Crystal chemistry and luminescence of the Eu 2 -activated alkaline earth aluminate phosphors

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1 Displays 19 (1999) Crystal chemistry and luminescence of the Eu 2 -activated alkaline earth aluminate phosphors D. Ravichandran*, Shikik T. Johnson, S. Erdei, Rustum Roy, William B. White Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Abstract Alkaline earth monoaluminate phosphors activated with Eu 2 were synthesized by microwave processing and alkaline earth hexaaluminate phosphors activated by Eu 2 were synthesized by hydrothermal reactions. The emission spectra of the monoaluminates are similar to phosphors prepared by high-temperature firing except for BaAl 2 O 4 which emits farther in the blue. The spectra of the hexaaluminate phosphors in general have their emission peak at different wavelengths than what are nominally the same compositions prepared by high-temperature firing Published by Elsevier Science B.V. All rights reserved. Keywords: Plasma display phosphors; Hexaaluminates; Eu-activation 1. Introduction An important class of fluorescent lamp and plasma display phosphors are based on compounds in the alkaline-earth rare-earth aluminate systems. These phosphors fall into three broad classes: 1. Binary alkaline earth aluminates such as SrAl 2 O 4 [1,2]. 2. Alkaline earth hexaaluminates related to magnetoplumbite and beta-alumina and their superstructures [3 11]. 3. Rare earth hexaaluminates with the magnetoplumbite structure [12 15]. A variety of activators were used of which the most important are Eu 2, Ce 3 /Tb 3, and to a lesser extent, Mn 2 [16 20]. This paper is concerned with the systematic relations between luminescence emission and crystal structure for Eu 2 -activated alkaline earth aluminates of classes (1) and (2) and with aspects of the synthesis of these refractory compounds. 2. Crystal chemistry of the aluminates The compounds MAl 2 O 4,MvCa, Sr, and Ba, are formed of a three-dimensional framework of corner-sharing AlO 4 tetrahedra. Each oxygen is shared with two aluminum ions so that each tetrahedron has one net negative charge. Charge * Corresponding author. Present address: U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA. balance is accomplished by the large divalent cations which occupy interstitial sites within the tetrahedral framework. The tetrahedral framework is isostructural with the SiO 2 polymorph, tridymite, so that these compounds are stuffed tridymite structures [21]. BaAl 2 O 4 has the ideal stuffed tridymite structure. SrAl 2 O 4 undergoes a phase transition from a low-temperature monoclinic distorted structure to the hexagonal tridymite structure at 650 C [22]. CaAl 2 O 4 has a stuffed tridymite structure but transforms to at least three other polymorphs at high pressures [23]. In the stuffed tridymite structure there are two sites for the large cation, each with 9-fold coordination. The binary hexaaluminates are compounds with the formula MAl 12 O 19. CaAl 12 O 19 (known by the mineral name of hibbonite) and SrAl 12 O 19 both have the magnetoplumbite structure [24,25]. In contrast, the compound BaAl 12 O 19 does not exist and instead there are two compounds, one deficient in barium and one with excess barium, that were shown to have the beta-alumina structure [26 28]. The large cation is 12-coordinated in the magnetoplumbite structure and 9-coordinated in the beta-alumina structure. When MgO is added as a third component, a set of more complicated superstructures appear, all based on stacking sequences of magnetoplumbite, beta-alumina, and spinel building blocks. Many compositions were investigated as potential phosphor hosts but the number of distinct compounds and their compositions were uncertain. Recent detailed crystallographic work [29 33] on these systems has shown that there are only two additional high alumina ternary phases in each MO MgO Al 2 O 3 system, though /99/$ - see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S (98)00050-X

198 D. Ravichandran et al. / Displays 19 (1999) 197 203 Table 1 Alkaline earth hexaaluminate compounds Code Composition Structure a a 0 b these phases do not all have the same compositions.

