FLUORESCENCE OF Eu2+-ACTIVATED ALKALINE-EARTH ALUMINATES

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1 R662 Philips Res. Repts 23, ,1968 FLUORESCENCE OF Eu2+-ACTIVATED ALKALINE-EARTH ALUMINATES by G. BLASSE and A. BRIL Abstract The fluorescence of Eu 2 + -activated aluminates with the general formulae MeAI Eu and MeA Eu (Me = Ca, Sr or Ba) is reported. The diffuse-reflection spectra and the excitation spectra of the fluorescence consist of a number of broad bands. The emission ofthe phosphors MeAI Eu and CaAI Eu peaks in the blue, that of the phosphors BaAI Eu and SrAI Eu in the green. The quantum efficiencies for ultraviolet excitation are high; the radiant efficiencies for cathode-ray excitation are of the order of 1-3%. 1. Introduetion The efficient fluorescence of Eu2+-activated alkaline-earth pyrophosphates and ternary silicates has recently been reported 1.2.3). This paper deals with i the fluorescence of phosphors MeAI Eu2+ and MeAI Eu2+ (Me = Ca, Sr, Ba). The efficient, green fluorescence ofsra Eu was already known in the patent literature 4).. 2. Experimental Samples were prepared by firing intimate mixtures of MeC03, EU203 and! Al(OH)3 for several hours at temperatures between 1350 and 1450 C in an atmosphere consisting of a mixture of N2 and H2 in the ratio 20 : 1. Final products were checked by X-ray analysis. The optical measurements were performed as described previously 5). 3. Results Unter ultraviolet and cathode-ray excitation the phosphors MeAl12019 (Me = Ca, Sr, Ba) and CaAI20 4 -Eu showastrong blue fluorescence, while the phosphors SrAI20 4 -Eu and BaAI20 4 -Eu showastrong green fluorescence. In table I some data on the emission and efficiency of these phosphors are given. Table IJ shows the dependence of the efficiency on the activator concentration. Figures 1 and 2 present the diffuse-reflection spectra and the excitation spectra of the light output of these phosphors. Figure 3 gives their spectral-energy distribution and fig. 4 the temperature dependence of the light output. It is possible to obtain phosphors with the emission band at a specified wave-

2 202 G. BLASSE and A. BRIL length position by an appropriate choice of the chemical composition. In the solid-solution series Srl_xBaxAI12019-Eu, for ex!lmple,the emission band shifts regularly to longer wavelengths with increasing value of x. In the series Srl_xBaxAl204-Eu the emission band shifts to shorter wavelengths with increasing value of x. It may be possible to obtain higher efficiencies by refining the preparation techniques. TABLE I Some data on the fluorescence of Eu 2 + -activated aluminates phosphor MeAI Eu MeAI Eu (Eu 2 + cone. 2 at.%) Me=Ca Me=Sr Me=Ba Me= Ca Me-e Sr Me=Ba position of the maximum of the emission band (nm) decay time (microseconds) radiant efficiency for cath ode-ray excitation (%; 20kV) quantum efficiency (%; nm excitation) maximum quantum effi ciency in u.v. region (%) reflection (%; 254 nm) TABLE 11 Concentration dependence of the quantum efficiency of the fluorescence of BaAI204-Eu and BaAI Eu composition x maximum quantum efficiency in the u.v. region (%) !

3 FLUORESCENCE OF EuH-ACTIVATED ALKALINE-EARTH ALUMINATES 203 Reflection ('Ye) Relative quantum W~15D ioo'rïr ,---~~~c=~~_,~----~~----, o Àexc(nm)- Fig. 1. Diffuse-reflection spectra and excitation spectra ofthe Eu 2 + fluorescence ofphosphors MeAI Eu.: El' E2 and E3 represent the excitation spectra of the fluorescence of CaAI20 4 -Eu, SrA1204- Eu and BaAI20 4 -Eu, respectively. Rl and R2 represent the diffuse-reflection spectra of CaAI20 4 -Eu and SrAI20 4 -Eu, respectively. That of BaAI20 4 -Eu is omitted for clarity; it resembles the reflection spectrum of SrAI20 4 -Eu (curve R2). Eu 2 + concentration in all figures is 2 at. %. Reflection (%) Relative quantum ootput 1 5D 100',.---r----:::--..,-""7"<O= , O~--~ ~------_L------_J Àexc (nm)- Fig. 2. Diffuse-reflection spectra and excitation spectra ofthe Eu 2 + fluorescence ofphosphors MeA1l20l9-Eu. El> E2 and E3 represent the excitation spectra of the fluorescence of CaA1 12 0l9-Eu, SrAI 12 0l9-Eu and BaA1 12 0l9-Eu, respectively. R2 and R3 represent the diffuse-reflection,spectra of SrAI 12 0l9-Eu and BaA1 12 0l9-Eu, respectively. That of CaA Eu is omitted for clarity; it resembles the reflection spectrum of SrAI Eu (curve R2) À (nm) Fig. 3. Spectral-energy distribution of the Eu 2 +-fluorescence of (1) CaAI 12 0 l9 -Eu, (2) BaAI Eu, (3) CaAI20 4 -Eu, (4) BaAI20 4 -Eu; 254-nm excitation. Along the ordinate the radiant power per constant wavelength interval (I) has been plotted in arbitrary units.

