中文摘要：

通过调控Mn2＋的掺杂浓度，在镥基纳米晶体成功地实现了六方、四方混合相到纯四方相的相位转变，并详细讨论了其相变机理. 时域和频域光谱的分析表明，立方相Na5Lu9F32：40% Mn2＋，20% Yb3＋，2% Ln3＋（Ln=Er3＋，Ho3＋）纳米晶体内的准纯红色荧光发射主要由Mn2＋和Ln3＋之间的两步能量转移引起. Mn2＋掺杂后引起了发光离子附近局域对称性的降低，使得电偶极跃迁的辐射速率明显增加，进而导致了上转换、下转换荧光的极大增强. 该研究结果在生物荧光成像、太阳能电池效率的提高方面具备潜在的、广阔的应用前景.

英文摘要：

Transformation from Lu-based nanocrystals in hexagonal and cubic mixed phases to pure cubic phase was observed through adjusting the doping concentration of Mn2＋. The mechanism for the phase transformation was discussed in detail. Studies on the time and frequency domain spectra indicated that the semi-pure red emissions in cubic Na5Lu9F32： 40% Mn2＋, 20% Yb3＋, 2% Ln3＋ （Ln=Er3＋, Ho3＋） nanocrystals were caused by a two-step energy transfer between Mn2＋ and Ln3＋ ions. After incorporating of Mn2＋ ions into the host lattices, the local symmetry around the luminescent ion was reduced, which induced the increase of radiative rates for transitions that were mainly contributed by electric dipole radiations. Considerable enhancements in upconversion and downconversion luminescence were accompanied. The result of the current study has great application potential in bioimaging and solar cells.