调控Al2O3晶型控制MgAl2O4:Mn4+荧光粉中Mn价态研究

Mn⁴⁺激活红光荧光粉是白光半导体发光二极管(wLEDs)领域当前研究热点之一。Mn⁴⁺离子²E→⁴A₂跃迁在铝酸盐中的最短发光波长是在MgAl₂O₄中实现的651 nm发光, 由于其结构中含有形成四面体或八面体配位的两种阳离子格位(Mg²⁺/Al³⁺), 易造成所掺杂锰元素存在多种价态(+2/+4/+3等)。本研究通过改变起始原料中Al₂O₃的晶型(γ/α比例)及退火处理来调控锰离子在MgAl₂O₄晶格中的占据格位, 对其主要存在价态实现调控。采用荧光光谱和紫外-可见-近红外漫反射光谱技术来表征所合成荧光粉中Mn离子的价态及其演变。研究发现, 高α/(α+γ)比铝源促进Mn²⁺形成, 而低α/(α+γ)比铝源促进Mn⁴⁺形成。通过使用高活性纳米γ-Al₂O₃为铝源, 有效抑制了锰离子在MgAl₂O₄中Mg²⁺格位的占据及Mn²⁺离子的形成, 经空气中1550 ℃保温5 h的一次高温热处理即可制备出在可见光区只有Mn⁴⁺红光发光的高纯高亮度MgAl₂O₄:Mn⁴⁺荧光粉。氧化铝晶型影响锰离子掺杂格位和掺杂价态的本质规律是: 氧化铝活性决定实际固溶掺杂反应步骤, 进而影响锰离子掺杂格位和价态。本研究提出的反应步骤调控作为反应气氛、电荷补偿剂、反应温度三种调控方法外的一种新方法, 为Mn⁴⁺激活铝酸盐荧光粉中锰离子掺杂价态调控提供了新思路。

Valence State Control of Manganese in MgAl2O4:Mn4+ Phosphor by Varying the Al2O3 Crystal Form

Mn⁴⁺ activated red phosphor is one of the current research hot-spots in the field of white light emitting diodes (wLEDs). The shortest emission of Mn^{4+ 2}E→⁴A₂ transition in aluminate is 651 nm realized in MgAl₂O₄, but the doped manganese ions exists in multiple valence states (+2/+4/+3, etc.) due to the fact that there exist two cationic sites (Mg²⁺/Al³⁺) forming tetrahedron or octahedron coordination in the spinel structure. In this study, variation of the Al₂O₃ polymorphs (γ/α ratio) in the starting materials and post-annealing were used to control the doping sites and valence state of manganese ions in the MgAl₂O₄ structure. The results show that a high α/(α+γ) ratio of starting Al₂O₃ favors the formation of Mn²⁺ while a low α/(α+γ) ratio of starting Al₂O₃ favors the formation of Mn⁴⁺ dopant. By using highly active nano-γ-Al₂O₃ as the Al³⁺-bearing source, the occupancy of manganese ions in the Mg²⁺ site and the formation of Mn²⁺ valence state were effectively suppressed. Bright and pure MgAl₂O₄:Mn⁴⁺ phosphors in which only the red luminescence from Mn⁴⁺ was observed in the visible spectral region were successfully prepared via once heat treatment at 1550 ℃ for 5 h in air. The intrinsic reason for the dependence of manganese doping valence state on the Al₂O₃ polymorph lies in that the reactivity of Al₂O₃ determined the sequences of doping reactions and then the doping site/valence of manganese ions in MgAl₂O₄:Mn. All the above data demonstrated that the control of reaction sequences was a new method to regulate the valence state of manganese in aluminate phosphors.