2 198 D. Ravichandran et al. / Displays 19 (1999) Table 1 Alkaline earth hexaaluminate compounds Code Composition Structure a a 0 b these phases do not all have the same compositions. Table 1 lists the accepted high alumina compounds of the alkaline earths. All are hexagonal or trigonal with very complex X- ray powder patterns, making difficult the identification of the phases and the determination of phase purity. c 0 b Space group CA 6 CaAl 12 O 19 Mp P6 3 /mmc CAM-I Ca 2 Mg 2 Al 28 O 46 2Mp, Sp R3m CAM-II CaMg 2 Al 16 O 27 Mp, Sp P6m2 SA 6 SrAl 12 O 19 Mp P6 3 /mmc SAM-I Sr 2 MgAl 22 O 36 Mp, Sp P6m2 SAM-II SrMgAl 10 O 17 ba P6 3 /mmc BA 6 -I 0.82BaO.6Al 2 O 3 ba P6 3 /mmc BA 6 -II 1.32BaO.6Al 2 O 3 ba P6m2 BAM-I BaMgAl 10 O 17 ba P6 3 /mmc BAM-II BaMg 3 Al 14 O 25 ba, Sp P6m2 a Mp ˆ magnetoplumbite block, Sp ˆ spinel block, ba ˆ betaalumina block. b Unit cell data mainly from Goebbels [30] and confirmed by our X-ray measurements. Fig. 1. (a) Hexagonal platelets of BaMgAl 10 O 17. The large hexagonal crystal is about 5 mm in diameter. (b) Equant crystals of Ca 2 Mg 2 Al 28 O Synthesis The alkaline earth aluminates are refractory compounds. Synthesis by usual ceramic processing methods requires firing temperatures in the range of 1600 C 1800 C. The compounds for which luminescence data were published represent the stable forms at high temperature. Two alternate synthesis methods were considered: (i) microwave processing which also achieves high temperature, and (ii) hydrothermal processing which allows synthesis at temperatures in the range of 700 C 800 C. The alkaline earth monoaluminate phosphors were synthesized by microwave processing using appropriate mixtures of CaCO 3, SrCO 3, and BaCO 3 with Dispersal (AlOOH in boehmite form). Ten gram batches of finelypowdered starting mixture containing 3 wt% Eu 2 O 3 were reacted in alumina crucibles in the microwave chamber. To maintain a reducing atmosphere, the chamber was continuously flushed with forming gas (95% N 2,5%H 2 ). Microwave energy at 2.54 GHz was injected into the chamber at an initial power of 100 W as the temperature increased. The power injection was raised to 2.5 kw which was sustained for 30 min. Temperatures, as measured by optical pyrometer, ranged from 1250 C to 1600 C. Well crystallized phosphor could be prepared in 30 min reaction time but the desired MAl 2 O 4 compound were mixed with small amounts of other known phases from the corresponding binary systems. For the hydrothermal experiments, precursor materials were prepared from Ba(NO 3 ) 2, Al(NO 3 ) 3 9H 2 O, Mg(NO 3 ) 2 6H 2 O (Baker analyzed reagent 99.98% purity), Eu(NO 3 ) 3, and Ga(NO 3 ) 3 9H 2 O (Aldrich Chemicals 99.99% purity). The nitrates were weighed stoichiometrically and dissolved in ml deionized water and stirred well for min to get a clear solution. The solutions were then titrated against 15 M NH 4 OH to completely precipitate the metals as hydroxides. These were washed with deionized water to remove the nitrate ion. The colloidal hydroxides were oven dried at 120 C 125 C and then calcined at 600 C between 10 and 12 h to completely remove the water and last traces of nitrates. The powder precursors were welded into gold capsules along with a known quantity of deionized water. A small amount of metallic aluminum was added to the mixture to maintain the europium in the reduced Eu 2 state. These were reacted in hydrothermal pressure vessels at temperatures in the range of 500 C 800 C under pressures of MPa for h. At the end of the reaction period, temperature and pressure were quenched, the capsules opened, and the specimens removed and dried for characterization and spectroscopic measurements. Reaction in the presence of hydrothermal fluids allows the growth of faceted crystals. Most of the hexaaluminates took the form of hexagonal platelets with well developed faces (Fig. 1a). An exception was Ca 2 Mg 2 Al 28 O 46 which appeared as more equant and smaller sized crystals (Fig.

