Journal of the Less-Common
SOME NEW CLASSES OF RARE EARTH ACTIVATED MATERIALS*
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Summary Gd3+ compounds are very promising materials for lamp phosphors. Examples are GdF3, GdB306 and GdMgB,Olz. The compound GdTa04 is an efficient host lattice for X-ray phosphors. The energy-transfer processes are complicated. A new host lattice with simple activation is Ln2BaZnOS (Ln = La, Gd). Eu3+ and Tb3+ yield efficient phosphors.
1. Introduction There is a continuous need for new and efficient phosphors. Here we will report on some new classes of rare earth activated phosphors. One of these holds a strong promise for a considerable improvement of the efficiency of fluorescent lamps, viz., the materials based on gadohnium compounds. An efficient X-ray phosphor may be expected in dense, isolating compounds. A new example here is GdTa04. Finally, recently reported compounds, viz., LnzBaZnOS (Ln = La, Gd) [ 1, 21, may be suitable host lattices for phosphors. The physical phenomena underlying the materials properties will be discussed. 2. Gadolinium compounds Gadolinium compounds can be efficient host lattices for luminescent materials if there are no energy levels below the excited Gd3+ levels. Since the absorption strength of the optical transitions on Gd3+ (e.g., ‘S + 6P, 61) is very low, it is necessary to introduce the excitation energy via a sensitizer. Suitable examples are Bi3+ and Ce3+. These ions .have allowed optical transitions in the short wavelength ultraviolet region. The excitation energy is transferred from the sensitizer to the Gd3+ sublattice. Rapid energy migration within this sublattice occurs and transports the excitation energy to an activator, e.g., Tb 3+. The processes can be represented as follows: *Paper presented at the International land, March 4 - 8,1985.
in The Netherlands
. . Bi3+ -
Let us consider some specific examples. A simple host lattice is GdF,. The composition Gd,,. 9sCe,,01Tb O.OIFsis an efficient green-emitting phosphor under 254 nm excitation [ 31. Excitation occurs in the Ce3+ ion via an allowed 4f-5d transition. In view of the perfect spectral overlap between the Ce3+ emission and Gd3+ absorption, the Ce3+ + Gd3+ energy transfer is very efficient. Via many Gd3+ + Gd3+ steps the excitation reaches the Tb3+ ion, from where emission occurs. In isoelectronic Y,,sCeo.oiTbo,oiF3, excitation into the Ce3+ ion results in mainly Ce3+ emission, because there is no “intermediary” between the Ce3+ and Tb3+ ions. Similar phenomena are observed in GdMgBsOi,, . The Ce3+ ion is a very suitable sensitizer. The Bi3+ ion acts only as a sensitizer at liquid helium temperatures. At room temperature the Bi‘3+ levels have shifted in such a way that backtransfer Gd3+ + Bi3+ also occurs. This is fatal for possible applications. Possible activators are Tb3+ on Gd3+ sites and Mn*+ on Mg*+ sites. The possible processes are characterized by their rates as follows:
1: -lo7 s-i; 2: -lo9 s-1; 3: 5 x 102 s-i; 4: -107 s-i; 5: 2 x lo6 s-i; 6: 4 X lo6 s-l. Although these data are not very accurate, they illustrate very well what is going on. Codoped GdMgBsOio:Ce, Tb, Mn has been shown to be an excellent lamp phosphor with a green (Tb3’) and a red (Mn2’) emission. The ratio between these two emissions can be varied by varying the activator concentrations . A third interesting host lattice is GdB306 . Here Bi3+ is an excellent sensitizer. The Ce3+ ion is now less suitable, since it emits about 15% of the absorbed excitation energy and transfers only 85% to the Gd3+ sublattice. This 15% is lost for conversion into visible radiation. Figure 1 shows the excitation spectrum of the Eu3+ emission of GdBsOb:Bi, Eu. Note especially band which shows that efficient Eu3+ emission the strong Bi3+ excitation occurs from Bi3+ excitation.
