Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum - 695 019, India.
Phys Chem Chem Phys. 2018 Oct 7;20(37):24287-24299. doi: 10.1039/c8cp03863e. Epub 2018 Sep 13.
Eu ions epitomize excellent structural probes for the detection of disorder induced by structural variations in stannate pyrochlores due to their relatively simple energy level structure. In this context, we have synthesized a series of Eu doped LnYSnNbO:0.15Eu (Ln = La, Gd, Y, and Lu) red phosphors via a high temperature solid state reaction method to explore the influence of phase evolution on the luminescence properties. The substitution of Ln ions in the A sites of the pyrochlore induces a structural transition from an ordered pyrochlore to a disordered fluorite structure with decreasing ionic radius. The diminishing trend of the characteristic superstructure peaks in the XRD pattern and the broadening of Raman modes with lanthanide substitution clearly substantiate the disorder induced in the cationic sublattice. These phosphors exhibit strong absorption in the near UV region and emit red luminescence under 392 nm excitation, which also correlates to their crystal structures. Their luminescence properties were enhanced with the increased ordering of cations from Lu to La. The more ordered structure of the La system favors a more uniform distribution of Eu ions, preventing cluster formation, and thus improving the luminescence properties. The splitting of the D-F transition of the Eu ions further provides a clue for the structural transition in support of the XRD and Raman analysis. The long range ordering, ligand polarizability, and covalent nature of the Eu bonding influence the emission probability and improve the quantum efficiency of the La host. Eu emissions resulting from D-F transitions are significantly improved with increases in the concentration of Eu ions in the La host. The distortion of the A site symmetry and the red shift of the charge transfer band with Eu doping in the LaYSnNbO system leads to an enhanced electric dipole transition. The extent of the distortion in the EuO polyhedra is reflected in the quantum efficiency values and J-O intensity parameters, Ω and Ω, which are a measure of the degree of polarizability of the Eu-O bonds in the lattice. Our results suggest that we can manipulate the Eu red luminescence in lanthanide stannate pyrochlores through order-disorder structural transitions.
Eu 离子是探测锡酸钇钙矿中结构变化引起的无序的理想结构探针，因为它们的能级结构相对简单。在这种情况下，我们通过高温固相反应法合成了一系列 Eu 掺杂的 LnYSnNbO:0.15Eu（Ln=La、Gd、Y 和 Lu）红色荧光粉，以探索相演变对发光性能的影响。Ln 离子在钙钛矿 A 位的取代会导致从有序钙钛矿到具有减小的离子半径的无序萤石结构的结构转变。随着镧系元素取代，XRD 图谱中特征超结构峰的减少趋势和拉曼模式的变宽清楚地证实了阳离子亚晶格中的无序。这些荧光粉在近紫外区域具有强烈的吸收，并在 392nm 激发下发射红色发光，这也与其晶体结构有关。随着阳离子有序度从 Lu 增加到 La，它们的发光性能增强。La 体系更有序的结构有利于 Eu 离子更均匀的分布，防止了团簇的形成，从而提高了发光性能。Eu 离子的 D-F 跃迁的分裂进一步为结构转变提供了线索，支持了 XRD 和拉曼分析。Eu 键的长程有序、配体极化率和共价性质影响发射概率并提高 La 主体的量子效率。随着 La 主体中 Eu 离子浓度的增加，Eu 离子的 D-F 跃迁发射得到显著改善。在 LaYSnNbO 体系中，A 位对称性的扭曲和电荷转移带的红移导致电偶极跃迁增强。EuO 多面体的扭曲程度反映在量子效率值和 J-O 强度参数Ω和Ω中，这是晶格中 Eu-O 键的极化率的度量。我们的结果表明，我们可以通过有序-无序结构转变来操纵镧系锡酸钇钙矿中的 Eu 红色发光。