3 D. Ravichandran et al. / Displays 19 (1999) as a symmetrical band at 512 nm, a slightly shorter wavelength than the 521 nm observed by Palilla et al. [1] and 518 nm, observed by Song et al. [34]. The excitation band is broader and the two-band structure is not well resolved. The greatest contrast between phosphor prepared by microwave processing and phosphor prepared by high-temperature ceramic firing is observed in BaAl 2 O 4. Palilla et al. [1] reported a somewhat asymmetrical band peaking at 500 nm. Poort et al. [2] measured their spectra at 4.2 K and resolved the emission band into a main component at 511 nm and the long wavelength tail inito a second band at 540 nm. The main emission band of the microwave processed phosphor occurs at 443 nm with an intense shoulder in the range of nm. Microwave processing of CaAl 2 O 4 and Sr Al 2 O 4 produces phosphors with properties comparable to phosphors prepared by firing at high temperature except that the microwave processing requires from 30 min to as little as 10 min. To account for the distinctly different emission characteristics of Ba Al 2 O 4, it may be assumed that there is a different distribution of Eu 2 over the two cation sites in the microwave processed material. Fig. 2. Emission and excitation spectra for CaAl 2 O 4, SrAl 2 O 4 and BaAl 2 O 4 prepared by microwave processing. Mole fraction Eu 2 ˆ (b)). Hydrothermal processing produces phosphors with excellent crystallinity although at the expense of some possible uptake of OH ion which can act to enhance non-radiative decay. 4. Luminescence of the monoaluminates Emission and excitation spectra for the 1 : 1 alkaline earth aluminates prepared by microwave processing are shown in Fig. 2. The emission band of CaAl 2 O 4 at 439 nm is in good agreement in both wavelength and band shape with the measurements of Palilla et al. [1] who observed the band at 440 nm. The excitation spectrum shows the expected two bands from the crystal field splitting of the Eu 2 d-orbitals. The emission from SrAl 2 O 4 was observed 5. Luminescence of the hexaaluminates The emission characteristics of the binary hexaaluminates are summarized in Table 2. The Ca and Sr-compounds have the magnetoplumbite structure and have their peak emission in the deep violet portion of the spectrum. The Ba-deficient and Ba-excess compounds with the beta-alumina structure have distinctly different emission wavelengths. The 9-coordinated site in the beta-alumina structure produces a stronger crystal field than the 12-coordinated site in the magnetoplumbite structure, thus lowering the emitting level of Eu 2 and producing a longer wavelength emission. The compounds identified as distinct superstructures (Table 1) were synthesized by hydrothermal reaction and activated by replacing 0.01 or 0.02 of the alkaline earth ion by Eu 2. The peak emission wavelengths of the ternary hexaaluminates compared with compounds prepared by high temperature ceramic reaction are summarized in Table 2 Emission characteristics of magnetoplumbite binary compounds Compound Emission wavelength Emission wavenumber Reference CaAl 12 O 19 :Eu Verstegen and Stevels [3]; Stevels and Schrama-de Pauw [7] SrAl 12 O 19 :Eu Verstegen and Stevels [3]; Stevels and Schrama-de Pauw [7] BaAl 12 O 19 :Eu Verstegen and Stevels [3] 0.82BaO 6Al 2 O 3 :Eu Smets et al. [9] 1.32BaO 6Al 2 O 3 :Eu Smets et al. [9]

4 200 D. Ravichandran et al. / Displays 19 (1999) Table 3 Emission characteristics of ternary hexaaluminate phosphors Compound a Synthesis Emission wavelength Emission wavenumber Reference CAM-I Ca 2 Mg 2 Al 28 O 46 Hydr 419, 439, 467 This work CAM-II CaMg 2 Al 16 O 27 Hydr This work SAM-I Sr 2 MgAl 22 O 36 Hydr This work SAM-II SrMgAl 10 O 17 Hydr This work SrMgAl 10 O 17 Ceram [7] Sr 2 Mg 2 Al 22 O 37 Ceram [3] BAM-I BaMgAl 10 O 17 Hydr This work BaMgAl 10 O 17 Ceram [7] BAM-II BaMg 3 Al 14 O 25 Hydr This work Ba 2 Mg 2 Al 22 O 37 Ceram [3] 2461 b BaMg 2 Al 16 O 27 Ceram Sylvania c BaMg 2 Al 17 O 28.5 BaMg 2 Al 16 O 27 Hydr This work a Nomenclature according to Goebbels [30]. b Nominal composition of Sylvania Type 2461 commercial blue phosphor as given in Sylvania catalog. c Composition of Sylvania Type 2461 as determined by chemical analysis. Table 