Experimental And Theoretical Study On Structural Stability And Electrical Transmission Characteristics Of Sn - Based Cage

The development of pollution-free and renewable sources have been attracting significant attention due to the growing problems of energy crisis and environment pollution, and thermoelectric（TE） materials has become the focal point all over the world, but the low conversion efficiency seriously hindered their practical application. Clathrates which satisfy the PGEC concept were considered as foreground TE materials. Among which the Sn-based clathrates can be potentially applied in thermoelectric electricity generation because of their low lattice thermal conductivity and high Seebeck efficient. The TE efficiency have been significant improved via high pressure and doping strategy. However, the ZT still keep around unity. It is extremely urgent to develop high-performance Sn-based clathrates.In this thesis, the electronic structure and thermoelectric property of Sn-based were probed via first-principle and experiment. Theoretical analysed the effect of high-pressure and doping on electronic structure. In order to explore the influence of doping on electronic structure and thermoelectric property, High-purity elements Ba（ingot, 99.9%）, Ga（ingot, 99.999%）, Sn（ingot, 99.999%）, and Cu（ingot, 99.999%） were weighed according to Ba: Ga: Cu: Sn = 8:16-x: x: 50（x = 0, 1, 2） to fabricate Iand VIII- Ba₈Ga_16-xCu_xSn₃₀（x=0,1,2） via Sn-flux method. and the main obtained results as follows.The theoretical results for Sr₈Ga₁₆Sn₃₀ revealed that the type-VIII Sr₈Ga₁₆Sn₃₀ is the stable phase while type-I is the metastable phase, and there have no high-pressure phase transformation trend between the type-I and type-VIII Sr₈Ga₁₆Sn₃₀ clathrates; type-I and type-VIII Sr₈Ga₁₆Sn₃₀ are both indirect band gap semiconductors. And the energy gap is 0.125 e V and 0.009 e V respectively. With pressure increase, both energy gap are increased ascribe to the conduction-band move to high energy, and the change of type-I is more notable compare with type-VIII.I-and VIII- Ba₈Ga_16-xCu_xSn₃₀（x=0, 1, 2） single-crystal were synthesized via Sn-flux strategy. The results of XRD indicated that Cu-doping raised the synthetic temperature of type-I, and the synthetic temperature is more high as the initial content of Cu increase; Cu-doping reduced the Seebeck coefficient while enhanced the conductivity ascribe to the increase of electron mobility; The theoretical results showed that the VIII- Ba8 Cu Ga15Sn30 emerge metallicity because that valence band move to high energy. The contribution of Cu4s is negligible, and Cu3 d emerged a poignant peak near Fermi level to effect the edge of energy band of clathrates.I-and VIII- Ba₈Ga_16-xCu_xSn₃₀（x=0, 1, 2） single-crystal were synthesized via Sn-flux strategy. The XRD and DTA were combined to analyze the thermostability of clathrates, and results indicated that I-Ba8Ga16Sn30 which were heating to 183℃ phase transformed to type-VIII. In contrast, VIII-Ba8Ga16Sn30 was stable during heating and cooling process; Cu-doping reduced the structure stability, the transformation temperature of I-Ba8Ga16Sn30 depress to 170℃, and VIII-Ba8Ga15Cu1Sn30 decompose to Sn and Ba（Ga/Sn）4 during cooling process; The electronic conductivity of type-I clathrate which cooling to ambient temperature emerged type-VIII characteristic.