The Controllable Self-reduction Of Eu³⁺ In Air And The Effect On The Luminescence Properties

Eu²⁺,as a common luminescent materials activator,has some disadvantages such as high energy consumption,high cost and potential risk since it need to be prepared in reduced conditions.In this paper,a series of Eu²⁺-activated phosphors with Eu2O3 as the activator raw materials were prepared in air by optimizing the matrix structure,preparing technology and controlling crystal field environment.This self-reduction process was investigated in detail and used for direct white light emitting phosphor.The novel ?-Mg₂Al₄Si₅O₁₈:Eu²⁺phosphor was prepared by high temperaturesolid state methodand the reduction of Eu³⁺ to Eu²⁺ in air was observed in ?-Mg₂Al₄Si₅O₁₈:Eu.Emission and excitation spectra were employed to detectthe presence of Eu²⁺ ions in the compound.Theself-reduction of Eu³⁺ to Eu²⁺ in?-Mg₂Al₄Si₅O₁₈ can be explained with charge compensation model,and thestructures of 3-D networks composed by [Si/Al O4] and [AlO4]tetrahedra are possible for maintaining of the reduction(Eu³⁺-Eu²⁺)when samples were prepared in air at high temperature.By investigating the valence state stability of Eu in SrAl₂Si₂O₈ at air atmosphere and LaSiO₂ N at reducting atmosphere,we found that the intensity of Eu²⁺ emission and the reduction process of Eu³⁺ were enhanced with the increase of Eu concentration and preparation temperature no matter at the air or reducting atmospherewhen the different valence substitution happened.Based on this relationship,the BaAl₂O₄ host,which can emit the cyan light when active by Eu²⁺ and emit the orange light with Eu³⁺ doped,was used for the host of direct white-emitting phosphor.By adjusting the amount of Eu and preparation temperature,the direct white-emitting BaAl₂O₄:Eu²⁺,Eu³⁺phosphor was prepared at 1200 oC with the 12 mol% Eu concentration.Based on the charge compensation model,the charge compensation process can be controlled by charge compensating agent.By changing the concentration of Li,the color of Ba₂MgSi₂O₇:Eu,which shows the cyan light and orange light with Eu²⁺ and Eu³⁺,can be tuned from green to white.When the concentration of Li was 4.5 mol%,the Ba₂MgSi₂O₇:Eu²⁺,Eu³⁺,Li coulddirectly emit the white light.The self-reduction also can be applicable to the Mn4+.TheEu²⁺ and Mn²⁺ co-doped ?-Mg₂Al₄Si₅O₁₈:Eu²⁺,Mn²⁺ phosphor was prepared by high temperaturesolid state methodin air,although the raw material of Mn is MnO₂.Theself-reduction of Mn4+ to Mn²⁺ in?-Mg₂Al₄Si₅O₁₈ was investigated in detail and also attribute to the charge compensation model and rigid structure.Based on the energy transfer between the Eu²⁺-Mn²⁺,the color of ?-Mg₂Al₄Si₅O₁₈:Eu²⁺,Mn²⁺ phosphor can be tuned by controlling the Mn concentration.The direct white-emitting?-Mg₂Al₄Si₅O₁₈:Eu²⁺,Mn²⁺phosphor can be prepared when the Eu concentration is 12 mol%.The Eu²⁺,Eu³⁺ and Mn4+ codoped Sr₄Al₁₄O₂5: Eu²⁺,Eu³⁺,Mn4+ was firstly prepared by combining the self-reduction and the [AlO6]octahedra in host which is suitable for the substitution of Mn4+.The ratio between the Mn4+ and Eu²⁺/Eu³⁺ can be controlled by amount of Mn,and the color of Sr₄Al₁₄O₂5: Eu²⁺,Eu³⁺,Mn4+ will locate on the white light region when the concentration is 0.2 mol%.