Structural basis for understanding phase transitions in 3-dimensional charge-density-wave compounds

向电荷密度波（CDW）态的相变通常与金属材料中电子带的低维特性有关。对于晶体结构和物理性质没有明显低维特征的物质，不能理解CDW相图。本课题主要研究三维CDW化合物Lu2Ir3Si5、NiV2S4 （Cr3S4结构型）和CuV2S4（尖晶石结构型）中CDW相变的温度依赖单晶x射线衍射。这些实验的一个重要结果将是与CDW相关的周期性晶格畸变（PLD）。PLD提供了关于CDW在晶体结构单位胞内的位置、受挫相互作用的重要性、CDW的对称性（1-q或2-q状态）以及可能参与CDW状态稳定的声子的结论性信息。衍射实验还将用于分析进一步的相变（例如可能发生的锁定相变）和确定序参量的温度依赖性。实验信息将用于理论考虑，以理解相变机制和解释CDW状态内物理性质的本质。

Phase transitions towards charge-density wave (CDW) states are often associated with the low-dimensional character of the electron bands in metallic materials. The CDW phase diagram is not understood for substances without obvious low-dimensional features of their crystal structures and physical properties. The proposed project is concerned with the study of CDW phase transitions in the 3-dimensional CDW compounds Lu2Ir3Si5, NiV2S4 (Cr3S4 structure type) and CuV2S4 (Spinel structure type) by means of temperature-dependent single-crystal X-ray diffraction. An important result of these experiments will be the periodic lattice distortion (PLD) associated with the CDW. The PLD provides conclusive information about the location of the CDW within the unit cell of the crystal structure, about the importance of frustrated interactions, about the symmetry (1-q or 2-q state) of the CDW, and about the phonons that may participate in the stabilisation of the CDW state. Diffraction experiments will also be used for analysing further phase transitions (for example a possible occurrence of a lock-in transition) and for the determination of the temperature dependence of the order parameter. The experimental information will be used for theoretical considerations towards understanding the mechanism of the phase transitions and explaining the nature of the physical properties within the CDW state.