Activated Alkaline Earth Aluminate Phosphors

Displays 19 (1999) 197–203

Crystal chemistry and luminescence of the Eu2ϩ-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 Eu2ϩ were synthesized by microwave processing and alkaline earth hexaaluminate phosphors activated by Eu2ϩ were synthesized by hydrothermal reactions. The emission spectra of the monoaluminates are similar to phosphors prepared by high-temperature ﬁring except for BaAl2O4 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. ᭧ 1999 Published by Elsevier Science B.V. All rights reserved.

Keywords: Plasma display phosphors; Hexaaluminates; Eu-activation

1. Introduction balance is accomplished by the large divalent cations which occupy interstitial sites within the tetrahedral framework. An important class of ﬂuorescent lamp and plasma The tetrahedral framework is isostructural with the SiO2 display phosphors are based on compounds in the alka- polymorph, tridymite, so that these compounds are stuffed line-earth–rare-earth–aluminate systems. These phosphors tridymite structures [21]. BaAl2O4 has the ideal stuffed fall into three broad classes: tridymite structure. SrAl2O4 undergoes a phase transition from a low-temperature monoclinic distorted structure to 1. Binary alkaline earth aluminates such as SrAl O [1,2]. 2 4 the hexagonal tridymite structure at 650ЊC [22]. CaAl O 2. Alkaline earth hexaaluminates related to magnetoplum- 2 4 bite and beta-alumina and their superstructures [3–11]. has a stuffed tridymite structure but transforms to at least three other polymorphs at high pressures [23]. In the stuffed 3. Rare earth hexaaluminates with the magnetoplumbite tridymite structure there are two sites for the large cation, structure [12–15]. each with 9-fold coordination. The binary hexaaluminates are compounds with the A variety of activators were used of which the most formula MAl12O19. CaAl12O19 (known by the mineral 2ϩ 3ϩ 3ϩ important are Eu ,Ce /Tb , and to a lesser extent, name of hibbonite) and SrAl12O19 both have the magneto- ϩ Mn2 [16–20]. plumbite structure [24,25]. In contrast, the compound This paper is concerned with the systematic relations BaAl12O19 does not exist and instead there are two between luminescence emission and crystal structure for compounds, one deﬁcient in barium and one with excess ϩ Eu2 -activated alkaline earth aluminates of classes (1) and barium, that were shown to have the beta-alumina structure (2) and with aspects of the synthesis of these refractory [26–28]. The large cation is 12-coordinated in the magne- compounds. toplumbite structure and 9-coordinated in the beta-alumina structure. When MgO is added as a third component, a set of more 2. Crystal chemistry of the aluminates complicated superstructures appear, all based on stacking The compounds MAl O ,MvCa, Sr, and Ba, are formed sequences of magnetoplumbite, beta-alumina, and spinel 2 4 building blocks. Many compositions were investigated as of a three-dimensional framework of corner-sharing AlO4 tetrahedra. Each oxygen is shared with two aluminum ions potential phosphor hosts but the number of distinct so that each tetrahedron has one net negative charge. Charge compounds and their compositions were uncertain. Recent detailed crystallographic work [29–33] on these systems * Corresponding author. Present address: U.S. Army Research Labora- has shown that there are only two additional high alumina tory, 2800 Powder Mill Road, Adelphi, MD 20783, USA. ternary phases in each MO–MgO–Al2O3 system, though

