调控Al_(2)O_(3)晶型控制MgAl_(2)O_(4):Mn^(4+)荧光粉中Mn价态研究

Valence State Control of Manganese in MgAl_(2)O_(4):Mn^(4+)Phosphor by Varying the Al_(2)O_(3)Crystal Form

Mn^(4+)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。Mn^(4+)离子^(2)E→^(4)A_(2)跃迁在铝酸盐中的最短发光波长是在MgAl_(2)O_(4)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(Mg^(2+)/Al^(3+)),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Al_(2)O_(3)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Mn^(2+)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-Al_(2)O_(3)为铝源,有效抑制了锰离子在MgAl_(2)O_(4)中Mg^(2+)格位的占据及Mn^(2+)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MgAl_(2)O_(4):Mn^(4+)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Mn^(4+)激活铝酸盐荧光粉中锰离子掺杂价态调控提供了新思路。

Mn^(4+)activated red phosphor is one of the current research hot-spots in the field of white light emitting diodes(wLEDs).The shortest emission of Mn4+2E→4A2 transition in aluminate is 651 nm realized in MgAl_(2)O_(4),but the doped manganese ions exists in multiple valence states(+2/+4/+3,etc.)due to the fact that there exist two cationic sites(Mg^(2+)/Al^(3+))forming tetrahedron or octahedron coordination in the spinel structure.In this study,variation of the Al_(2)O_(3)polymorphs(γ/αratio)in the starting materials and post-annealing were used to control the doping sites and valence state of manganese ions in the MgAl_(2)O_(4)structure.The results show that a highα/(α+γ)ratio of starting Al_(2)O_(3)favors the formation of Mn^(2+)while a lowα/(α+γ)ratio of starting Al_(2)O_(3)favors the formation of Mn^(4+)dopant.By using highly active nano-γ-Al_(2)O_(3)as the Al^(3+)-bearing source,the occupancy of manganese ions in the Mg^(2+)site and the formation of Mn^(2+)valence state were effectively suppressed.Bright and pure MgAl_(2)O_(4):Mn^(4+)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 Al_(2)O_(3)polymorph lies in that the reactivity of Al_(2)O_(3)determined the sequences of doping reactions and then the doping site/valence of manganese ions in MgAl_(2)O_(4):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.