Structure And Thermoelectric Properties Of MgAgSb Compound

MgAgSb-based alloy system is a promising thermoelectric(TE)material near room-temperature and is paid increasing attention because of its abundant constituent elements in earth's crust.However,there are still some problems to be solved.Firstly,the complicated phase transitions make it difficult to obtain pure ?-MgAgSb phase.A high content of secondary phase Ag3Sb(?19%)was present in the ?-MgAgSb.Secondly,the most striking feature of a-MgAgSb is its low thermal conductivity.The origin of the intrinsically low thermal conductivity,however,is much less-studied.Thirdly,Theoretical calculations show that ?-MgAgSb has a metallic character,while experimental studies suggest that it behaves as a semiconductor.In the present work,the structure and thermoelectric properties of MgAgSb-based alloy were studied.The main results are summarized as follows:(1)Bulk a-MgAgSb with improved phase purity has been obtained and its carrier concentration has been optimized by In doping.We calculate the band structure of a-MgAgSb by first principles calculations.The result shows that a-MgAgSb is a narrow band-gap semiconductor with the calculated indirect gap of-0.26 eV,and the Nv of valence band degeneracy of a-MgAgSb is 8.Single parabolic band(SPB)model is applied to analyze the charge carrier transport in a-MgAgSb.The density-of-state effective mass m*is calculated to be m*= 2.0me and single band mass mb*= 0.5me.A deformation potential E = 20 eV is found for a-MgAgSb alloy.Hole concentration of a-MgAgSb can be optimized by In doping.A maximum zT of?1.1 was achieved at 525 K for ?-MgAgSb0.99In0.01 with the optimal carrier concentration of 8?9×1019cm-3.(2)We elucidate the coexistence of global and local weak chemical bonds as the origin of the intrinsically low lattice thermal conductivity of non-caged structure Nowotny-Juza compound,a-MgAgSb.The global weak bonds of the compound lead to a low sound velocity.The unique three-centered Mg-Ag-Sb bonds in a-MgAgSb vibrate locally,and induce low frequency optical phonons,resulting in "rattling-like"thermal damping to further reduce the lattice thermal conductivity.The result of the low-temperature heat capacity of a-MgAgSb verify the existence of low frequency localized vibrations.Further studies show that the low valence count induced weak chemical bondings can be generalized to other Nowotny-Juza compounds.Lattice dynamics calculations for two other Nowotny-Juza compounds(CdCuSb and CaAgSb)testify their low sound velocities and the existence of low-frequency optic phonons.(3)The crystal structure and structural disorder of MgAgSb have been studied by multi-temperature high-resolution synchrotron radiation powder X-ray diffraction.Trace impurities of Sb and Ag3Sb appear at elevated temperatures of 400-600 K,and the impurities content increases with temperature.A tendency of increasing structural symmetry with temperature is observed for a-MgAgSb,which may significantly contribute to the temperature evolution of the thermoelectric properties.Two MgAgSb polymorphs(?-MgAgSb and y-MgAgSb)coexist at 700 K,but only the ?-MgAgSb erystalline phase is found at high temperatures(800-1000 K).The content of ?-MgAgSb phase decreases with temperature,and the sample is only 43.8%crystalline at 1000 K.The liquid impurities largely affect the stability of ?-MgAgSb.The high resolution powder data of ?-MgAgSb can be fitted using both type ?(with Mg,Ag,and Sb on the 4b,4c,and 4a sites,respectively)and type ?(with Mg,Ag,and Sb on the 4a,4b,and 4c sites,respectively)half-Heusler crystal structure models.Maximum Entropy Method analysis carried out on the extracted factors shows that type ? gives a more physically reasonable electron density.The high temperature transport data suggest a semiconducting y-MgAgSb phase in agreement with theoretical calculation for the type? structure.