3. GdTa04 Due to its high density (d = 8.81 g cmW3) GdTa04 is interesting as an Xray phosphor . The transfer processes have recently been elucidated by Lammers and Blasse [S]. Figure 2 gives the energy levels involved.
Fig. 1. The excitation spectrum of the Eu3+ emission relative quantum output is denoted by qr.
Eu at 300
Fig. 2. Schematic representation of transfer processes in GdTa04-Tb3+ (see also text). Energy levels involved in sharp-line transitions are indicated as lines; those involved in broad-band transitions by pairs of blocks (unrelaxed and relaxed excited state). EXC: excitation; EM: emission; RLX: relaxation; CR: cross relaxation.
Excitation into the tantalate (Ta04) group is followed by the following processes: (a) Relaxation to the luminescent Tao4 level. (b) Energy transfer from this level to the Gd3+ sublattice (the tantalate groups serve as a sensitizer of the Gd3+ sublattice); the energy is accepted in the 6P level. (c) Energy migration among the Gd3+ ions. (d) Energy transfer from Gd3+ to the activator (in this case Tb3+). (e) (Blue) emission from the Tb3+ ‘D3 level. (f) If the Tb3+ concentration is high enough, (green) emission from the Tb3+ ‘De level after cross-relaxation (transfer of the energy difference between the ‘D3 and ‘Da level to another Tb3+). However, if excitation is into the 61 level of Gd3+, different processes occur: (a) relaxation from the 61 level to the 6P level of Gd3+ does not occur; (b) the excitation energy migrates among the Gd3+ ions (via the ‘j1 level) ; (c) energy transfer from the Gd 3+ 61 level to the 4f75d state of Tb3+; (d) relaxation in the 4f75d state of Tb3+ bypassing the ‘D, level; the system ends up in the ‘D, level from where: (e) (green) emission occurs. The total transfer process is rather complicated. Nevertheless, an efficient phosphor results.
4. Ln,BaZnOs Title compounds were prepared by Raveau et al. [I, 2 ] who also determined the crystal structure. The Eu3’ ion is a very efficient emitter in LazBaZn05 and GdzBaZnOs . The maximum of the charge-transfer transition is at 310 nm and 275 nm, respectively. X-rays are unable to determine the La3+/Ba2+ distribution. The luminescence measurements show that there is a certain amount of disorder. The Tb3” ion only shows efficient luminescence in LazBaZnO,. However, this phenomenon is not stable in air, so that we have to assume that Tb3+ becomes easily tetravalent in these compounds. The phonon cutoff is only some 500 cm -r for these lattices. As a eonsequence luminescence from higher states is efficient. In fact it is possible to prepare blue-emitting Eu3+ activated samples (Fig, 3).
Fig. 3. Emission spectrum of Gd*BaZnOs:Eu (0.2%) at LHeT. Excitation figures indicate the value of J of the emitting 5Ds level.
380 nm. The
We conclude that the discovery of new, rare earth activated is still going on and that new surprises are possible.
References 1 2 3 4 5 6 7 8 9
C. Michel, L. Er-Rakho and B. Raveau, J. Solid State C&em., 42 (1982) 176. C. Michel and B. Raveau, J. Solid State Chem., 49 (1983) 150. G. Blasse, Phys. Status So~idi A, 73 (1982) 205. M. Leskelti, M. Saakes and G. Blasse, Muter. Res. Bull., 19 (1984) 151. J. Th. W. de Hair and J. T. C. van Kemenade, 3rd Int. Conf. Science and Technology of Light Sources, Toulouse, 1983, Paper 54. Hao Zhiran and G. Blasse, J. Lumin., 31/32 (1984) 817; Mater. Chem. Phys., 12 (1985) 257. L. H. Brixner and H. Y. Chen, J. Electrochem. Sot., 130 (1983) 2435. M. J. J. Lammers and G. Blasse, Mater. Res. Bull., 19 (1984) 759. M. J. J. Lammers, H. Donker and G. Blasse, Mater. Chem. Phys., to be published.