3. All emit in the blue but the wavelength varies both between compounds and with method of synthesis. The Ca-compounds, CAM-I and CAM-II, are composed of a superstructure of magnetoplumbite blocks with an additional spinel block. Only CAM-I (Fig. 3) is luminescent with a series of weak bands centered on 440 nm. There is some question of the stability of the calcium hexaaluminates in the presence of water during the quenching of the hydrothermal experiments. Two superstructures were identified with Sr 2 as the large cation. SAM-I has a structure based on magnetoplumbite with intermediate spinel blocks while SAM-II has a betaalumina structure (Table 1). Surprisingly, the two compounds have very similar emission and excitation spectra. However, the high temperature ceramic preparations have their peak emission at 465 nm (Table 3) while the hydrothermal preparations emit at nm (Fig. 4). Most puzzling of the ternary hexaaluminates are the Ba 2 compounds. A variety of materials were synthesized, all with slightly different compositions. The two compounds that were established as specific superstructures are BAM- I and BAM-II (Table 1). BAM-I prepared by high temperature ceramic processing has a Eu 2 emission peak at 450 nm and is used as a practical blue phosphor. The same composition prepared at 800 C and 71 MPa by hydrothermal reaction emits at 430 nm (Fig. 5). BAM-II is reported as having an extended beta-alumina structure with additional spinel Fig. 3. Emission and excitation spectra of CAM-I, Ca 2 Mg 2 Al 28 O 46 (Eu 2 ˆ 0.02). Fig. 4. Emission and excitation spectra of SAM-I, Sr 2 MgAl 22 O 36 (Eu 2 ˆ 0.02 and SAM-II, SrMgAl 10 O 17 (Eu 2 ˆ 0.01).

5 D. Ravichandran et al. / Displays 19 (1999) practical phosphors. Smets et al. [36] claim that BaMg 2 Al 16 O 27 is the same as BaMgAl 10 O 17 (BAM-I). A comparison between the spectrum of the commercial product and the same composition prepared by hydrothermal reaction is shown in Fig. 6. Both compounds have the same excitation wavelength, 334 nm, but the emission of the hydrothermal material appears at 463 nm rather than 450 nm. Otherwise, band shapes for both emission and excitation are very similar. 6. Excitation Fig. 5. Emission and excitation spectra of BAM-I, BaMgAl 10 O 17 (Eu 2 ˆ 0.01) and BAM-II, BaMg 3 Al 14 O 25 (Eu 2 ˆ 0.01). blocks in the stacking sequence. Hydrothermal synthesis of the ideal composition, BaMg 3 Al 14 O 25 activated with Eu 2 produces the blue-emitting phosphor with a peak at 467 nm (Fig. 5). The literature is somewhat ambiguous with respect to the composition of the efficient blue-emitting phosphor used in lamps and plasma displays. Phosphors with similar but not identical composition that were prepared by high temperature ceramic processing include Ba 2 Mg 2 Al 22 O 37 which emits at 449 nm [3] and BaMg 2 Al 16 O 27 (Sylvania Type 2461) which emits at 450 nm [35] (Table 3). Both are used as Reviewing the Eu 2 excitation spectra for the entire family of monoaluminate and hexaaluminate phosphors, it is apparent that the most efficient excitation is the direct pumping of the lowest component of the 5-d manifold, the exact energy of which depends on the crystal field of the Eu 2 coordination polyhedron. This state varies from 250 to 350 nm. The higher energy crystal field component of the 5-d generally provides weaker excitation or no excitation at all. Use of the alkaline earth aluminate phosphors in plasma display devices requires efficient excitation in the vacuum ultraviolet, especially near 147 nm where the xenon plasma has its strongest emission line. The most dramatic comparison is between the commercial phosphor, Sylvania Type 2461, and a material with the same nominal composition prepared by hydrothermal synthesis (Fig. 7). The excitation spectrum for Sylvania 2461 exhibits two broad excitation bands giving quantum efficiencies in the range of 85% 95%. The hydrothermal preparation has a sharp, narrow band excitation spectrum but the quantum efficiency of only 15% 20%. 7. Conclusions The synthesis and spectroscopic properties of a series of Fig. 6. Emission and excitation spectra of BaMg 2 Al 16 O 27, comparing hydrothermal preparation with reference phosphor, Sylvania 2461 (Eu 2 ˆ 0.04).

