Potassium europium molybdate, KEu(MoO₄)₂, is an intriguing material known for its efficient luminescence properties attributed to Eu³⁺ ions and its polymorphic nature. Despite its significance, research on the metastable γ-phase has been limited, with no prior reports on its structure, particle morphology, and luminescence characteristics. In this study, both the stable α-phase and the metastable γ-phase of KEu(MoO₄)₂ were synthesized using solid-state and hydrothermal reaction methods, and their crystal structures, particle morphologies, and luminescence properties were comprehensively investigated. X-Ray diffraction analysis confirmed the formation of the triclinic α-phase and the orthorhombic γ-phase, with factor analysis results consistent with theoretically optimized structures, facilitating accurate structural determination. Both phases exhibited typical photoluminescence (PL) spectra of Eu³⁺ ions. However, in the γ-phase, the ⁵D₀ → ⁷F₂ transition appeared as a non-split peak with minimal Stark effect, attributed to differences in structural symmetry between the phases. A red-shift in the PL excitation edge was observed in the α-phase, which was attributed to a narrowed band gap resulting from the broadening of the O-p orbital in the valence band, as indicated by density functional theory (DFT) calculations. Additionally, KEu(MoO₄)₂ was doped with Y³⁺ ions to form K(Eu,Y)(MoO₄)₂, revealing a solid solubility limit of 40% in the α-phase, while the γ-phase displayed no solid solution limit. The estimated critical distance of Eu³⁺ ions suggested that Eu–Eu interactions contributing to concentration quenching are minimal in the stacking direction but significant in the in-plane direction, operating in the second shell.

中文翻译：

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阐明水热合成亚稳斜方结构γ-KEu(MoO4)2微晶荧光粉的结构特征和光致发光性能以及Y3+稀释导致的结构转变导致的发光性能差异

钼酸钾铕，KEu(MoO ₄ ) ₂，是一种有趣的材料，以其Eu ³⁺离子的高效发光特性及其多晶型性质而闻名。尽管亚稳态γ相具有重要意义，但其研究仍然有限，之前没有关于其结构、颗粒形貌和发光特性的报道。本研究采用固态和水热反应方法合成了KEu(MoO ₄ ) ₂的稳定α相和亚稳态γ相，并全面研究了它们的晶体结构、颗粒形貌和发光性能。 X射线衍射分析证实了三斜晶α相和斜方γ相的形成，因子分析结果与理论优化结构一致，有利于准确的结构测定。两个相均表现出典型的 Eu ³⁺离子光致发光 (PL) 光谱。然而，在γ相中，⁵ D ₀ → ⁷ F ₂转变表现为具有最小斯塔克效应的非分裂峰，这归因于相之间结构对称性的差异。密度泛函理论 (DFT) 计算表明，在 α 相中观察到 PL 激发边发生红移，这是由于价带 Op 轨道变宽而导致带隙变窄。此外，KEu(MoO ₄ ) ₂掺杂Y ³⁺离子形成K(Eu,Y)(MoO ₄ ) ₂，在α相中显示出40%的固溶度极限，而γ相则没有显示出固溶度极限。固溶体极限。 Eu ³⁺离子的估计临界距离表明，有助于浓度猝灭的Eu-Eu相互作用在堆积方向上最小，但在面内方向上显着，在第二个壳层中起作用。