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Luminescence of Eu3+-doped lanthanum titanate (La2TiO5), a system with one-dimensional energy migration
J. Phys. Chem. So&is Vol. 53, No. 5, pp. 677480. Printed in Great Britain.
@X2-3697/92 SS.00 + 0.00 Q 1992 F=ergsmon Press pk
1992
LUMINESCENCE OF Eu3 +-DOPED LANTHANUM TITANATE (La,TiO,), A SYSTEM WITH ONE-DIMENSIONAL ENERGY MIGRATION J. ALARCON? and G. BLASSE~ tDept. de Quinica Inorganica, Universitat de Valencia, 46100 Burjassot (Valencia), Spain $Debye Research Institute, Utrecht University, POB 80.000, 3508 TA Utrecht, The Netherlands (Received 9 August 1991; accepted 23 October
1991)
Abstract-The
titanate and Eu 3+ luminescence of a composition La,TiO,-Eu are reported as a function of temperature. At 4.2 K the quantum efficiency is very high, because the titanate excitation energy is localized. At 300 K there is delocalization and quenching centres are easily reached, so that the quantum efficiency becomes low. This is discussed in terms of the crystal structure, as are the peculiarities of the ELI’+ spectra. ICeywords: Luminescence, La,TiO, , Eu3 + .
1.
INTRODUCTION
Eu3 +-activated phosphors are applied widely for lighting and display [l]. Their physical properties have been intensively studied. We were tempted to investigate the Eu3 + luminescence in La,TiOS, because the crystal structure of this host lattice has some remarkable characteristics. Although the formula La,TiO, suggests that the trigonal bipy~madai Ti05 groups in this structure do not share oxygen ions, this is not the case, since one of the oxygen ions is coordinated by La3+ only [2]. The TiO, groups form linear chains along the c-axis by sharing apexes resulting in an arrangement 0-Ti-O-Ti-0 with Ti+Ti angles close to 180”. Such an arrangement is expected to promote strong interaction between the titanate groups, with SrTiOj as the most famous example. In SrTiO, the coupling of the titanate groups is three-dimensional, in La2Ti05 it is onedimensional. Because of this structural property it seemed worthwhile to investigate the luminescence of La,TiO,-Eu, and to compare the results with those for other host lattices in which energy migration is known to occur.
2. EXPERIMENTAL
Starting materials were Laz03 (99.99%), Eu203 (99.9%) and TiOz ( > 99.9%). Intimate mixtures were fired at final temperatures of 1300°C. The products were found to be single phase using X-ray powder diffraction. In this paper we present results for the compositions La,., Eu,, TQ and nominally pure La,TiO, . The optical measurements were performed 677
with instruments described before [3]. The sample temperature could be varied between liquid helium temperature (LHeT) and room temperature (RT). 3. RESULTS
The diffuse reflection spectra (at RT) show an optical absorption edge at about 290 nm which can be ascribed to the titanate absorption. Apart from very weak features due to Eu3+ in the europium containing sample, the visible region shows a weak and broad absorption band for wavelengths < 500 nm. This absorption is ascribed to transition metal ions present as an impurity in the starting material TiOz. Such absorption bands are well known in titanates and have been ascribed to charge-transfer transitions [4]. Using the Kubelka-Munk formula, the concentration of the impurities is estimated to be 700 ppm. This is in good agreement with the data of the supplier. At 300 K the undoped sample does not luminesce at all, the 5% Eu3+ sample shows under ultraviolet excitation a weak red emission with a quantum efficiency of a few per cent. At 4.2 and 78 K the samples show a strong near-white emission for short wavelength U.V. excitation, and a red emission for long wavelength U.V. excitation. The quantum efficiency of the former was estimated to be some 80% at 4.2 K. Figure 1 shows the emission spectrum of the Eu3+ emission in La,TiO,. It consists of the well-known 3Do-‘F, transitions. At shorter wavelengths the emission lines from the ‘D# = 3, 2, I) level could also be observed. Their intensity is about two orders of magnitude lower than the intensity of the 3D,
678
J. ALARCON and G. BLA~~E
700nm
600
