Crystal And Electronic Structures Control Of TiNiSi Structure Related Zintl Phase Thermoelectric Materials

Thermoelectric materials,which can realize power generation by converting waste heat into reusable electricity,are promising candidates for the green and sustainable development.Among numerous thermoelectric materials,Zintl phases have alreadly shown great potential for practical applications.However,high thermoelectric performance seems only limited in a few structures according to present studies.Among them,Zintl phases related to the TiNiSi structure type are especially interesting.Herein,a series of Zintl phases based on the TiNiSi structure were prepared by metal flux reactions and solid-state syntheses,and their crystal structures,electronic band structures and thermoelectric properties were systematically studied.The main contents are as follows:1.The TiNiSi-type Zintl phases with the nominal formulae of ALiSb（A=Ca,Sr,Eu,Yb）were for the first time investigated on their thermoelectric properties.With the help of theoretical calculations,the electronic band structures and theoretical electrical properties of CaLiSb and SrLiSb were analyzed,indicating that this series of compounds are potential thermoelectric materials.The ALiSb materials were experimentally synthesized and then the thermoelectric properties were studied,showing good electrical properties and low thermal conductivity.As a result,the pristine ALiSb materials featured decent zT values of 0.4～0.7 at 823 K.It is worth noting that although the structure of ALiSb materials is relatively simple,the materials have a low intrinsic lattice thermal conductivity.The atomic displacement parameters（ADPs）of CaLiSb were analyzed by single crystal X-ray diffraction.The result indicates that the ultra-low lattice thermal conductivity in ALiSb originates from the rattling-like scattering behavior of Li atoms.The thermoelectric performance can be further optimized by preparing Sr1-xEuxLiSb（0≤x≤1）solid solutions.The addition of Eu significantly reduces the resistivity of SrLiSb,which leads to a significant improvement of PF.Also,the difference in atomic mass between Sr and Eu and the structural disorder caused by the mixing of cation sites enhance the phonon scattering and result in a low lattice thermal conductivity.With the electrical and thermal transport properties further optimized by configuring the solid solutions such as Sr0.8Eu0.2LiSb,the material approached a maximum zT up to 1.2 at the same temperature.Different from other TiNiSi-type compounds,these researches indicate that ALiSb materials possess excellent thermoelectric properties.At the same time,this work shows that some simple structural materials containing light elements can also have outstanding thermoelectric properties,breaking the previous inherent cognition in the design of Zintl-phase thermoelectric materials.2.Based on previous studies on the Ca_1-xRE_xAg_1-ySb（RE=La,Ce,Pr,Nd,Sm）and CaZn（1-x）/2AgxSb（0<x<1）system,we continutede to tune the Zn/Ag contents in the CaZn（1x）/2AgxSb series,and discovered a metastable compound CaZn0.5Sb.The structure refinements indicate that this compound has a similar crystal structure to CaZn_0.4Ag_0.2Sb.However,CaZn0.5Sb is metastable and changes to Ca9Zn4+xSb9 after annealing.By doping Li into the cation sites,the metastable CaZn0.5Sb become stable Ca_1-xLixZn（1+x）/2Sb（x=0.1,0.125,0.15）phases.Thermoelectric properties of Ca1xLixZn（1+x）/2Sb were investigated and the results showed that the compounds exhibited extremely low thermal conductivity in the range of 300～823 K（<1 W·m-1·K-1）and large Seebeck coefficient.As a result,the zT value at room temperature was 0.3 and also reached 0.92 at 823 K,which is about 50%higher than that of CaZn_0.4Ag_0.2Sb at the same temperature.3.CaZn_0.4Ag_0.2Sb with the LiGaGe structure type has shown high zT plateaus in a wide temperature range.While isomorphic LiZnSb has been predicted to be good material in thermoelectric applications for a long time.However,experimental work indicated that the synthesized LiZnSb materials were p-type,and the maximum zT value is only 0.08 at 525 K.By comparing their crystallographic and electronic band structures,it’s evident that the interlayered distances in both compounds have great effects on the regulation of corresponding electrical transport properties.When alloying CaZn_0.4Ag_0.2Sb with LiZnSb,solid solutions form within a specific range,which results in significant enhancement on Seebeck coefficient through the orbital alignment and carrier concentration optimization.In addition,low thermal conductivity was obtained owing to the reduced electronic component.With above optimization,a maximum zT of～1.3 can be realized for(CaZn_0.4Ag_0.2Sb)0.87（LiZnSb）0.13 at 873 K,more than twice of the pristine CaZn_0.4Ag_0.2Sb and about tenfold compared to LiZnSb.This work not only discovers a new type of thermoelectric material with high zT value,but also provides new lights on the optimization of thermoelectric properties based on Zintl phases.In particular,the use of cations of different sizes to adjust the anion structure effectively achieves the valence band convergence at the Fermi level,which is an effective way to improve the thermoelectric properties of Zintl phase compounds.4.In Ca_1-xCexAg_1-ySb,successive phase transitions were observed with the increase of Ag defect concentration.In consideration of the flexibility of LiGaGe structure,Cu atoms were also considered for such studies.Through the regulation of Cu defects,a new ytterbium copper antimonide,YbCu0.52（2）Sb,was synthesized from the Pb-flux reactions,and its crystal structure was determined by the single-crystal X-ray diffraction method.The compound crystallizes in the trigonal space group R3m（No.166）,a=4.4494（4）?,c=33.767（7）?,which adopts the same structure as Ca0.61（1）Ce0.39（1）Ag0.60（1）Sb.The crystal structure of this compound rationalized as a combination of the YbCuSb and YbSb structures.In addition,the formula of this compound can be rewritten as a sum of these two components,[（Yb2+）（Cu+）（Sb3-）]0.52[（Yb3+）（Sb3-）]0.48.The magnetic susceptibility and X-ray photoelectron spectroscopy measurements also supported the coexistence of Yb2+ and Yb3+ in the structure.The temperature dependent resistivity measurements indicated the metallic nature of the material and Seebeck coefficient measurements showed characteristic n-p conversion.Besides,a new compound Yb8Cu15.67（2）Sb12 was found during the phase exploration of the YbCu1-xSb system,which was proved to crystallize in the space group P62m（No.189）with mixed-valence Yb cations in the structure as well.