基于同位素效应的高性能IVA族笼状物热电材料研究

In IVA-group element constructed clathrate compounds, it is sugested that heat transfer is suppressed by phonon scattering, even though electric transport remains high in the Bloch lattice, which can give rise to high thermoelectric figure of merit. Therefore, it is regarded as a very promising new green functional material for enviroment protection. Type-I clathrate M8Ga16Ge30 show high thermoelectricity. However, their crystal structures and thermoelectric mechanism are not fully clarified and thus need to be further improved. In this proposal, the crystal structure and electron-phonon interaction of p/n-Ba8Ga16Ge30, n-Sr8Ga16Ge30 are investigated based on Ge isotope effect, using neutron scattering, Raman and physical property measurement system and etc, combined with first principle theoretical calculation. These investigation can not only effectively differentiate Ga/Ge occupation in M8Ga16Ge30 crystals, but also clarify the electron-phonon interaction between guest atom and host fromework, and thus resolve the long-debated question about the thermal conductivity difference between p- and n-Ba8Ga16Ge30. This will be very significant to improve thermoelectric mechanism for clathrate compounds, and will provide guidance to design high-performance thermoelectric materials.

IVA 族元素构成的笼状物热电材料晶体结构中，客体原子在主体多面体内作非简谐运动，因而对载热声子有强散射作用导致热导率急剧下降, 但对导电性能无显著影响，被认为是极具潜力的废热利用、无氟制冷的新型绿色环保功能材料。I型笼状热电材料表现出高热电性，但其晶体结构及热电机理仍需要进一步研究完善。本项目将利用Ge同位素效应，结合中子衍射、Raman、基本物性测量以及第一性原理理论计算，研究I型笼状物热电材料M8Ga16Ge30晶体结构及电子-声子相互作用对I型笼状物热电性质的影响。本研究不但能有效区别p-和n-Ba8Ga16Ge30 (BGG) 笼状物中Ga和Ge原子占位，还可以研究客体原子和主体多面体结构间的电声相互作用对其电性的影响，可望成为解决国际学术界存在长期争议的p-和n-BGG热导率上存在巨大差异的决定因素，将对于完善笼状物材料热电理论，设计高效热电材料具有非常重要的指导意义。

本研究巧妙地利用Ge-76同位素效应并结合三单色器Raman、超分辨电镜（STEM）等表征研究了p-、n-BGG热电材料客体Ba原子和Ga-Ge主体多面体结构间的电子-声子相互作用机理。通过比较研究通过p-、n-BGG 和pi-、ni-BGG的三单色器Raman光谱，发现T2g模式下，笼振动峰在Ge-76同位素替代后，振动频数均减小；对于笼内Ba原子，p- BGG在Ge-76替代前后，Ba原子的的振动频数没有发生变化，而n- BGG在Ge-76替代后，Ba原子的的振动频数增加。这表明p-、n-BGG晶体中(Ga-Ge)46在晶格上占位存在差别，从而导致笼内Ba原子与笼状结构间的相互作用方式也存在差异。通过球差透射电镜（STEM）实现了笼状热电材料原子尺度成像，并观察到Ge-76同位素替代与Ge天然丰度笼状热电材料相反的同位素效应。对p-BGG而言，无论Ge-76同位素替代与否，笼状结构的尺度保持不变；而对n-BGG，笼的晶格尺度小于ni-BGG，Ge-76同位素替代后，笼在100方向发生畸变。这说明p/n-BGG结构是导致其电子-声子相互作用差异的重要因素。该研究结果对研究笼状热电材料的热电转换机制，完善PGEC理论具有重要意义。