6 202 D. Ravichandran et al. / Displays 19 (1999) Fig. 7. Vaccum UV excitation spectra of BaMg 2 Al 16 O 27 comparing hydrothermal preparation with reference phosphor, Sylvania Eu 2 -activated alkaline earth aluminate phosphors were investigated. The monoaluminates of Ca, Sr, and Ba were synthesized by microwave processing. Well crystallized phosphor could be obtained in min in the microwave chamber with spectroscopic properties comparable to those obtained only by much longer firing at high temperature. The spectrum of microwave-processed BaAl 2 O 4 :Eu 2 was distinctly different from the spectrum of ceramic-processed phosphor, suggesting different distribution of activator ion over the possible Ba 2 sites. Examples of the ternary alkaline earth hexaaluminates were synthesized by hydrothermal reaction at temperatures in the range of 700 C 800 C. These reactions give well crystallized products but with emission at distinctly different wavelengths than the same compositions reacted by high-temperature firing. Two explanations are possible for the distinctively different spectroscopic behavior: (i) A greater degree of structural ordering or perhaps a different low-temperature structure in the hydrothermally prepared phosphors and (ii) a different distribution of activator ion over the alkaline earth sites. As Eu 2 gives a broad band emission in all structures, the emission band shape is not sensitive to degree of ordering. The much sharper and more detailed vacuum UV excitation spectrum for a Ba-hexaaluminate is suggestive of increased structural order. Acknowledgements This work was supported by the Defense Advanced Research Projects Agency (DARPA) through the Phosphor Technology Center of Excellence (PTCOE) under Grant no. MDA We are indebted to Dr. Matthias Goebbels for discussions of hexaaluminate crystal chemistry and to Prof. Richard Meltzer, University of Georgia, for measurement of the vacuum UV excitation spectra. References [1] F.C. Palilla, A.K. Levine, M.R. Tomkus, Fluorescent properties of alkaline earth aluminates of the type MAl 2 O 4 activated by divalent europium, J. Electrochem. Soc. 115 (1968) [2] S.H.M. Poort, W.P. Blokpoel, G. Blasse, Luminescence of Eu 2 in barium and strontium aluminate and gallate, Chem. Mater. 7 (1995) [3] J.M.P.J. Verstegen, A.L.N. Stevels, The relation between crystal structure and luminescence in b-alumina and magnetoplumbite phases, J. Luminescence 9 (1974) [4] J.M.P.J. Verstegen, J.L. Sommerdijk, A. Bril, Line emission of SrAl 12 O 19 :Eu 2, J. Luminescence 9 (1974) [5] J.M.P.J. Verstegen, A survey of a group of phosphors, based on hexagonal aluminate and gallate lattices, J. Electrochem. Soc. 121 (1974) [6] A.L.N. Stevels, J.M.P.J. Verstegen, Eu 2! Mn 2 energy transfer in hexagonal aluminates, J. Luminescence 14 (1976) [7] A.L.N. Stevels, A.D.M. Schrama-de Pauw, Eu 2 luminescence in hexagonal aluminates containing large divalent or trivalent cations, J. Electrochem. Soc. 123 (1976) [8] A.L.N. Stevels, Effect of non-stoichiometry on the luminescence of Eu 2 -doped aluminates with the b-alumina type crystal structure, J. Luminescence 17 (1978) [9] B. Smets, J. Rutten, G. Hoeks, J. Verlijsdonk, 2SrO 3Al 2 O 3 :Eu 2 and 1.29(Ba, Ca)O 6Al2O3: Eu 2, two new blue-emitting phosphors, J. Electrochem. Soc. 136 (1989) [10] R. Roy, D. Ravichandran, W.B. White, Hydrothermal hexaaluminate phosphors, J. Soc. Information Display 4 (1996) [11] D. Ravichandran, R. Roy, W.B. White, S. Erdei, Synthesis and characterization of sol-gel derived hexa-aluminate phosphors, J. Mater. Res. 12 (1997) [12] J.M.P.J. Verstegen, J.L. Sommerdijk, J.G. Verriet, Cerium and terbium luminescence in LaMgAl 11 O 19, J. Luminescence 6 (1973) [13] J.L. Sommerdijk, J.M.P.J. Verstegen, Concentration dependence of the Ce 3 and Tb 3 luminescence of Ce 1 x Tb x MgAl 11, J. Luminescence 9 (1974)