Fig. 1. Spectral energy distribution of the 5D0-7F, emission of Eu3+ in La,TiO, at LHeT. Excitation at 391 nm (7F0-5L6). The value of J is indicated. emission lines. They are a factor of two weaker for titanate excitation than for Eu3+ excitation at 395 nm. Figure 2 shows the excitation spectrum of the Eu’+ emission of La, TiO,-Eu’ + at LHeT. The broad band in the U.V. corresponds with the optical absorption observed in the reflection spectrum and is, therefore, due to titanate excitation. This shows that energy transfer from titanate to europium takes place. The sharp lines are the intraconfigurational transitions of the 4f6 configuration of the Eu3+ ion. At room temperature the ultraviolet band is much weaker relative to the lines. The difference is more than one order of magnitude, corresponding to the difference in quantum efficiency at the two temperatures. The inset of Fig. 2 shows the excitation line corresponding to the ‘F,-‘D, transition of the Eu’+ ion. It shows also vibronic transitions on the shorter wavelength side [5]. Due to the large splitting of the electronic lines, it is hard to analyse the vibronic lines exactly, but it is clear that there are a more intense group at about 250 cm-’ and a less intense group at about 750 cm-’ from the electronic lines. The former are ascribed to coupling with the Eu-0 vibrational stretching mode, the latter to coupling with the Ti-0
--A
44
300
L60nm
SOOnm
Fig. 2. Excitation spectrum of the Eu’+ emission of La,,,Eu,,,TiO, at LHeT. qr gives the relative quantum output. The inset gives the 7F0-5D, excitation transition stressing the vibronic transitions.
stretching mode. In tungstates and molybdates these groups have usually comparable intensities (Ref. 5 and references cited therein). Excitation into the titanate at LHeT yields not only Eu3+ emission, but also broad band titanate emission [6]. Their integrated intensity ratio is about 1:2. At RT this titanate emission is quenched. The spectra of La,TiO, consist of broad bands only. The emission band has its maximum at about 475 nm, the excitation band is the same as for Eu3+ with a maximum at about 280 nm. Upon increasing the temperature, the emission intensity started to drop above 40 K and is quenched above 150 K.
4. DISCUSSION 4.1. The Eu3+ emission The lines in the Eu’+ spectra are well known and do not need a further assignment. At first sight there seems to be a discrepancy between the very low site symmetry (C,) and the number of possible sites for the lanthanide ion (two) on one hand, and the very simple emission spectrum which, in good approximation, shows two lines for the ‘D,,-‘F, and for the 5Do-7F, transitions. However, not only are the two independent sites very much alike [2], but also the seven coordination of the lanthanide ions can be considered as an octahedral one elongated along a trigonal axis, to which a seventh ion has to be added at somewhat larger distance. The trigonally distorted octahedron is then responsible for the splitting into two levels of the ‘F, level and into three levels of the ‘F2 level (approximate site symmetry D,&. This symmetry allows two lines for both transitions, explaining the emission spectrum in first approximation. The seventh ion lowers the symmetry to the real one, viz. C,. This ion causes a linear crystal field on the Eu3+ ion, a necessary condition for the observation of the ‘Do-‘F, transition with the observed intensity. It will also bring about a further crystal field splitting. From the emission spectrum it is clear that the longer wavelength components of the 5Do-7F,,, transitions are not single lines at all. In this way the experimental emission spectrum agrees completely with the crystallographic data [2]. Simultaneously it is a nice illustration of the fact that small changes in the surroundings of the Eu’ + ion influence the intensities much stronger than the crystal field splittings [7]. Apart from sharpening which makes small splittings better observable, there is no essential difference between the Eu3 + excited Eu3 + luminescence at RT and LHeT. Especially the luminescence intensity
619
Luminescence of Eu’+-doped lanthanum titanate does not vary with temperature. This is completely different for titanate excitation. But first we discuss the titanate luminescence itself.
