铕取代的锆硼钨多金属氧酸盐的制备和发光特性

制备了高发射效率的稀土多金属氧酸盐BPOM:Eu及ZrPOM:Eu荧光体,并对其结构及发光性质进行了研究。结果表明样品具有Keggin结构,Eu在两类样品中均有肉眼可见的在红光区的强发射。但在BPOM:Eu中以⁵D₀→⁷F₁磁偶极跃迁发射为主,在ZrPOM:Eu中以⁵D₀→⁷F₂电偶极跃迁发射为主。铕在ZrPOM:Eu中的对称性低于在BPOM:Eu中的对称性。这种差异也使铕离子的激发光谱具有不同的特征。铕在两种多金属氧酸盐中均有浓度猝灭,在BPOM:Eu中浓度为2.0%时发射强度最大;在ZrPOM:Eu中浓度为2.4%时发射强度最大。在ZrPOM:Eu中有较弱的配体到中心铕离子的能量传递,而在BPOM:Eu中却未观测到能量传递现象。

Preparations and the Luminescent Properties of BPOM:Eu and ZrPOM:Eu

Two kinds of europium-substituted Keggin type polyoxometalates of BPOM:Eu and ZrPOM:Eu have been synthesized,which are highly luminescent.Their Keggin type structure has been characterized by IRand XRDspectra.The emission and excitation spectra have been measured at room temperature.The emission spectra show Eu gives strong emission in red light region,but the magnetic dipole transition⁵D₀→⁷F₁ domains others in BPOM:Eu;while a hypersensitive electric dipole transition⁵D₀→⁷F₂ domains others in ZrPOM:Eu.The⁵D₀→⁷F₁ and⁵D₀→⁷F₂ transitions are extremely sensitive to the chemical surroundings in the vicinity of the Eu³⁺ ions.The⁵D₀→⁷F₂ transition is not only permissive in asymmetry environment but also is of hypersensitive forbidden transition which is extremely sensitive to the chemical surroundings in the vicinity of the Eu³⁺ ions.When the Eu³⁺ ions’ sites hold asymmetry centers,the⁵D₀→⁷F₂ transition of Eu³⁺ ions is permissive while in symmetrical environment the⁵D₀→⁷F₁ transition is permissive.Judged from the spectra results the Eu site symmetry in BPOM:Eu surpasses that in ZrPOM:Eu.This contrast stems from different nature (radius and polarative ability) of the boron and zirconium heteroatoms.The site difference also influnce on Eu³⁺ ions’ excitation spectra.The spectrum of ZrPOM:Eu has four main groups of peaks:the highest peak laying at 395 nm,the others laying at 381～385 nm,464 and 535 nm with similar intensity.The spectrum of BPOM:Eu has two main groups of peaks:the highest peak lying at 395 nm,the second highest peak laying at 381～385 nm,the others are very weak.There exits energy transfer among Eu³⁺ ions.Resonant transfer is important ways for lanthanide especially the same kinds of lanthanide ions due to more spectra overlapping.This function leads to typical fluorescence quenching,the maximum intensity lies at Eu concentration of 2.0% in BPOM:Eu and at Eu concentration of 2.4% in ZrPOM:Eu respectively.There exist two characteristic absorption bands of Keggin anions at nearly 200 and 260 nm in ultraviolet region ascribeed to O_d→W charge transition.There may exist possibility of energy transfer within short distance.There exists low intramolecular energy transfer from the excited O_d→W charge transition to Eu³⁺ in ZrPOM :Eu,but doesn’t in BPOM:Eu at room temperature.