Table 1 3. Synthesis Alkaline earth hexaaluminate compounds

a b b The alkaline earth aluminates are refractory compounds. Code Composition Structure a0 c0 Space group Synthesis by usual ceramic processing methods requires CA6 CaAl12O19 Mp 5.564 21.892 P63/mmc firing temperatures in the range of 1600ЊC–1800ЊC. The CAM-I Ca2Mg2Al28O46 2Mp, Sp 5.571 79.770 R3m compounds for which luminescence data were published CAM-II CaMg Al O Mp, Sp 5.599 31.297 P6m2 2 16 27 represent the stable forms at high temperature. Two alter- SA6 SrAl12O19 Mp 5.562 21.972 P63/mmc SAM-I Sr2MgAl22O36 Mp, Sp 5.583 22.225 P6m2 nate synthesis methods were considered: (i) microwave SAM-II SrMgAl10O17 bA 5.620 22.400 P63/mmc processing which also achieves high temperature, and (ii) BA6-I 0.82BaO.6Al2O3 bA 5.588 22.769 P63/mmc hydrothermal processing which allows synthesis at tempera- BA6-II 1.32BaO.6Al2O3 bA 5.600 22.922 P6m2 tures in the range of 700ЊC–800ЊC. BAM-I BaMgAl O bA 5.627 22.658 P6 /mmc 10 17 3 The alkaline earth monoaluminate phosphors were BAM-II BaMg3Al14O25 bA, Sp 5.638 31.988 P6m2 synthesized by microwave processing using appropriate a Mp magnetoplumbite block, Sp spinel block, bA beta- mixtures of CaCO3, SrCO3, and BaCO3 with Dispersal alumina block. (AlOOH in boehmite form). Ten gram batches of finely- b Unit cell data mainly from Goebbels [30] and confirmed by our X-ray measurements. powdered starting mixture containing 3 wt% Eu2O3 were reacted in alumina crucibles in the microwave chamber. To maintain a reducing atmosphere, the chamber was continuously flushed with forming gas (95% N2,5%H2). these phases do not all have the same compositions. Table 1 Microwave energy at 2.54 GHz was injected into the cham- lists the accepted high alumina compounds of the alkaline ber at an initial power of 100 W as the temperature earths. All are hexagonal or trigonal with very complex X- increased. The power injection was raised to 2.5 kW ray powder patterns, making difficult the identification of which was sustained for 30 min. Temperatures, as measured the phases and the determination of phase purity. by optical pyrometer, ranged from 1250ЊC to 1600ЊC. Well crystallized phosphor could be prepared in 30 min reaction time but the desired MAl2O4 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(NO3)2, Al(NO3)3·9H2O, Mg(NO3)2·6H2O (Baker analyzed reagent 99.98% purity), Eu(NO3)3, and Ga(NO3)3·9H2O (Aldrich Chemicals 99.99% purity). The nitrates were weighed stoichiometri- cally and dissolved in 15–20 mL deionized water and stirred well for 20–25 min to get a clear solution. The solu- tions were then titrated against 15 M NH4OH 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 120ЊC–125ЊC and then calcined at 600ЊC 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 Eu2ϩ state. These were reacted in hydrothermal pressure vessels at temperatures in the range of 500ЊC–800ЊC under pressures of 50– 70 MPa for 12–48 h. At the end of the reaction period, temperature and pressure were quenched, the capsules opened, and the specimens removed and dried for charac- terization 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

Fig. 1. (a) Hexagonal platelets of BaMgAl10O17. The large hexagonal faces (Fig. 1a). An exception was Ca2Mg2Al28O46 which crystal is about 5 mm in diameter. (b) Equant crystals of Ca2Mg2Al28O46. appeared as more equant and smaller sized crystals (Fig. D. Ravichandran et al. / Displays 19 (1999) 197–203 199 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 BaAl2O4. 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 500–510 nm. Microwave processing of CaAl2O4 and Sr Al2O4 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 Al2O4, it may be assumed that there is a different distribution of Eu2ϩ over the two cation sites in the microwave processed material.

Fig. 2. Emission and excitation spectra for CaAl2O4, SrAl2O4 and BaAl2O4 prepared by microwave processing. Mole fraction Eu2ϩ 0.02. 5. Luminescence of the hexaaluminates

1(b)). Hydrothermal processing produces phosphors with The emission characteristics of the binary hexaaluminates excellent crystallinity although at the expense of some are summarized in Table 2. The Ca and Sr-compounds have possible uptake of OHϪ ion which can act to enhance the magnetoplumbite structure and have their peak emission non-radiative decay. 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-coor- 4. Luminescence of the monoaluminates dinated site in the beta-alumina structure produces a stron- ger crystal field than the 12-coordinated site in the Emission and excitation spectra for the 1 : 1 alkaline magnetoplumbite structure, thus lowering the emitting earth aluminates prepared by microwave processing are level of Eu2ϩ and producing a longer wavelength emission. shown in Fig. 2. The emission band of CaAl2O4 at 439 nm The compounds identified as distinct superstructures is in good agreement in both wavelength and band shape (Table 1) were synthesized by hydrothermal reaction and with the measurements of Palilla et al. [1] who observed activated by replacing 0.01 or 0.02 of the alkaline earth ion the band at 440 nm. The excitation spectrum shows the by Eu2ϩ. The peak emission wavelengths of the ternary expected two bands from the crystal field splitting of the hexaaluminates compared with compounds prepared by 2ϩ Eu d-orbitals. The emission from SrAl2O4 was observed high temperature ceramic reaction are summarized in

Table 2 Emission characteristics of magnetoplumbite binary compounds

Compound Emission wavelength Emission wavenumber Reference

2ϩ CaAl12O19 :Eu 410 24 390 Verstegen and Stevels [3]; Stevels and Schrama-de Pauw [7] 2ϩ SrAl12O19 :Eu 395 25 320 Verstegen and Stevels [3]; Stevels and Schrama-de Pauw [7] 2ϩ ‘‘BaAl12O19 :Eu ’’ 443 22 570 Verstegen and Stevels [3] 2ϩ 0.82BaO·6Al2O3 :Eu 470 21 260 Smets et al. [9] 2ϩ 1.32BaO·6Al2O3 :Eu 440 22 700 Smets et al. [9] 200 D. Ravichandran et al. / Displays 19 (1999) 197–203