7 D. Ravichandran et al. / Displays 19 (1999) [14] J.L. Sommerdijk, J.A.W. Van der Does de Bye, P.H.J.M. Verberne, Decay of the Ce 3 luminescence of LaMgAl 11 O 19 :Ce 3 and of CeMgAl 11 O 19 activated with Tb 3 or Eu 3, J. Luminescence 14 (1976) [15] A.L.N. Stevels, Ce 3 luminescence in hexagonal aluminates containing large divalent of trivalent cations, J. Electrochem. Soc. 125 (1978) [16] A. Bergstein, W.B. White, Manganese activated luminescence in SrAl 12 O 19 and Ca Al 12 O 19, J. Electrochem. Soc. 118 (1971) [17] A. Bergstein, W.B. White, Luminescene and site distribution of Mn 2 in beta-al 2 O 3, J. Inorg. Nucl. Chem. 33 (1971) [18] J.M.P.J. Verstegen, Luminescence of Mn 2 in SrGa 12 O 19, LaMg- Ga 11 O 19 and BaGa 12 O 19, J. Solid State Chem. 7 (1973) [19] J.M.P.J. Verstegen, J.L. Sommerdijk, Mn 2 and Tl luminescence in b-aluminas, J. Luminescence 10 (1975) [20] A.L.N. Stevels, Red Mn 2 luminescence in hexagonal aluminates, J. Luminescence 20 (1979) [21] F.P. Glasser, L.S.D. Glasser, Crystal chemistry of some AB 2 O 4 compounds, J. Amer. Ceram. Soc. 46 (1963) [22] S. Ito, S. Banno, K. Suzuki, M. Inagaki, Phase transitions in SrAl 2 O 4, Zeits. Physik. Chem. 105 (1977) [23] S. Ito, K. Suzuki, M. Inagaki, S. Naka, High pressure modifications of CaAl 2 O 4 and CaGa 2 O 4, Mater. Res. Bull. 15 (1980) [24] K. Kato, H. Saalfeld, Verfeinerung der Kristallstrucktur von CaO 6Al 2 O 3, Neues Jahr. Mineral. Abh. 109 (1968) [25] A.J. Lindop, C. Mathews, D.W. Goodwin, The refined structure of SrO 6Al 2 O 3, Acta Cryst. B31 (1975) [26] S. Kimura, E. Bannai, I. Shindo, Phase relations relevant to hexagonal barium aluminates, Mater. Res. Bull. 17 (1982) [27] N. Iyi, Z. Inque, S. Takekawa, S. Kimura, The crystal structure of barium hexaaluminate phase I (barium b-alumina), J. Solid State Chem. 52 (1984) [28] J.-G. Park, A.N. Cormack, Crystal/defect structures and phase stability in Ba hexaaluminates, J. Solid State Chem. 121 (1996) [29] N. Iyi, S. Takekawa, S. Kimura, Crystal chemistry of hexaaluminates b-alumina and magnetoplumbite structures, J. Solid State Chem. 83 (1989) [30] M. Goebbels, Phasen der b-tonerde-und Magnetoplumbit-Familien met 2-wertigen charakteristischen Kationen (Ca, Sr, Ba) Phasenbeziehungen, Kristallchemie und Struktur, Ph.D. Thesis, Technische Hoschschule, Aachen, 1994, 203pp. [31] M. Goebbels, E. Woermann, J. Jung, The Al-rich part of the system CaO Al 2 O 3 MgO. Part I. Phase relationships, J. Solid State Chem. 120 (1995) [32] N. Iyi, M. Goebbels, Y. Matsui, The Al-rich part of the system CaO Al 2 O 3 MgO. Part II. Structure refinement of two new magnetoplumbite phases, J. Solid State Chem. 120 (1995) [33] N. Iyi, M. Goebbels, Crystal structure of the new magnetoplumbiterelated compound in the system SrO Al 2 O 3 MgO, J. Solid State Chem. 122 (1996) [34] Y.K. Song, S.K. Choi, H.S. Moon, T.W. Kim, S.I. Mho, H.L. Park, Phase studies of SrO Al 2 O 3 by emission signatures of Eu 2 and Eu 3, Mater. Res. Bull. 32 (1997) [35] T.E. Peters, R.G. Pappalardo, R.B. Hunt Jr, Lamp Phosphors, in: A.H. Katai (Ed.), Solid State Luminescence, Chapman and Hall, London, 1993, pp [36] B.M.J. Smets, J.G. Verlijsdonk, The luminescence properties of Eu 2 and Mn 2 doped barium hexaaluminates, Mater. Res. Bull. 21 (1986)

Eu 2+ activated long persistent strontium aluminate nano scaled phosphor prepared by precipitation method

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