4 204 G. BLASSE and A.BRIL Fig. 4. Temperature dependence of the light output of the fluorescence of. (1) CaAI Eu, (2) SrAI Eu and BaAl Eu, (3) CaAI Eu and SrAI Eu, (4) BaAI Eu; 254-nm excitation. The curves for SrAI Eu and BaAI Eu coincide within the experimental error and are represented by one curve for clarity. The same is true for CaAI Eu and BaAI Eu. 4. Discussion In a previous paper on Eu2+-activated silicates a general discussion was devoted to the absorption and emission ofthe Eu2+ ion in solids 3). The present results can be interpreted in the same terms. The reflection and excitation spectra (figs 1 and 2) consist of a number of broad bands corresponding to the crystal-field components of the 5d level in I the excited 4j6 5d configuration of the Eu2+ ion. These spectra should depend not only on the crystal structure of the host lattice, but also on the choice of the alkaline-earth ion. This choice determines the lower-symmetrical crystal field at the Eu2+ ion and, therefore, the position of the absorption bands. The compounds MeAI (Me = Ca, Sr, Ba) have magnetoplumbite structure 6). The reflection and excitation spectra of CaAI and SrAI Eu are similar, but differ from those of BaAI 120WEu (fig. 2). A possible reason for this may be the following. In the magnetoplumbite structure the alkaline-earth and oxygen ions form a close-packed system. The Ba2+ ion is large enough to I fit in the packing (the ionic radii of Ba2+ and are roughly equal), but the Ca 2 + and Sr2+ ions are too small so that distortions result. Such an effect has also been found in other structures e.g. in the perovskite structure 7). These distortions influence the crystal field at the Eu2+ site, and therefore also the reflection and excitation spectra. Note the importance of such an effect for the fluorescence: the fluorescence of BaAI Eu can be excited reasonably well by 365-nm excitation; that of CaAI 12 0wEu and SrAI Eu is not excited by this radiation (fig. 2). The reflection and excitation spectra of BaAI Au and SrAI Eu are more or less similar. The host lattices are isomorphous (stuffed tridymite with hexagonal unit cell 8)). The spectra of CaAI204-Eu differ from those of the analogous Ba and Sr phosphors. The monoclinic structure of CaAl can be

5 FLUORESCENCE OF Eu2+-ACTIVATED ALKALINE-EARTH ALUMINATES 205 considered as a strongly distorted stuffed tridymite lattice 8,9), so that a crystallographic distortion seems to be responsible in this case too for a difference in :fluorescent properties. These differences between BaAI Eu on the one hand and CaAI Eu on the other and between BaAI20 4 -Eu and SrAI20~-Eu on the one hand and CaAI Eu on the other are also apparent in the other :fluorescent properties. The emission of BaAI Eu occurs at longer wavelengths than that of SrAI Eu and CaAI12019-Eu, and the quenching temperature of the :fluorescence of BaAI Eu is considerably higher than that of the Sr and the Ca phosphors (fig. 4). On the other hand the emission of BaAI20 4 -Eu and SrAI Eu is situated at longer wavelengths than that of the analogous Ca phosphor; the efficiency of CaAI20 4 -Eu is lower than that of the Sr and Ba phosphors; and the quenching temperature of the Ca phosphor is lower than that of the Sr and Ba phosphors (fig. 4). We noted earlier that the quenching temperature of isomorphous Eu2+-activated Ca, Sr and Ba phosphors increases in this sequence 3). The decay time of the present Eu 2 +-activated phosphors is short, but not as short as should be expected for an allowed transition. This has been ascribed by us to the interaction of octet and sextet states in the excited configuration 3). Concentration quenching of the Eu 2 + :fluorescenceoccurs at concentrations in the order of a few atomic per cent (see table II). Finally it is interesting to compare the :fluorescence of BaAI20 4 -Eu and BaMgSi04-Eu. The latter phosphor was reported on earlier 3). These host lattices are isomorphous (stuffed tridymite structure 10)). The Si4+ and Mg 2 + ions are distributed statistically among the AP+ sites. The in:fluence of such a substitution on the :fluorescence of the Eu2+ ions is drastic. Although the re:flection and excitation spectra resemble each other, the emission maximum shifts from 505 nm (BaAI20 4 -Eu) to 440 nm (BaMgSi0 4 -Eu) and the u.v, quantum efficiencydecreases from 60 % (BaAI20 4 -Eu) to 20 % (BaMgSi0 4 -Eu). S. Acknowledgement The authors are greatly indebted to Miss M. P. Bol, Miss D. J. M. Trumpie and Messrs C. J. Loyen, J. A. de Poorter and J. de Vries for their assistance in the experiments. Eindhoven, November 1967 REFERENCES 1) G. Blasse, W. L. Wanmaker and J. W. ter Vrugt, J. electrochem. Soc., to be published. 2) W. L. Wan maker and J. W. ter Vr u gt, Philips Res. Repts 22, 355, ) G. Blasse, W. L. Wanrnaker, J. W. ter Vru gt and A. Bril, Pilips Res. Repts 23, , ) U.S.A. patent nr , 1966.

6 206 G. BLASSE and A. BRIL 5) A. Bril and W. L. Wanmaker, J. electrochem. Soc. 111, 1363, G. Blasse and A. Bril, Appl. Phys, Letters 11, 53, ) R. W. G. Wyckoff, Crystal structures, Interscience, New York, 1965, 2nd ed., vol. 4. 7) See e.g. A. F. Wells, Structural inorganic chemistry, Clarendon Press, Oxford, ) F. P. Glasser and L. S. Dent Glasser, J. Am. ceram. Soc. 46,377, ) M. W. DougiIl, Nature 180, 292, ) C. Do Dinh and A. Durif, Bull. Soc. franç. MinéraI. Crist. 87, 108, 1964.