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Submitted on 1 Jan 1980
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Laser spectroscopy of defect equilibria in fluorite materials
J. Wright
To cite this version:
J. Wright. Laser spectroscopy of defect equilibria in fluorite materials. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-434-C6-435. �10.1051/jphyscol:19806111�. �jpa-00220018�
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-434
Résumé. — Une technique pour exciter sélectivement des sites précis dans les cristaux alcalinoterreux dopés aux terres rares sera décrite. La technique a permis la caractérisation des états d'équilibre des imperfections. Les variations, avec la concentration et la température, de la distribution des sites seront décrites.
Laser spectroscopy of defect equilibria in fluorite materials
J. C. Wright
Department of Chemistry, University of Wisconsin, Madison, WI 53706, U.S.A.
Abstract. — A new method using laser spectroscopy has been deveioped and applied to the study of defects in alkaline earth fluoride crystals. The concentrations of ail the defects hâve been determined as a function of concen- tration and température. The measurements show that the simple defect equilibria previously assumed for fluorites is incorrect and a new model is proposed.
Crystals that possess the fluorite structure hâve played an important rôle in the understanding of defect equilibria. Despite the excellent agreement that has existed between theory and experiment for a variety of studies in thèse materials, there hâve been several experiments that hâve shown marked disagreements. At low concentrations, EPR measure- ments in alkaline earth fluorides doped with trivalent cations hâve shown anomalous behavior for the ratio of associated to dissociated defect pairs as a function of annealing température and dopant concentrations.
At high concentrations, neutron scattering experi- ments in CaF2 by Cheetham et al. [1] and in U02
by Willis [2] show defect clusters of anion interstitials where the séparations are anomalously small and the net charge expected in the clusters is anomalously high. Catlow [3] suggested that covalent interactions between anion interstitials could explain the high concentration results.
We hâve deveioped a technique which allows us to observe and measure the concentrations of ail of the différent defect structures encountered by trivalent lanthanide dopants in CaF2, SrF2, and BaF2 [4-13].
A tunable dye laser is used to selectively excite spécifie absorption lines of spécifie lanthanide sites. This technique allows an absorption spectrum to be systematically taken apart into the component site spectra that make it up. We hâve identified 16 différent sites that are associated with defect clustering in CaF2 in addition to the tetragonal, trigonal, and cubic sites associated with the single lanthanides and their fluoride interstitials that are normally observed.
It is commonly thought that only the three sites having single lanthanides are the important ones in understanding the defect equilibria in the fluorites.
Our measurements show that this view is incorrect.
The absolute concentrations of each of the différent types of lanthanide sites hâve been measured as a function of both the annealing température of the crystals and the concentration of lanthanide dopants.
Furthermore, two-body and three-body energy trans- fer processes hâve been used to distinguish between those sites that correspond to dimers of lanthanide- fluoride interstitial pairs and higher order clusters involving more than two lanthanide ions. Our experi- ments hâve led us to a new model for the control of defect equilibria in fluorite materials in which the formation of clusters détermines the behavior of ail defect equilibria. The formation of clusters is observed to be the controlling influence at concentrations as low as 1 0- 2 mole % lanthanide dopant, a région where it is commonly assumed one can neglect the influence of clusters.
In this model, one must assume that the clusters scavenge fluoride interstitials in the lattice, perhaps by the covalent interactions proposed by Catlow [3].
The scavenging of fluoride interstitials may be part of the mechanism by which the clusters are built. In any case, the removal of fluoride interstitials induces the dissociation of the simple tetragonal and trigonal pairs and the formation of cubic lanthanide sites.
This model explains the anomalous behavior observed by previous EPR measurements and it also is consistent with the more detailed behavior observed in our experiments [9]. It is believed that the technique of sélective laser excitation (or site sélective spectroscopy) is a new tool which will greatly enhance the knowledge that can be obtained about defects in materials.
Acknowledgments. — This research was supported by grants DMR-7707765 and DMR-7906788 from the National Science Foundation.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806111
LASER SPECTROSCOPY O F DEFECT EQUILIBRIA IN FLUORITE MATERIALS C6-435
DISCUSSION
Question. - P. W. M. JACOB.
D o you have a value for the association energy of an interstitial F- with a dimer ?
Reply. - J. C . WRIGHT.
We cannot obtain a value for the association energy until we have completed our study of the important equilibria. Until that study is completed, any value could be erroneous because of competing equilibria.
Comment. - J. J . FONTANELLA.
I'd like to point out that we've recently been able to correlate our electrical relaxation spectrum with the selective laser excitation spectrum of Wright and co-workers (J. Chem. Phys., to be published).
All of our data, both concentration and thermal cycling effects are consistent with his interpretation.
It should also be emphasized that, his dimer (C-site) is a negative dimer ion i.e. two rare-earths and three fluorines. Such a dimer is preferable for explaining the analogous electrical relaxation since such a cluster will most likely be asymmetrical and be dipolar and hence relax electrically.
References
CHEETHAM, A. K., FENDER, B. E. F. and COOPER, M. J. J., J. Phys. C : Solid State Phys. 4 (1971) 3107.
WILLIS, B. T. M., Proc. Br. Ceram. Soc. 1 (1964) 9.
CATLOW, C. R. A., J. Phys. C : Solid State Phys. 9 (1976) 1859.
. . TALLANT, D. R. and WRIGHT, J. C., Proc. 11th Rare Earth Research Conf (1974).
[S] TALLANT, D. R. and WRIGHT, J. C., J. Chem. Phys. 63 (1975) 2075.
[6] TALLANT, D. R., MILLER, M. P. and WRIGHT, J. C., J. Chem.
Phys. 65 (1976) 510.
[7] KURZ, M. D. and WRIGHT, J. C., J. Lumin. 15 (1977) 169.
[8] MILLER, M. P., TALLANT, D. R., GUSTAFSON, F. J. and WRIGHT, D. C., Anal. Chem. 49 (1977) 1474.
[9] TALLANT, D. R., MOORE, D. S. and WRIGHT, J. C., J. Chem.
Phys. 67 (1977) 2897.
[lo] WRIGHT, J. C., Spectroscopic des Eldments de Transition et des ElPments Lourds dans les Solides (C.R.N.S., Parks, 1977).
[ l l ] MILLER, M. P, and WRIGHT, J. C., J. Chem. Phys. 68 (1978) 1548.
[12] MILLER, M. P. and WRIGHT, J. C., Phys. Rev. B 18 (1978) 3753.
[13] MILLER, M. P. and WRIGHT, J. C., J. Chem. Phys. 71 (1979) 324.
