EFFECT OF THE ONE-DIMENSIONAL STRUCTURE ON THE ENERGY TRANSFER IN Li6Gd (BO3)3

Transcription

1 EFFECT OF THE ONE-DIMENSIONAL STRUCTURE ON THE ENERGY TRANSFER IN Li6Gd (BO3)3 C. Garapon, B. Jacquier, Y. Salem, R. Moncorge To cite this version: C. Garapon, B. Jacquier, Y. Salem, R. Moncorge. EFFECT OF THE ONE-DIMENSIONAL STRUC- TURE ON THE ENERGY TRANSFER IN Li6Gd (BO3)3. Journal de Physique Colloques, 1985, 46 (C7), pp.c7-141-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C7, supplément au n 10, Tome 46, octobre 1985 page C7-141 EFFECT OF THE ONE-DIMENSIONAL STRUCTURE ON THE ENERGY TRANSFER IN Li 6 Gd(B0 3 ) 3 C.T. Garapon, B. Jacquier, Y. Salem and R. Moncorge Labovatoiice de Physico-Chimie des Matériaux Luminescents, VA 442 C.N.R.S., Université Lyon I, 43 Bd du 11 novembre 1918, Villeurbanne, France Résume - L'étude de la fluorescence des ions Gd + dans LigGd(B0,), montre qu'il y a diffusion de l'énergie parmi les ions Gd3+ et piégeage par des impuretés à haute température et par des ions Gd 3+ perturbés à basse température. Le passage d'un régime de diffusion rapide à un régime de transfert limité par la diffusion à basse température est attribué à la structure quasi-unidimensionnelle du cristal. Abstract - The Gd + fluorescence has been studied in LigGd(B0 3 )3. Energy migration among Gd 3+ and trapping by impurities at high temperature and perturbed Gd 3+ ions at low temperature are observed. The cross-over from a fast diffusion regime to a diffusion-limited regime at low temperature is attributed to the quasi one-dimensional crystal structure. INTRODUCTION It has often been observed that energy migrates easily among Gd ions in Gd compounds and this property has been used in phosphors materials to transfer energy from a sensitizer to an activator /1,2/. The aim of the study of Li6Gd(B0 3 )3 is to see how the energy transfer among Gd 3+ ions is influenced by the crystal structure which is 0 here one-dimensional. Gd 3+ ions belong to chains separated by a distance of 6,65 A for an intrachain distance of 3.88 A /3/. The intrachain interactions should be dominant over the interchain interactions and the transfer might be expected to be one-dimensional. No magnetic measurements at very low temperature have been performed at the present time. However, the shortest interatomic distance along the chain is comparable to the ones in magnetic order material such as GdCl3 or Gd(0H) 3. EXPERIMENTAL The synthesis and single crystal growths have been achieved in the laboratoire de Chimie du Solide de Bordeaux /3/. Most of the results have been obtained with crystal 1 prepared with Gd 2 03, 3N. Some additional results concern crystal 2 prepared with Gd203 4N. The fluorescence spectra and decays have been obtained after selective pulsed excitation with.a doubled dye laser pumped by a doubled neodymium YAG laser. Details of the apparatus have been described elsewhere /3/. RESULTS 1) High temperature (T> 40 K) - The fluorescence spectrum of the transition from the~first"excited"manyfold 6 P7/2 to the ground state 8 S7/2 contains four lines as expected for Gd 3+ ions in a low symmetry site ((4). The fluorescence decay of the line I4 lying at lowest energy is exponential with a time constant at room temperature of about 360 fcs for the crystal 1 and 820 j*s for crystal 2. These values are much smaller than the radiative life time which in this material is estimated as 4.35 ms (see later). Article published online by EDP Sciences and available at

3 C7-142 JOURNAL DE PHYSIQUE From this exponential and fast decay we conclude that fast diffusion of the energy takes place among the Gd3+ ions nd that the energ gets trapped by impurities (EU~+ for example whose lines at due to emission ~1 are observed). Up to now the behavior of LigGd(B03)3 is very similar to that of other Gd3+ compounds and no information about the dimensionality of the energy diffusion may be obtained as we are dealing with a fast diffusion regime. As the temperature is decreased from 40 K to 1.5 K the intrinsic fluorescence decreases at the advantage of new emissions at lower energy labelled Po to P10 (figure 1). Fig. 1 - Intrinsic (14) and trap (Pa) fluorescences from the 6 transition of Gd3+ at 1.5 K (crystal 1). The 1 ines correspond to the fluorescence of Gd3+ ions located on sites perturbed by impurities or defects which act as traps. All the emission lines exhibit the sa e excitation spectrum with a linewidth of 3 cm-l, ten times broader than in a Gd' dilute compound. No im ortant changes of the lineshape or of the position of the absorption line 8s7/2+ P712 have been observed at very low temperature which could be related to a possible magnetic ordering of this material. Each of these traps is characterized by a trap depth relative to the intrinsic level 14. The temperature at which each trap fluorescence appears decreases as the trap depth

