 W·m-1·K-1,almost reaching the low value limit of solid materials.Therefore,a decent zT value of～0.56 was obtained at 723 K.The discovery of such A2MnSb2(A=Ca,Yb)phases completes the critical missing members of the A-Mn-Sb ternary family,which is crucial to understanding the structures and properties related to these novel phases.2.Ca2ZnSb2,the Zn analogue of A2MnSb2(A=Ca,Yb)compounds,was prepared here.The impacts of size effects based on cation sites were systematically studied on crystal,electronic structures and thermoelectric properties.Ca2ZnSb2 compound was metastable and underwent phase transition to Ca9Zn4+xSb9 after annealing at medium temperatures.When smaller Li+was incorporated at cation sites in Ca2ZnSb2 and Yb2MnSb2,two new structures Ca1.84(1)Li0.16(1)Zn0.84(1)Sb2 and Yb1.82(1)Li0.18(1)Mn0.96(1)Sb2 with the P63/mmc space group were discovered,which can be viewed as derivatives of LiGaGe-type structures.Owing to the reduced interlayered distance and the optimization of the cation coordination geometry,the structural stability of the new compounds was improved compared with Ca2ZnSb2 and Yb2MnSb2.Electronic calculations illustrated that the energy band degeneracy of valence bands near the Fermi level was quite sensitive to the size of cations.The highly disordered structure resulted in the ultralow thermal conductivity of 0.47 to 0.79 W·m-1·K-1 in Yb1.82Li0.18Mn0.96Sb2;in addition,the high degeneracy of the bands contributed to a remarkable Seebeck coefficient of～270.77 μV·K-1 at 723 K.The discovery of Ca2ZnSb2 phase enriches the Sb-based 2-1-2 map,and the size effects induced by cations provide new ideas for the improvement of structural stability and property modulation.3.Systematic investigations on the structure evolution and thermoelectric properties of Yb-Mn-Sb based Yb9Mn4+xSb9 and Yb14MnSb11 compounds,were carefully executed.The narrow phase width of medium-temperature Yb9Mn4+xSb9 has brought difficulties to the modulation of thermoelectric properties.In this work,through external doping at cation and transition metal sites,the structure evolution and the origin of narrow phase width have been systematically explored by single crystal X-ray diffraction.The introduction of larger Sr2+ at cation sites enlarged the unit cells,doubling the concentration of interstitial Mn atoms.The lattice distortion introduced new defects,leading to the splitting of Mn in the polyanionic frameworks.By doping Ag at the transition metal sites,Yb9Mn4+xSb9 underwent structural reorganization,and new phases Yb2(Mn0.86(2)Ag0.28(3))Sb2 and Yb10(Mn4.84(1)Ag2.32(1))Sb12 were discovered.The former was determined as LiGaGe-type monolayered structure,and the latter as Eu10Cd6Bi12-type bilayered structure.The synthesized Yb14MnSb11 materials often contains metallic Yb-Sb side products,which deteriorates the high-temperature thermoelectric properties.Herein,a highly pure Yb14MnSb11 polycrystalline material was successfully synthesized by high-energy ball milling and high-temperature annealing approaches,and a high zT value of～1.3 was obtained at 1123 K.This value is comparable to the highest level reported in the current literatures.Using CoSb3 as a precursor,the Co nanoparticles were in-situ formed in Yb14MnSb11,effectively improving the electrical conductivity of the materials.More importantly,the mechanical properties of the materials were improved without damaging thermoelectric properties.