Crystal Structures and Photoluminescence across the La₂Si₂O₇–Ho₂Si₂O₇ System

It is well-known that when an RE2Si2O7 matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho3+-substituted La2Si2O7. Several compositions across the La2Si2O7–Ho2Si2O7 system were synthesized using the sol–gel method followed by calcination at 1600 °C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)2Si2O7 which extends to the La0.6Ho1.4Si2O7 composition. Compositions richer in Ho3+ show a two-phase domain (G+δ), while δ-(La,Ho)2Si2O7 is the stable phase for Ho3+ contents higher than 90% (La0.2Ho1.8Si2O7). Anomalous diffraction data interestingly indicated that the La3+ for Ho3+ substitution mechanism in the G-(La,Ho)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Ho3+ for the RE2 site is observed. The Ho3+-doped G-La2Si2O7 phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho3+ content of 10%.众所周知，当RE₂Si₂O₇基质掺杂活性镧系离子（lanthanide ions）时，其展现出适用于光学应用的优异发光性能。该硅酸盐基质所采用的晶体结构，以及掺杂剂在可及的稀土晶体学位置中的分布，对这类化合物的发光产率具有显著影响。本研究旨在分析钬离子（Ho³⁺）取代型二硅酸镧（La₂Si₂O₇）的结构行为与发光性能。本研究通过溶胶-凝胶法（sol–gel method）合成了La₂Si₂O₇-Ho₂Si₂O₇体系中的多种组分，并于1600℃下进行煅烧处理。采用X射线衍射（X-ray diffraction）与中子衍射（neutron diffraction）技术对所得粉体进行表征，以确定该体系的相稳定性。结果表明，G-(La,Ho)₂Si₂O₇存在固溶区（solid solubility region），其组分范围可延伸至La₀.₆Ho₁.₄Si₂O₇。钬离子含量更高的组分则呈现双相区（two-phase domain）(G+δ)；当钬离子占比高于90%（对应组分为La₀.₂Ho₁.₈Si₂O₇）时，δ-(La,Ho)₂Si₂O₇为稳定相。有趣的是，反常衍射数据显示，G-(La,Ho)₂Si₂O₇多晶型（polymorph）中镧离子（La³⁺）与钬离子的取代机制并非均匀进行，而是观测到钬离子优先占据RE₂位。钬离子掺杂的G-La₂Si₂O₇荧光粉（phosphors）在446nm波长激发下展现出强烈的绿色发光。寿命测试（lifetime measurements）结果显示，钬离子掺杂量为10%的荧光粉性能最优。