Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate

Abstract

An atomistic simulation method has been used to study the equilibrated geometry, defect structure, and phase mixing of barium magnesium aluminate ͑BAM͒ and related compounds. The calculated lattice energies are used to study the stability of defect structures in the ␤-alumina lattice, particularly in the presence of excess alumina in the starting materials. It is shown that excess alumina in the starting compounds could lead to the formation of Reidinger defects in BAM. The calculated structural modification of the lattice due to a Reidinger defect is in excellent agreement with the experimental results. These defects are responsible for generation of F + and F-type color centers by ͑vacuum͒ ultraviolet radiation and bombardment of energetic particles from the discharge. BAM could be made resistant to the formation of color centers by adhering to a strictly stoichiometric formulation of this phosphor.Barium magnesium aluminate ͑BaMgAl 10 O 17 ͒ activated by divalent europium ions is a blue-emitting phosphor. This phosphor is popularly known as BAM in lighting applications. It is used as a blue-emitting phosphor in triband phosphor coatings in low-pressure Hg-discharge lamps. Unlike the other components of triband phosphors, namely, the red-emitting yttrium oxide phosphor activated by trivalent europium, and the green-emitting lanthanum phosphate phosphor activated by terbium, BAM is relatively unstable during high-temperature processing ͑ϳ400°C͒ of certain fluorescent lamps in an oxidizing environment. 1,2 Similar processing conditions are also encountered in plasma display panel applications with similar results. BAM also tends to degrade faster during the lamp life under high loading conditions, a leading to "unacceptable" color shifts during the lamp life. The last few years have seen intense activities in the luminescent community toward developing an understanding of the nature of the thermal degradation process. It is believed that the thermal degradation of BAM results from the oxidation of europium ions. 3 However, similar efforts to understand the nature of the degradation process during the lamp life are still lacking. This paper presents the results of a theoretical investigation of the structure and origin of defects that could contribute to the generation of color centers during the lamp life.One of the reasons for phosphor degradation in the lamp environment is the generation and growth of color centers in phosphors due to lamp operation. Inside the fluorescent lamps, the phosphors exist in a very harsh environment of an arc discharge. During the lamp operation, the surfaces of microcrystalline phosphor particles are continuously subjected to bombardment of energetic electrons, ions, and neutral atoms and irradiation by vacuum ultraviolet ͑VUV͒ photons. The color centers are induced by these discharge-related processes.The lamp phosphors are usually made from complex inorganic hosts. The color center literature is sparse on the investigation of such systems. The haloapatite phosphors are the only cl...

A highly Mn2+-doped narrowband green phosphor toward wide color-gamut display applications

“…To maintain charge balance, a series of point defects and vacancies spontaneously emerged and dispersed randomly across the mirror plane. 30,[36][37][38] In the hexaaluminate structure, Mn 2+ prefers to occupy the Al(2) site in the [Al(2)O 4 ] tetrahedron due to the lowest formation energy. 29 Additionally, the Mn 2+ doping sites in the host must be separated by a relatively large distance to attenuate the energy migration between Mn 2+ ions, thus suppressing the PL concentration quenching effect and achieving a higher Mn 2+ doping concentration.…”

Theoretical Investigation of Intercalated Water Molecules and Hydroxyl Groups in BAM (BaMgAl[sub 10]O[sub 17]:Eu[sup 2+]) Phosphor and Associated Degradation Processes

“…Classical atomistic simulation.-Interatomic potentials.-In a previous paper, 5 the lattice dynamics based atomistic simulation approach for calculating the equilibrated geometry of a defect lattice was discussed in detail. However, simulating the structure of water or hydroxyl groups inside an ionic crystal presents a unique challenge for a simulation approach based on classical models of interatomic potentials.…”

“…The potential parameters used for rest of the crystal are same as those used earlier in our simulation study of BAM and related compounds. 5,8 In this work, the atomistic simulations of the optimized structures of BAM with intercalated water molecules and hydroxyl groups have been performed using the SHELL simulation package. 9 The code implements a lattice dynamics approach for calculating the temperature dependence of the thermodynamic properties of solids.…”

Novel blue-emitting phosphor Sr2.979Eu0.021Al10−xBxSiO20 (x=0, 0.25, 0.5, 0.75 and 1) for PDP applications

“…The electron transfer process responsible for reduction of Eu 2+ ion (present in Sr 2+ sites) is inhibited. This is due to the framework structure of AlO 6 [17]. The formation of anionic tetrahedra BO 4 5À network, which facile the electron transfer process for the reduction of Eu 3+ -Eu 2+ .…”

“…The blue phosphor BaMg 10 O 17 :Eu 2+ (BAM) has been of particular interest, because it is considerably less stable than the red and green-emitting phosphors, in terms of panel fabrication (thermal damage) and panel operation (VUV damage). The issues have already been well documented by numerous researchers [2][3][4][5][6]. There are some approaches that have already been used to improve the stability of BAM phosphor.…”