4.2. The titanate luminescence Titanate luminescence involves charge-transfer transitions [6]. This luminescence has been observed for tetrahedral, octahedral and tetragonal pyramidal titanate groups. To this we now add the trigonal bipyramidal titanate group. At LHeT the quantum efficiency of the luminescence is high, but it starts to drop above 40 K. There are two models to explain this decrease. The first is the appearance of non-radiative transitions in the titanate group which compete with the radiative transitions. In this model all transitions occur in one and the same titanate group. This model is always valid for diluted systems. The second model considers energy transfer between titanate groups, so that the excitation energy can migrate to quenching centres. Here the excitation gets lost non-radiatively. By diluting the concentration of luminescent ions it is possible to distinguish between the two models [8]. Unfortunately, this is not possible in the case of La,TiO, in view of the specific titanium coordination. Therefore we compare our system with results from the literature on other compounds with an aim to solve our problem in this way. An important fact to start with is the observation that the optical absorption of islolated titanate groups is usually situated at about 250 nm, i.e. some 40,000 cm-’ [6, 8, 91. In La,TiO, its position is N 35,000 cn-I, which indicates interaction between the titanate groups via the linear Ti&Ti bridge. Due to band broadening the absorption energy is reduced [lo]. This interaction extends in one direction. In Sr,TiO, it does so in two dimensions and the absorption band is at N 30,000 cm-’ [1 11. In SrTiO,, finally, the connections are threedimensional and the absorption band is at only N 27,000 cm-’ [12]. Although it cannot be excluded that the relevant bond distances and angles in these compounds are slightly different, the enormous shift must, in our opinion, be ascribed in the first place to the interaction between titanate groups and its dimensionality. The stronger the interaction, the less the relaxation in the excited state [lo, 121. This is measured by the Stokes shift of the emission. Interestingly enough, the Stokes shift decreases with increasing dimensionality of the interaction: La,TiO, 14,000 cm-‘, Sr*TiO, 11,000 cm-’ (1 l), and SrTiO, 8000 cm-’ [12]. At LHeT the emission of these titanates can be ascribed to self-trapped exciton emission on one of
the titanate groups. The mobility of the excitons can be neglected. The SrTiO, emission already starts to be quenched above 10 K. De Haart et al. [12] have shown that the exciton becomes mobile and reaches impurities. In the case of SrTiO,: CIJ+ the Cr’+ emission builds up at the expense of the titanate emission. Therefore we ascribe the quenching of the luminescence of La,TiO, above 40 K to exciton migration to quenching sites. Probably these consist of the impurity transition metal ions (see above). Since these are expected to be built into the chain, they can be effective quenchers of the titanate emission. Finally we note that in LaWO,Cl (13), La,NbO,Cl, and La,TaO,Cl, (14) there are linear W04, Nb04 and Tao, chains of exactly the same constitution as the TiO, chain in La,TiO,. The luminescence of these compounds [13,14] is very similar to that of La,TiO,.
4.3. Titanate-Eu3 + La, TiO, : Eu’ +
energy
transfer
in
First we consider the situation at LHeT: the quantum efficiency of the total luminescence upon titanate excitation is high and the emission comprises onethird of Eu3+ emission. If the excitation remains localized on the excited titanate group, the ratio of titanate to Eu3+ emission intensity can be derived as follows. Each titanate group in La, TiO, has eight nearest La3 + neighbours. With a Eu3+ concentration of 5% the probability to find eight La’+ ions around the TiOs group in La,,VEu,,,TiO, is 0.95’ = 0.66. Since it is very probable that the excited titanate group transfers to Eu3 + if there is a Eu3+ ion on one of the eight nearest neighbour sites [15], this simple model predicts for La,,gEu,,, TiOS upon titanate excitation at LHeT 34% Eu3+ and 66% titanate emission, in excellent agreement with experiment. This confirms the self-trapped exciton model suggested above. Since the titanate emission band is so broad, all higher emitting levels of the Eu3+ ion are populated by the transfer process. This explains why the amount of higher level emission from Eu3+ is higher for ‘Ls excitation than for titanate excitation, since the latter also populates the lower levels. Now we consider the quenching region. Since quenching starts above 40 K due to linear exciton migration and the quenching site concentration ( _ 650 ppm) is nearly two orders of magnitude lower than the Eu’+ concentration (SO!), it is clear that trapping of the mobile exciton by the quenchers must be much more effective than trapping by the Et?+
680
J. ALARCONand G. BLASE
ions. Also the hopping time of the exciton must be shorter than the transfer time to EL?+. An important factor is undoubtedly the fact that the quenchers are situated in the linear titanate chain (see above), whereas the Eu’+ ions are not. Further we note that the oxygen in the Ti-O-T&-O-Ti chain is the seventh oxygen of the La’+ coordination mentioned above, i.e. it is the one with the largest La3+a2distance. For the smaller Eu’+ ion this may be even more pronounced. Since this type of transfer is assumed to occur by exchange [15], the titanate-Eu’+ transfer may be relatively slow, although faster than the radiative decay time of the TiO, group. Also the weak vibronic coupling of the Eu’+ transitions with the titanate vibrations points to a relatively weak interaction. We are planning further investigations on this one-dimensional system.
