低热导率三元稀土硫族化合物的热电特性调制

，The latest progress of condensed matter physics reveals the existence of a new state of matters, topological insulators, and their unique physical characteristics. Very differently from the ordinary insulators, energy bands of this kind of insulators have energy gaps for carrier excitation within the bulk crystal, but there are gapless excited metallic states at surface, which are protected by time-reversal symmetry. The surface states supply a back-scattering-free transport channel for electrons. Most topological insulators are good thermoelectric materials. The state-of-art thermoelectric materials of Bi2Te3、Bi2Se3 and related compounds are considered to be the prototype family of three-dimensional topological insulators. Revealing the physical fundamentals of this new topological states and their inherent correlation with heat and electrical transport offers a novel way to increase the power factor of thermoelectric materials. We plan to experimentally investigate the coexistence of the two different electronic states of a series ofternary rare earth compounds RXTe3 (R = Rare earth, and X = Sb or Bi). We will grow single crystals with Bridgman method, deposit thin film and superlattice of them with PLD and magnetron sputtering technology. The electronic structures will be tuned with element substitution and high-pressure high temperature synthesis. We will seek the relationship between the novel electronic states and thermoelectric performances. By exploring the formation mechanism of the surface metallic state and energy gap of bulk Te based materials, we search for higher thermoelectric performance materials with large Seebeck coefficient and high electrical conductivity. Novel thermoelectric materials are expected due to the similar layered structure to Bi2Te3, which leads to very low thermal conductivity for these Te-based ternary compounds.

凝聚态物理的最新进展揭示了拓扑绝缘体这一新物质形态及其独特的物理特性。它们的能带与普通绝缘体有本质区别，其中的载流子在晶体内部有带隙，但是在表面存在受时间反演不变性保护的无能隙的边缘激发态，表面态的电子存在无背散射的电输运通道。绝大多数拓扑绝缘体是性能良好的热电材料，尤其是Bi2Te3和Bi2Se3等是最典型的三维拓扑绝缘体。揭示该新物质形态的拓扑特性与热电特性的内在规律，为调制热电材料的功率因子提供了新的途径。我们以三元稀土硫族化合物RXTe3为对象，用布里奇曼法生长高质量单晶体、用磁控溅射技术和激光脉冲沉积技术制备薄膜和超晶格材料，用元素替代、高压合成等途径调控材料的电子态特性，实验研究它们的拓扑性质与热电特性之间的联系，探索表面金属态调控的方法，实现材料的高塞贝克系数与高电导性兼具的物性。它们具有与Bi2Te3类似的层状晶体结构和低热导率的特点，期望通过电子结构调控获得高性能材料。

近几年的研究表明，以Bi2Te3、Bi2Se3、Sb2Te3等化合物等为代表的多数性能优异的热电材料同时表现出新颖的拓扑绝缘体特征。探究拓扑绝缘体特性与良好热电特性之间的关联，为寻找新型高效热电材料的研究提供线索，我们以三元稀土硫族化合物RXTe3为对象，用固态反应、高速球磨加等离子体烧结方法制备了它们的多晶块体材料，并进行了热电特性表征。它们具有层状晶体结构和非常低热导率。采用溶剂热发和布里奇曼法生长它们的单晶体没有成功，拓扑特性研究无法深入展开。利用第一性原理计算方法研究了相关体系材料的电子结构和热电特性。发现在300 K，AlSiTe3功率因子可以达到0.38μW/(m.K2)，热电优值（ZT值）有望达到0.76。计入自旋轨道耦合后，预测了新化合物LuBiTe3具有良好的热电特性。用高温高压合成法制备研究了几种包合物，获得了一些创新性成果。高温高压合成法制备样品的效率高，部分样品的热电性能显著提高。拓扑绝缘体的边界或表面总是存在导电的边缘态，这是它有别于普通绝缘体的最独特的性质。这样的导电边缘态在时间反演对称性的保护下是稳定存在的，而且不同自旋的导电电子的运动方向是相反的，所以信息的传递可以通过电子的自旋来传递。发现了一种高居里温度的半导体，具有潜在应用前景，需要深入研究其物理机理。