The red or orange emission spectra of tripositive europium have led to thousands of publications and to phosphor applications. The orange emission mainly arises from the D → F transition (A), whereas in other cases, the red emission is due to the D → F transition (B), with a different mechanism. The intensity ratio B/A, which is commonly labelled 2, has been termed the asymmetry ratio and has been used as a probe of europium site symmetry in a material. We demonstrate that 2 is not an exclusive indicator of site symmetry, although it does vary if there is change in the structural symmetry of the coordination environment. We propose the new term ‘hypersensitivity ratio’ to replace ‘asymmetry ratio’. The concepts of crystallographic and spectroscopic site symmetry are reviewed, together with the theoretical basis of the electronic spectra of Eu. Structures of europium complexes with coordination numbers between 6 and 12 are illustrated and previous interpretations of hypersensitivity are discussed. A tabulation of spectroscopic and crystallographic data is given and analyzed for 262 europium compounds. Moreover, new calculations of the ratio 2 are presented for selected examples from the table, and these possess good agreement with the experimental data. Other factors which can affect the value of 2 are discussed.

中文翻译：

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铕(III)配位化学：结构、光谱和超敏性


三正铕的红色或橙色发射光谱已导致数千种出版物和荧光粉应用。橙色发射主要由 D → F 跃迁 (A) 引起，而在其他情况下，红色发射则由 D → F 跃迁 (B) 引起，但机制不同。强度比 B/A 通常标记为 2，被称为不对称比，并已被用作材料中铕位点对称性的探针。我们证明 2 并不是位点对称性的唯一指标，尽管如果配位环境的结构对称性发生变化，它确实会发生变化。我们提出新术语“超敏比率”来取代“不对称比率”。回顾了晶体学和光谱位点对称性的概念，以及Eu电子能谱的理论基础。图中展示了配位数为 6 至 12 的铕配合物的结构，并讨论了之前对超敏反应的解释。给出并分析了 262 种铕化合物的光谱和晶体学数据。此外，针对表中选定的示例，给出了比率 2 的新计算，并且这些计算与实验数据具有良好的一致性。讨论了可能影响 2 值的其他因素。