结合同步辐射实验和第一性原理的理论对"半晶化"过程中的晶体结构,电子结构和热力学性质进行研究

Recently, we studied the structural and bonding properties of Cu-based materials by means of first-principles calculations and experimental measurement. The results demonstrate that Cu atoms are embedded in a flattened potential energy surface due to the weak bonding interaction with the framework. This leads to quite large atom displacement parameters (ADPs) for Cu atoms, and the ADPs are so large that the corresponding Cu-sublattice can be considered to be “melted”. This “melted” phenomenon also can be observed from finite-temperature molecular dynamics simulations. As a result, the Cu sublattice can be regarded as “melting sublattice” and the whole system is thus in a mixed part-crystalline part-liquid state, containing a crystalline rigid part and a fluctuating noncrystalline substructure (Cu sublattice). Concerning this state, currently, the pair distribution function, the electronic structure, and thermodynamic properties as a function of temperature have rarely been studied. These properties are important for understanding and tuning the conversion efficiency of energy materials. Since conventional instruments and measurements can not meet the needs of the current job, the synchrotron radiation techniques including PDF, XAFS,ARPES and RIXS etc. are the key to measure these important parmateters. In this project, we plan to study these properties by combining first-principles calculations with synchrotron techniques.

最近，通过第一性原理的理论计算和实验相结合的办法,我们对一类Cu基材料进行了研究，结果表明:因Cu原子具有比较平坦的势能面,当温度升高时, Cu原子将变为无序的状态(融化状态), 而其它原子还处在一般的晶体振动状态(束缚的状态)，因此该材料部分处在晶态,而局部处在液态的有趣的“半晶态”状态。处在“半晶态”的材料会表现出反常的热输运行为和极低晶格热导率等。当前,有关这类材料的研究比较少,特别是随温度升高时,局部从有序到无序的原子变化过程,及其在此过程中电子结构和热力学性质的变化情况尤为重要,这些性质与能源材料的转换效率密切相关。因常规仪器和测量手段无法满足这些工作的需要,而同步辐射实验技术可以帮助实现对这些关键参数的测量(特别是PDF, XAFS,ARPES和RIXS等手段),所以本项目计划结合同步辐射技术和第一性原理的理论计算来研究这些问题。

通过第一性原理的理论计算和实验相结合的方法,我们最近对一类特殊的Cu基材料进行了研究，研究结果表明:因Cu原子具有比较平坦的势能面,当温度升高时,Cu原子将变为无序的状态, 而其它原子还处在一般的晶体振动状态(即：束缚的状态，在平衡位置振动)，因此该材料部分处在晶态,而局部处在液态的有趣的“半晶态”状态。处在“半晶态”的材料会表现出反常的热输运行为和极低晶格热导率等。为了进一步降低晶格热导率，以达到调控热输运和电输运性能，从而提高热电优值ZT的目的，我们系统地研究了Ag3SbSe3, Cu3BiSe3,和Cu3SbTe3的热输运和电输运性能，这三个新的材料是对原型材料Cu3SbSe3进行重元素替换所得到的。研究结果表明：通过重元素替换得到的这三种材料仍属于“半晶态”材料，都具有极低的晶格热导率，其中Ag3SbSe3和Cu3BiSe3的晶格热导率甚至比原型Cu3SbSe3更低。电子结构研究表明这四种材料都具有较小的带隙1eV左右，都可能具有比较好的电输运性能。结合热输运和电子输运相关的性质，在700 K的温度下，这四个“半晶态”材料Cu3SbSe3，Ag3SbSe3，Cu3BiSe3和Cu3SbTe3的热电优值分别为1.14、2.00、1.41和0.93。这些结果表明Ag3SbSe3和Cu3BiSe3是潜在的热电材料。