Table 3 Emission characteristics of ternary hexaaluminate phosphors

Compounda Synthesis Emission wavelength Emission wavenumber Reference

CAM-I Ca2Mg2Al28O46 Hydr 419, 439, 467 This work CAM-II CaMg2Al16O27 Hydr — — This work SAM-I Sr2MgAl22O36 Hydr 490 20 410 This work SAM-II SrMgAl10O17 Hydr 484 20 660 This work SrMgAl10O17 Ceram 465 21 505 [7] Sr2Mg2Al22O37 Ceram 464 21 530 [3] BAM-I BaMgAl10O17 Hydr 430 23 260 This work BaMgAl10O17 Ceram 450 22 220 [7] BAM-II BaMg3Al14O25 Hydr 467 21 410 This work Ba2Mg2Al22O37 Ceram 449 22 260 [3] b 2461 BaMg2Al16O27 Ceram 450 22 220 Sylvania c BaMg2Al17O28.5 BaMg2Al16O27 Hydr 463 21 600 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 with slightly different compositions. The two compounds between compounds and with method of synthesis. that were established as speciﬁc superstructures are BAM- The Ca-compounds, CAM-I and CAM-II, are composed I and BAM-II (Table 1). BAM-I prepared by high tempera- of a superstructure of magnetoplumbite blocks with an addi- ture ceramic processing has a Eu2ϩ emission peak at 450 nm tional spinel block. Only CAM-I (Fig. 3) is luminescent with and is used as a practical blue phosphor. The same composi- a series of weak bands centered on 440 nm. There is some tion prepared at 800ЊC and 71 MPa by hydrothermal reac- question of the stability of the calcium hexaaluminates in tion emits at 430 nm (Fig. 5). BAM-II is reported as having the presence of water during the quenching of the hydro- an extended beta-alumina structure with additional spinel thermal experiments. Two superstructures were identiﬁed with Sr2ϩ as the large cation. SAM-I has a structure based on magnetoplumbite with intermediate spinel blocks while SAM-II has a beta- alumina 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 484–490 nm (Fig. 4). Most puzzling of the ternary hexaaluminates are the Ba2ϩ compounds. A variety of materials were synthesized, all

2ϩ 2ϩ Fig. 3. Emission and excitation spectra of CAM-I, Ca2Mg2Al28O46 (Eu Fig. 4. Emission and excitation spectra of SAM-I, Sr2MgAl22O36 (Eu 2ϩ 0.02). 0.02 and SAM-II, SrMgAl10O17 (Eu 0.01). D. Ravichandran et al. / Displays 19 (1999) 197–203 201 practical phosphors. Smets et al. [36] claim that BaMg2Al16O27 is the same as BaMgAl10O17 (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

Reviewing the Eu2ϩ 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 ϩ Eu2 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 2ϩ Fig. 5. Emission and excitation spectra of BAM-I, BaMgAl10O17 (Eu ultraviolet, especially near 147 nm where the xenon plasma 2ϩ 0.01) and BAM-II, BaMg3Al14O25 (Eu 0.01). has its strongest emission line. The most dramatic comparison is between the commercial phosphor, Sylvania Type blocks in the stacking sequence. Hydrothermal synthesis of 2461, and a material with the same nominal composition 2ϩ prepared by hydrothermal synthesis (Fig. 7). The excitation the ideal composition, BaMg3Al14O25 activated with Eu produces the blue-emitting phosphor with a peak at spectrum for Sylvania 2461 exhibits two broad excitation 467 nm (Fig. 5). bands giving quantum efficiencies in the range of 85%– The literature is somewhat ambiguous with respect to the 95%. The hydrothermal preparation has a sharp, narrow composition of the efficient blue-emitting phosphor used in band excitation spectrum but the quantum efficiency of lamps and plasma displays. Phosphors with similar but not only 15%–20%. identical composition that were prepared by high temperature ceramic processing include Ba Mg Al O which emits 2 2 22 37 7. Conclusions at 449 nm [3] and BaMg2Al16O27 (Sylvania Type 2461) which emits at 450 nm [35] (Table 3). Both are used as The synthesis and spectroscopic properties of a series of

2ϩ Fig. 6. Emission and excitation spectra of BaMg2Al16O27, comparing hydrothermal preparation with reference phosphor, Sylvania 2461 (Eu 0.04). 202 D. Ravichandran et al. / Displays 19 (1999) 197–203

Fig. 7. Vaccum UV excitation spectra of BaMg2Al16O27 comparing hydrothermal preparation with reference phosphor, Sylvania 2661.

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