REFERENCES
1. Blasse G. (Ed.), Mat. Chern. Phvs. 16, (1987): Chem. Mater. 1, i94 (i989). 2. Guillen M. and Bertaut E. F., C. R. AC. Sci. Paris, B262, 962 (1966). 3. Hazenkamp M. F., Blasse G. and Sabbatini N., J. Phys. Chem. 95. 783 (1991). 4. Blasse G.; Suucrure and Bonding 76, 153 (1991). 5. Blasse G., Znorg. Chim. Acta 169, 33 (1990). 6. Blasse G., Structure and Bonding 42, 1 (1980). I. Blasse G., Chem. Phys. Letters 20, 573 (1973). 8. Kroger F. A., Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam (1948). 9. Macke A. J. H., J. Solid State Chem. 18, 337 (1976). 10. Blasse G., Progr. Solid State Chem. 18, 19 (1988). 11. Macke A. J. H., Thesis, University of Utrecht (1976). 12. de Haart L. G. J., de Vries A. J. and Blasse G. J. Solid Stale Chem. 59, 291 (1985).
13. Blasse G., Bokkers G., Dirksen G. J. and Brixner L. H., J. Solid State Chem. 46, 215 (1983). 14. Blasse G., Lammers M. J. J., Verhaar H. C. G., Brixner L. H. and Torardi C. C., J. Solid State Chem. 60, 258 (1985).
A. thanks D. G. I. C. Y. T. of the Spanish Ministry for Education for financial support.
Acknowledgement-J.
15. Powell R. C. and Blasse G., Structure and Bonding 42, 43 (1980).
@X2-3697/92 SS.00 + 0.00 Q 1992 F=ergsmon Press pk
1992
LUMINESCENCE OF Eu3 +-DOPED LANTHANUM TITANATE (La,TiO,), A SYSTEM WITH ONE-DIMENSIONAL ENERGY MIGRATION J. ALARCON? and G. BLASSE~ tDept. de Quinica Inorganica, Universitat de Valencia, 46100 Burjassot (Valencia), Spain $Debye Research Institute, Utrecht University, POB 80.000, 3508 TA Utrecht, The Netherlands (Received 9 August 1991; accepted 23 October
1991)
Abstract-The
titanate and Eu 3+ luminescence of a composition La,TiO,-Eu are reported as a function of temperature. At 4.2 K the quantum efficiency is very high, because the titanate excitation energy is localized. At 300 K there is delocalization and quenching centres are easily reached, so that the quantum efficiency becomes low. This is discussed in terms of the crystal structure, as are the peculiarities of the ELI’+ spectra. ICeywords: Luminescence, La,TiO, , Eu3 + .
1.
INTRODUCTION
Eu3 +-activated phosphors are applied widely for lighting and display [l]. Their physical properties have been intensively studied. We were tempted to investigate the Eu3 + luminescence in La,TiOS, because the crystal structure of this host lattice has some remarkable characteristics. Although the formula La,TiO, suggests that the trigonal bipy~madai Ti05 groups in this structure do not share oxygen ions, this is not the case, since one of the oxygen ions is coordinated by La3+ only [2]. The TiO, groups form linear chains along the c-axis by sharing apexes resulting in an arrangement 0-Ti-O-Ti-0 with Ti+Ti angles close to 180”. Such an arrangement is expected to promote strong interaction between the titanate groups, with SrTiOj as the most famous example. In SrTiO, the coupling of the titanate groups is three-dimensional, in La2Ti05 it is onedimensional. Because of this structural property it seemed worthwhile to investigate the luminescence of La,TiO,-Eu, and to compare the results with those for other host lattices in which energy migration is known to occur.
