Microstructure Evolution Of Transition Metal Doped Cu₂SnS₃ Thermoelectric Materials

Multinary compounds in the ?-?-?-? system??=Cu or Ag;?=Zn,Cd,Ni,Co,Fe,or Mn;?=Si,Ge,or Sn;and ?=S,Se,or Te?with many advantages including their abundance,low cost,and reduced environmental impact,possess excellent electrical,optical,magnetic and thermal properties.Thus,these materials have numerous potential applications in the fields of solar photovoltaic cell,thermoelectric conversion,photocatalysts and nonlinear optics.Generally,the crystal structure of such materials obeys the octet rule,in which the cations occupy the same sites and different ordered structures can be formed as the composition changes.Due to this structural feature,a variety of impurity phases,a wide range of solid solutions,and large numbers of defects are often formed in such materials,which increase the difficulty of controllable preparation of materials,but provide abundant microstructural variables to tailor their physical properties.However,from the perspective of microstructure research,since such materials are composed of very similar metal-sulfur tetrahedral structural units,conventional XRD,Raman and neutron diffraction characterization methods are difficult to detect the nanoscale microstructure information,which leads to insufficient understanding of the microstructure evolution and structure-activity relationship of such materials.In recent years,the development of spherical aberration-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive spectroscopy provides an opportunity for in-depth investigation of the microstructure evolution and microstructure-performance relationship of ?-?-?-? multinary chalcogenides at the atomic scale.Here,the microstructures of Cu₂SnS₃ thermoelectric materials doped with different amounts of Zn,Ni or Co elements,including crystal phases,cation ordering,crystal defects,nanoprecipitates and interface structure were investigated systematically by a combination of multiple electron microscopy techniques to reveal the microstructure evolution with different doping elements and contents.In addition,combined with first-principles calculations and classical perturbed molecular dynamics simulations,the physical mechanisms responsible for the very low lattice thermal conductivity and good electrical properties of these materials were established.?1?The Cu₂Zn_0.2Sn_0.8S₃ ceramic was investigated using atomic-resolution high angle annular dark field?HAADF?image of spherical aberration-corrected scanning transmission electron microscope?STEM?and energy dispersive spectroscopy?EDS?mapping,the unique mosaic-type domain nanostructure in the matrix grains comprising well-defined 10 nm wide cation-disordered domains coherently bonded to a surrounding 5 nm wide network phase with semi-ordered Cu₄ZnSn₂S₇ was revealed.This new phase in the Cu-Zn-Sn-S system derives from Cu₂ZnSnS₄ structure.First-principles calculations show that the Sn sites in the Cu₄ZnSn₂S₇ structure is completely ordered,while the Cu and Zn sites are weakly ordered,forming a unique semi-ordered structure.Meanwhile,Cu₄ZnSn₂S₇ is a p-type semiconductor with a direct band gap and high carrier mobility.Molecular dynamics simulations further revealed that the cation disordering in the central region of the mosaic structure and the coherent interface with the Cu₄ZnSn₂S₇ semi-ordered structure have a strong phonon scattering effect,which is the main reason for the materials exhibiting extremely low lattice thermal conductivity.Thus,this mosaic-type domain nanostructure represents a new kind of phonon-glass electron-crystal.?2?The Cu₂Zn_0.2Sn_0.8S₃ sample was characterized by selected area electron diffraction?SAED?and high-resolution transmission electron microscope?HRTEM?.It is found that the plate-like tetragonal metastable Cu₂S nanoprecipitates were embedded in a mosaic nanostructure.These metastable Cu₂S nanoprecipitates show clear orientation relationships with the matrix that the plates align with three crystal axes of cubic lattice and a mismatched strain field was produced between the matrix and nanoprecipitates.A combination of conductive atomic force microscope and Kelvin probe force microscope reveals that the nanoprecipitates have higher carrier density than the matrix,which can inject into the matrix and enhance the total electric conductivity of the sample.Meanwhile,the occupation disorder of Cu atoms in Cu₂S nanoprecipitates,coherent heterointerfaces between Cu₂S and matrix,and the extended strain field in the matrix regions adjacent to the Cu₂S nanoprecipitates can scatter the phonons with different wavelengths,which further reduces the thermal conductivity of the sample.?3?The microstructures of Cu₂SnS₃ ceramic samples doped with different amounts of Zn?5 mol%,10 mol%,15 mol%and 20 mol%?were investigated by a combination of SAED and HAADF techniques.It is found that mosaic-type domain nanostructures formed in all the samples.With the increase of Zn substitution,a series of semi-ordered phases formed at the boundary area of the mosaic-type nanostructures as CuInS₂-like phase?Zn<5 mol%?,Cu₆ZnSn₃S₁₀?Cu₂SnS₃:ZnS=3:1?and Cu₄ZnSn₂S₇?Cu₂SnS₃:ZnS=2:1?,respectively.These semi-ordered structures derive from the zinc blende structure?201?superlattice of-?Cu–S?₂?Zn–S??Sn–S?-in the kesterite Cu₂ZnSnS₄ and complement the Cu₂SnS₃-ZnS pseudo-binary phase diagram.Meanwhile,point defects,dislocations,stacking faults,and finally Cu_2-xS nanoprecipitates are formed sequentially to compromise the excessive Cu ions when the Zn contents increase from 5 mol%to 20 mol%.The competition between cation ordering and disordering at the nanoscale as well as the evolution and equilibrium among various crystal defects provides important guidance for microstructural modulation and property optimization in the Cu-Zn-Sn-S quaternary system.?4?We analyzed the microstructure features of the Ni-doped Cu₂SnS₃ ceramic samples,it is found that Ni²⁺formed Cu₂NiSn₃S₈,mosaic-type domain nanostructure and a series of cation semi-ordered nanostructures corresponding to Zn doped samples,while Ni³⁺formed Cu_26-xNi_4+xSn₄S₃₂2 of germanite structure.Similarly,a large number of crystal defects,such as Cu-rich dislocations,stacking faults,and nanoprecipitations also exist in the sample.These microstructures of different sizes and types improve the phonon scattering ability,resulting in a low lattice thermal conductivity and a significant increase in thermoelectric performance.?5?In the Cu₂SnS₃ ceramic samples doped with Co,Co²⁺formed mosaic-type domain nanostructure and a series of semi-ordered cations,while,Co³⁺formed Cu_26-xCo_4+xSn₄S₃₂2 of germanite structure.Comparing the nanostructures of Cu₂SnS₃ceramic samples doped with Zn,Ni and Co elements,respectively,it can be concluded that the nanoscale disorder-ordered phase separation,different cation ordering states and various defect types and numbers can be modulated by the content and valence state of doping elements under octet rule and lowest total energy of the system.The atomic-scale microstructure evolution can be extended to other ?-?-?-? multinary chalcogenides and other complicated compounds to design novel nanostructures and optimize physical properties.