4 gets smaller. The relative intensities of the trap lines suggest that the shallow traps P1 and P2 are more concerned with rare earth impurities while P7 and P8 could be related to defect centers since they behave differently for crystals 1 and 2. In this temperature range the energy mostly absorbed by the intrinsic ions is spread over the whole crystal before getting trapped in perturbed ions. This is also confirmed by the fluorescence decay analysis. The decay of the intrinsic fluorescence 14 is no more exponential. We have checked that this is not due to the pump intensity or other sources /3/. A simple trapping model assuming fast diffusion among intrinsic ions and trapping by the most important traps (P7 and Pg down to 10 K and P1 and P2 for 4.4 K and 1.5 K) has been used. The intrinsic and trap fluorescence decays are correctly fitted respectively by a sum and a difference of two exponential functions as illustrated in figure 2 a,b. Fig. 2 - a : Decay of the intrinsic fluo- Fig. 2 - b : Decay curve of the trap rescence line 14 at 4.4 K. The slow com- line P at 4.4 K. The decay time is ponent is due to back tansfer from trap P2. equal 80 the radiative lifetime s. The decay rates for the longtime slow components have the same order of magnitude. The slight difference could be related to direct transfer from perturbed ~ d ions ~ +

5 C7-144 JOURNAL DE PHYSIQUE (say Pi) to fluorescing traps such as ~ b or ~ EU~+ + which are 1st nn or 2nd nn. In this model the decay rate of the short time fast component is equal to trapping rate to the trap. From the intensity of the trap lines relative to the intensity of the intrinsic line at very low temperature an estimation of this trapping rate is obtained (K s-1 at 1.5 K for crystal 1). This value is not consistent with the value obtained from the decay rate (K S-1 for crystal 1). Thisxiscrepancy, which is observed for both samples, leads us to the conclusion that this 3+ short time component does not correspond to trapping but to diffusion between Gd intrinsic ions which is then the limiting step in the transfer. To account for the intensities we expect a still faster component a t shorter times that those sampled in our experiment that is less than 2 s. Preliminary results at shorter time (time scale 0,2ps) seem to indicate tkt the decay rate becomes indeed faster. This cross over from a fast diffusion regi~e to a diffusion-limited Egime at low temperature has never been observed in ~d~ compounds to our knowledge and might be attributed to the one-dimensional structure. DISCUSSION It has been proposed that for a one-dimensional energy migration the fluorescence decay should be exp(-a \rt) and exp(-a fi - Kt) if a three dimensional component is present /4/. In our case the short component deviates only slightly from an exponential time dependence as shown in figure 3. Fig. 3 - Decay of the intrinsic fluorescence I4 at 1.5 K showing weak non exponential time dependence with a mean decay rate of 80 ps. The decay rate expected from the intensities is 30 ps.

6 tiowever without data at very short times it is not possible to say wether only this 3D diffusion occurs or wether is the long time component of a quasi 1-D diffusion. The results may be explained as follow : - at high temperature the perturbed ions are inefficient and fast diffusion of the energy takes place, probably along the chains as strongly suggested by the crystal structure. - at low temperature the perturbed ions act as energy traps the probability of back transfer to the intrinsic ions decreases as temperature decreases and this hinders the diffusion along the chains which becomes as slow as to be the limiting step in the transfer so that diffusion limited transfer takes place and as to be of the same order of magnitude the slow interchain energy transfer so that 3D diffusion is observed. This cross over from fast diffusion to diffusion limited transfer might be considered as an indication that the fast diffusion could be ID. If it were 3D it could not be prevented by such a small amount of shallow traps (which has been evaluated as about 10-3 /3/) and fast diffusion should remain probably as long as intrinsic ~d3+ concentration is more than 30 % (order of magnitude of a percolation threshold at 3D). Our understanding of this system is limited by the fact that the perturbed ~ d ions ~ + act both as blocking and trapping ions depending on temperature and have not clearly defined concentrations. In order to improve it should be interesting to use blocking ions like ~ 3 + in known concentrations and acceptors, in the chains or out the chains, such as the trapping rate is very fast as to favor the diffusion limited regime which is the only one where dimensionality effects can be seen. ACKNOWLEDGEMENTS We wish to thank J.P. CHAMINADE for providing new samples and G. BLASSE for several discussions. REFERENCES /1/ Mahiou,R., Jacquier, 6. and Linares, C., J.O.S.A. 73 (1983) /2/ Blasse, G., Phys. St. Sol. (a) 73 (1982) 205. /3/ Garapon, C.T., Jacquier, B., Chzinade, J.P. and Fouassier, C., J. of Lum. (accepted paper). /4/ Wieting, R.D., Fayer, M.D. and Dlott, D.D., J. : Chem. Phys. 69 (1978) 1996.