2. EXPERIMENTAL
Starting materials were Laz03 (99.99%), Eu203 (99.9%) and TiOz ( > 99.9%). Intimate mixtures were fired at final temperatures of 1300°C. The products were found to be single phase using X-ray powder diffraction. In this paper we present results for the compositions La,., Eu,, TQ and nominally pure La,TiO, . The optical measurements were performed 677
with instruments described before [3]. The sample temperature could be varied between liquid helium temperature (LHeT) and room temperature (RT). 3. RESULTS
The diffuse reflection spectra (at RT) show an optical absorption edge at about 290 nm which can be ascribed to the titanate absorption. Apart from very weak features due to Eu3+ in the europium containing sample, the visible region shows a weak and broad absorption band for wavelengths < 500 nm. This absorption is ascribed to transition metal ions present as an impurity in the starting material TiOz. Such absorption bands are well known in titanates and have been ascribed to charge-transfer transitions [4]. Using the Kubelka-Munk formula, the concentration of the impurities is estimated to be 700 ppm. This is in good agreement with the data of the supplier. At 300 K the undoped sample does not luminesce at all, the 5% Eu3+ sample shows under ultraviolet excitation a weak red emission with a quantum efficiency of a few per cent. At 4.2 and 78 K the samples show a strong near-white emission for short wavelength U.V. excitation, and a red emission for long wavelength U.V. excitation. The quantum efficiency of the former was estimated to be some 80% at 4.2 K. Figure 1 shows the emission spectrum of the Eu3+ emission in La,TiO,. It consists of the well-known 3Do-‘F, transitions. At shorter wavelengths the emission lines from the ‘D# = 3, 2, I) level could also be observed. Their intensity is about two orders of magnitude lower than the intensity of the 3D,
678
J. ALARCON and G. BLA~~E
700nm
600
Fig. 1. Spectral energy distribution of the 5D0-7F, emission of Eu3+ in La,TiO, at LHeT. Excitation at 391 nm (7F0-5L6). The value of J is indicated. emission lines. They are a factor of two weaker for titanate excitation than for Eu3+ excitation at 395 nm. Figure 2 shows the excitation spectrum of the Eu’+ emission of La, TiO,-Eu’ + at LHeT. The broad band in the U.V. corresponds with the optical absorption observed in the reflection spectrum and is, therefore, due to titanate excitation. This shows that energy transfer from titanate to europium takes place. The sharp lines are the intraconfigurational transitions of the 4f6 configuration of the Eu3+ ion. At room temperature the ultraviolet band is much weaker relative to the lines. The difference is more than one order of magnitude, corresponding to the difference in quantum efficiency at the two temperatures. The inset of Fig. 2 shows the excitation line corresponding to the ‘F,-‘D, transition of the Eu’+ ion. It shows also vibronic transitions on the shorter wavelength side [5]. Due to the large splitting of the electronic lines, it is hard to analyse the vibronic lines exactly, but it is clear that there are a more intense group at about 250 cm-’ and a less intense group at about 750 cm-’ from the electronic lines. The former are ascribed to coupling with the Eu-0 vibrational stretching mode, the latter to coupling with the Ti-0
--A
44
300
L60nm
SOOnm
Fig. 2. Excitation spectrum of the Eu’+ emission of La,,,Eu,,,TiO, at LHeT. qr gives the relative quantum output. The inset gives the 7F0-5D, excitation transition stressing the vibronic transitions.
stretching mode. In tungstates and molybdates these groups have usually comparable intensities (Ref. 5 and references cited therein). Excitation into the titanate at LHeT yields not only Eu3+ emission, but also broad band titanate emission [6]. Their integrated intensity ratio is about 1:2. At RT this titanate emission is quenched. The spectra of La,TiO, consist of broad bands only. The emission band has its maximum at about 475 nm, the excitation band is the same as for Eu3+ with a maximum at about 280 nm. Upon increasing the temperature, the emission intensity started to drop above 40 K and is quenched above 150 K.
