Study On Lattice Dynamics And Electrical-thermal Transport Properties In Novel Silver-based Thermoelectric Materials

The dissertation aims to utilize the concept of synergistic structure to explore novel high-performance thermoelectric materials,and it is found three kinds of routes for achieving synergistic structure by the cases of three types of silver-based materials.The study has discovered the spatial-domain characteristic of multiscale microstructures along with the time-domain characteristic of anharmonicity in the diamond-like pseudo-cubic structure AgIn₅Se₈,liquid-like phase-change cubic structure Ag₈GeSe₆,Ag₈SnS₆,and tetrahedrite-like cubic structure Ag₆Ge₁₀P₁₂compounds.All compounds are synthesized via the traditional melting-annealing methods and are studied by a variety of characterizations and analysis approaches to reveal the relationships among crystal structure,chemical bonding,coordination characteristics,elastic constant,lattice dynamics and thermal-electrical transport properties.The dissertation provides us some new insights in synergistically regulating thermoelectric properties of AgIn₅Se₈-based materials via Ag excess,enhancing thermoelectric performance of Ag₈SnS₆-based materials through expanding the high-temperature phase regime,and realizing excellent electrical transport properties in Ag₆Ge₁₀P₁₂-based materials by“plate-like”Fermi surface from stereochemically active lone pair electrons.The detailed of main research contents,results and conclusions are briefly summarized as follow:（1）The diamond-like pseudo-cubic AgIn₅Se₈ compound is selected as our first objective in our study,and its overall cubic-like along with the locally distorted crystal structure is deemed as the first approach to access the synergistic structure of thermoelectric materials.Based on the temperature-dependent PXRD Rietveld refinements,low-temperature isobaric heat capacity along with sound velocity measurements,the low speed sound,low-energy Einstein modes arising from the weak Ag1-Se2 and In2-Se1 bonds,and anharmonicity arising from the distorted Se1-In2-Se1tetrahedra at high temperatures are attributed to the low lattice thermal conductivity of the pristine AgIn₅Se₈.The Zn,Cd,and Ag dopants are intentionally adopted to optimize the electrical transport properties of AgIn₅Se₈-based material,and all dopants tend to effectively increase the carrier concentrations and boost the thermoelectric properties.The Ag excess strategy can synergistically regulate thermoelectric performance by introducing more twin boundaries and stacking faults.A maximum zT value～0.67 at883 K is obtained in Ag_1.03In₅Se₈ sample,while an average zT value of 0.34 is achieved from 323 to 883 K for Ag_1.03In₅Se₈ compound,which is the highest value for AgIn₅Se₈-based materials.The Ag excess method provides some guidance in optimizing other diamond-like materials.（2）The liquid-like Argyrodite Ag₈GeSe₆,Ag₈SnS₆ is selected as our second objective in our study,and its overall coexistence of dynamic disorder cations and high-symmetry rigid conductive framework is deemed as the second approach to access the synergistic structure of thermoelectric materials.Taking Ag₈GeSe₆ as an example,The Ag₈GeSe₆-based materials go through a phase transition from orthorhombic to cubic structure through the high-temperature PXRD Rietveld refinements and the occupancy of Ag ions are systematically decreasing with rising temperature,suggesting that the Ag ions undergo a crossover from order to disorder.The coupling between these Einstein modes and heat-carrying phonons,strong anharmonicity,and small sound velocity are responsible for the intrinsically low lattice thermal conductivity.The Se vacancy strategy is adopted to effectively optimize the carrier concentrations and power factors of Ag₈GeSe₆-based materials,especically,a maximum zT value of 0.55 is achieved at 923 K in Ag₈GeSe_5.88.Argyrodites exhibit excellent thermoelectric performance with the coexistence dynamic disorder cations and high-symmetry crystal structure at high-temperatures.Therefore,it is feasible to extend the high temperature cubic phase regime of Argyrodites for further boosting thermoelectric performances.Herein,the Se-doped sulfide-type Ag₈Sn(Se_1-xS_x)₆materials are studied.It is found that Se doping shifts the orthorhombic-cubic phase transition to lower temperature regime,thereby expanding the temperature range of the thermoelectrically favored cubic phase based on the high-temperature isobaric heat capacity measurements.The pristine Ag₈SnS₆ material exhibit an intrinsically low lattice thermal conductivity from 32 K to 773 K,with values ranging from 0.61 W/mK to 0.31 W/mK.According to the results of PXRD structure refinements,low-temperature isobaric specific heat capacity and calculated phonon structure,it is thought that the distorted local crystal structure,relatively weak chemical bonding,rattler-like Ag atoms,low-lying optical modes,and dynamic disorder of Ag ions at high temperatures are responsible for the ultra-low lattice thermal conductivity of the pristine Ag₈SnS₆.Low-temperature isobaric heat capacity measurement and estimated phonon mean free path uncover that Ag₈SnS₆ has low-energy localized Einstein vibration modes that prevent heat transport by disrupting the heat-carrying long-wavelength acoustic phonon modes.In addition,the theoretical calculation of phonon structure along with ELF and COHP,confirm the ratting-like,low lying optical modes and weakly bounded behavior of Ag atoms.Furthermore,upon alloying Se on S site of Ag₈SnS₆,the high-temperature cubic phase is extended to the lower temperature,favoring enhancing the power factors of Ag₈Sn(S_1-xSe_x)₆（x=0.01,0.03,0.05,0.10）compounds.A figure of merit zT value～0.80 has been achieved at 773 K in the Ag₈Sn(S_1-xSe_x)₆（x=0.03）sample,which is the record zT value reported among sulfide-type Argyrodites materials.Expanding the high-temperature phase regime in Argyrodites is an effective route to optimize thermoelectric properties.（3）The tetrahedrite-like cubic structure Ag₆Ge₁₀P₁₂ is selected as our third objective in our study,and its cubic structure along with the localized cluster is deemed as the third approach to access the synergistic structure of thermoelectric materials.The room-temperature powder XRD,XPS and high-temperature heat capacity are employed to analyze the phase and valence state of the pristine Ag₆Ge₁₀P₁₂.From the measurements of low-temperature isobaric specific heat capacity,high-temperature total thermal conductivities and phonon dispersion calculations,it is found that experimental along with theoretically predicted results clarify the relatively low thermal conductivity of about 1 W/mK arising from the rattling vibrations associated with Ag₆ clusters,which create low-frequency localized optical phonons in the acoustic region and thus enable strong anharmonic phonon scattering.The Ga-doped samples utilize the stereochemically active lone pair of electrons on the Ge（1）atom to realize the excellent electrical transport properties with PF maintained in the range of 10.5–13 mW/cmK²from room temperature to high temperature after optimizing carrier concentration.The relatively low lattice thermal conductivity and favorable PF result in a maximum zT value of 0.65,which is the highest value for polycrystalline phosphide compounds.Utilizing stereochemically active lone pair of electrons to cause the highly directional carrier pockets to provide us a new insight for realizing the excellent electrical performance of thermoelectric materials.