The hexagonal-phase (β) of NaREF₄ upconversion nanocrystals (RE = rare earth elements) has been widely employed because of the outstanding luminescence performance, yet less is known about the essence of this superior property. The current understanding of this issue is raised from the advantage of weak electron–vibration interactions in fluoride systems, while the interpretability of this statement is controversial and contradictory results are commonly reported. One feasible way to solve this puzzle is from the aspect of “structure–property” relationship, yet even after decades of investigation, the structural details of β-NaREF₄ are still under debate. Herein, the reported results relevant to this topic are reviewed, and the conflicting viewpoints are summarized. The similarities and differences between different lattice templates are assessed, and the reasons underlying the divergence are analysed. Based on these discussions, it is realized that the crystal structure of β-NaREF₄ should be more reliably depicted as one flexible lattice framework with complex characteristics, and the structural disorder induced by atom displacements in the lattice is probably the key to supporting the superior luminescence properties of β-NaREF₄ nanocrystals.

中文翻译：

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六方相 NaREF4 上转换纳米晶体：晶体结构问题

NaREF ₄上转换纳米晶体（RE = 稀土元素）的六方相 (β)因其出色的发光性能而被广泛采用，但人们对这种优越性能的本质知之甚少。目前对这个问题的理解是从氟化物系统中弱电子振动相互作用的优势提出的，而这一陈述的可解释性是有争议的，并且经常报道相互矛盾的结果。解决这一难题的一个可行方法是从“结构-性质”关系方面，但即使经过数十年的研究，β-NaREF ₄的结构细节仍在辩论中。在此，回顾了与该主题相关的报告结果，并总结了相互矛盾的观点。评估了不同晶格模板之间的异同，并分析了产生分歧的原因。基于这些讨论，人们意识到 β-NaREF ₄的晶体结构应该更可靠地描述为一种具有复杂特性的柔性晶格框架，晶格中原子位移引起的结构无序可能是支持优越性的关键。 β-NaREF ₄纳米晶体的发光特性。