4. DISCUSSION 4.1. The Eu3+ emission The lines in the Eu’+ spectra are well known and do not need a further assignment. At first sight there seems to be a discrepancy between the very low site symmetry (C,) and the number of possible sites for the lanthanide ion (two) on one hand, and the very simple emission spectrum which, in good approximation, shows two lines for the ‘D,,-‘F, and for the 5Do-7F, transitions. However, not only are the two independent sites very much alike [2], but also the seven coordination of the lanthanide ions can be considered as an octahedral one elongated along a trigonal axis, to which a seventh ion has to be added at somewhat larger distance. The trigonally distorted octahedron is then responsible for the splitting into two levels of the ‘F, level and into three levels of the ‘F2 level (approximate site symmetry D,&. This symmetry allows two lines for both transitions, explaining the emission spectrum in first approximation. The seventh ion lowers the symmetry to the real one, viz. C,. This ion causes a linear crystal field on the Eu3+ ion, a necessary condition for the observation of the ‘Do-‘F, transition with the observed intensity. It will also bring about a further crystal field splitting. From the emission spectrum it is clear that the longer wavelength components of the 5Do-7F,,, transitions are not single lines at all. In this way the experimental emission spectrum agrees completely with the crystallographic data [2]. Simultaneously it is a nice illustration of the fact that small changes in the surroundings of the Eu’ + ion influence the intensities much stronger than the crystal field splittings [7]. Apart from sharpening which makes small splittings better observable, there is no essential difference between the Eu3 + excited Eu3 + luminescence at RT and LHeT. Especially the luminescence intensity
619
Luminescence of Eu’+-doped lanthanum titanate does not vary with temperature. This is completely different for titanate excitation. But first we discuss the titanate luminescence itself.
4.2. The titanate luminescence Titanate luminescence involves charge-transfer transitions [6]. This luminescence has been observed for tetrahedral, octahedral and tetragonal pyramidal titanate groups. To this we now add the trigonal bipyramidal titanate group. At LHeT the quantum efficiency of the luminescence is high, but it starts to drop above 40 K. There are two models to explain this decrease. The first is the appearance of non-radiative transitions in the titanate group which compete with the radiative transitions. In this model all transitions occur in one and the same titanate group. This model is always valid for diluted systems. The second model considers energy transfer between titanate groups, so that the excitation energy can migrate to quenching centres. Here the excitation gets lost non-radiatively. By diluting the concentration of luminescent ions it is possible to distinguish between the two models [8]. Unfortunately, this is not possible in the case of La,TiO, in view of the specific titanium coordination. Therefore we compare our system with results from the literature on other compounds with an aim to solve our problem in this way. An important fact to start with is the observation that the optical absorption of islolated titanate groups is usually situated at about 250 nm, i.e. some 40,000 cm-’ [6, 8, 91. In La,TiO, its position is N 35,000 cn-I, which indicates interaction between the titanate groups via the linear Ti&Ti bridge. Due to band broadening the absorption energy is reduced [lo]. This interaction extends in one direction. In Sr,TiO, it does so in two dimensions and the absorption band is at N 30,000 cm-’ [1 11. In SrTiO,, finally, the connections are threedimensional and the absorption band is at only N 27,000 cm-’ [12]. Although it cannot be excluded that the relevant bond distances and angles in these compounds are slightly different, the enormous shift must, in our opinion, be ascribed in the first place to the interaction between titanate groups and its dimensionality. The stronger the interaction, the less the relaxation in the excited state [lo, 121. This is measured by the Stokes shift of the emission. Interestingly enough, the Stokes shift decreases with increasing dimensionality of the interaction: La,TiO, 14,000 cm-‘, Sr*TiO, 11,000 cm-’ (1 l), and SrTiO, 8000 cm-’ [12]. At LHeT the emission of these titanates can be ascribed to self-trapped exciton emission on one of
the titanate groups. The mobility of the excitons can be neglected. The SrTiO, emission already starts to be quenched above 10 K. De Haart et al. [12] have shown that the exciton becomes mobile and reaches impurities. In the case of SrTiO,: CIJ+ the Cr’+ emission builds up at the expense of the titanate emission. Therefore we ascribe the quenching of the luminescence of La,TiO, above 40 K to exciton migration to quenching sites. Probably these consist of the impurity transition metal ions (see above). Since these are expected to be built into the chain, they can be effective quenchers of the titanate emission. Finally we note that in LaWO,Cl (13), La,NbO,Cl, and La,TaO,Cl, (14) there are linear W04, Nb04 and Tao, chains of exactly the same constitution as the TiO, chain in La,TiO,. The luminescence of these compounds [13,14] is very similar to that of La,TiO,.
4.3. Titanate-Eu3 + La, TiO, : Eu’ +
energy
transfer
in
First we consider the situation at LHeT: the quantum efficiency of the total luminescence upon titanate excitation is high and the emission comprises onethird of Eu3+ emission. If the excitation remains localized on the excited titanate group, the ratio of titanate to Eu3+ emission intensity can be derived as follows. Each titanate group in La, TiO, has eight nearest La3 + neighbours. With a Eu3+ concentration of 5% the probability to find eight La’+ ions around the TiOs group in La,,VEu,,,TiO, is 0.95’ = 0.66. Since it is very probable that the excited titanate group transfers to Eu3 + if there is a Eu3+ ion on one of the eight nearest neighbour sites [15], this simple model predicts for La,,gEu,,, TiOS upon titanate excitation at LHeT 34% Eu3+ and 66% titanate emission, in excellent agreement with experiment. This confirms the self-trapped exciton model suggested above. Since the titanate emission band is so broad, all higher emitting levels of the Eu3+ ion are populated by the transfer process. This explains why the amount of higher level emission from Eu3+ is higher for ‘Ls excitation than for titanate excitation, since the latter also populates the lower levels. Now we consider the quenching region. Since quenching starts above 40 K due to linear exciton migration and the quenching site concentration ( _ 650 ppm) is nearly two orders of magnitude lower than the Eu’+ concentration (SO!), it is clear that trapping of the mobile exciton by the quenchers must be much more effective than trapping by the Et?+
680
J. ALARCONand G. BLASE
ions. Also the hopping time of the exciton must be shorter than the transfer time to EL?+. An important factor is undoubtedly the fact that the quenchers are situated in the linear titanate chain (see above), whereas the Eu’+ ions are not. Further we note that the oxygen in the Ti-O-T&-O-Ti chain is the seventh oxygen of the La’+ coordination mentioned above, i.e. it is the one with the largest La3+a2distance. For the smaller Eu’+ ion this may be even more pronounced. Since this type of transfer is assumed to occur by exchange [15], the titanate-Eu’+ transfer may be relatively slow, although faster than the radiative decay time of the TiO, group. Also the weak vibronic coupling of the Eu’+ transitions with the titanate vibrations points to a relatively weak interaction. We are planning further investigations on this one-dimensional system.
REFERENCES
1. Blasse G. (Ed.), Mat. Chern. Phvs. 16, (1987): Chem. Mater. 1, i94 (i989). 2. Guillen M. and Bertaut E. F., C. R. AC. Sci. Paris, B262, 962 (1966). 3. Hazenkamp M. F., Blasse G. and Sabbatini N., J. Phys. Chem. 95. 783 (1991). 4. Blasse G.; Suucrure and Bonding 76, 153 (1991). 5. Blasse G., Znorg. Chim. Acta 169, 33 (1990). 6. Blasse G., Structure and Bonding 42, 1 (1980). I. Blasse G., Chem. Phys. Letters 20, 573 (1973). 8. Kroger F. A., Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam (1948). 9. Macke A. J. H., J. Solid State Chem. 18, 337 (1976). 10. Blasse G., Progr. Solid State Chem. 18, 19 (1988). 11. Macke A. J. H., Thesis, University of Utrecht (1976). 12. de Haart L. G. J., de Vries A. J. and Blasse G. J. Solid Stale Chem. 59, 291 (1985).
13. Blasse G., Bokkers G., Dirksen G. J. and Brixner L. H., J. Solid State Chem. 46, 215 (1983). 14. Blasse G., Lammers M. J. J., Verhaar H. C. G., Brixner L. H. and Torardi C. C., J. Solid State Chem. 60, 258 (1985).
A. thanks D. G. I. C. Y. T. of the Spanish Ministry for Education for financial support.
Acknowledgement-J.
15. Powell R. C. and Blasse G., Structure and Bonding 42, 43 (1980).