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DEFECTS AFTER NEUTRON CAFTURE AND
INTRINSIC FLUORINE SELF-DIFFUSION IN CaF2
M. Grupp, H. Ackermann, D. Dubbers, H. Grupp, P. Heitjans, H. Stöckmann
To cite this version:
C7-538 JOURNAL DE PHYSIQUE Colloque C7, suppliment au no 12, Tome 37, Dicembre 1976
DEFECTS AFTER NEUTRON CAFTURE
AND
INTRINSIC FLUORINE SELF-DIFF'USION IN CaF,
M. GRUPP, H. ACKERMANN, D. DUBBERS, H. GRUPP, P. HEITJANS and H. J. STOCKMANN
Physikalisches Institut der Universitat, 69 Heidelberg, FRG and
Institut Laue-Langevin 156 Centre de Tri, 38042 Grenoble-Cedex, France
Rbsumb. - La production de dkfauts intrinskques dans CaFz par la capture de neutrons ther- miques est examinee dans un large domaine de tempkrature pour dew temps typiques aprks la crkation des dkfauts.
L'autodiffusion anionique intrinskque est ktudike par des mesures de la relaxation nuclkaire dependant du champ magnktique extkrieur et de l'orientation du cristal.
Abstract. - The production of intrinsic defects in CaFz by capture of thermal neutrons is inves- tigated in a wide temperature range in a short- and a long-time period after defect creation.
Intrinsic anion self-diffusion is studied by nuclear relaxation measurements in dependence of external magnetic field and crystal orientation.
Polarized 'OF ( T ~ = 16.2 S) nuclei are produced in CaF, single crystals by capture of polarized neu- trons. The 'OF nuclei act as probes, their polarization
- and thus any phenomenon destroying this pola- rization - being monitored via the asymmetry of the P-decay of 'OF. AS a special feature of this expe- riment the 'OF probes see their own defects, created by the recoil (E,,,,,, = several 100 eV) they suffered during their activation process.
Four different defect properties of CaF, are studied (l) :
1) A short time
(2
10-' to 10-l0 S) after defect creation all 'OF probe nuclei are found in the neigh- bourhood of defects which can be divided in two classes of defects showing different hyperfine inter- actions. The ratio of the creation probabilities may be identified as the equivalent of the @/F-ratio in the alkali halides and falls from 10 K to 140 K by one order of magnitude (a, F : creation probabi- lities for vacancies and F-centres, resp.).2) Several seconds after defect creation the stable defect configuration is measured at T < 70 K : the registered NMR spectrum of the 'OF nuclei shows a
(1) The results of 2 and 3 have been published previously in
ref. [l].
quadrupolar splitting due to electric field gradients caused by diamagnetic defects. The analysis of the spectrum identifies the defects as 19F interstitials in a [ I l l ] direction next to the 'OF ions stopped on normal lattice sites.
3) The annealing of defects is observed directly
in a transient way around 80 K. The corresponding activation energy is found to be 0.17
+
0.04 eV.4) Thermally activated diffusion is observed bet- ween 650 K and 800 K via its influence on the spin- lattice-relaxation time T l of 'OF (below 650 K the phonon induced quadrupolar relaxation is the domi- nant mechanism).
Earlier experiments using tracer techniques or relaxation of 19F showed extrinsic diffusion to be dominant below 900 K. Presumably due to the absence of spin diffusion between 19F and 'OF intrinsic diffusion is observable in our case down to 700 K.
The relaxation due to fluorine diffusion is found to be 1.6 rt_ 0.3 times more effective with the external magnetic field parallel to a [l 1 l] direction then with the field parallel to a [l001 direction. The dependence of the relaxation on the magnetic field strenth is as expected for relaxation due to diffusion.
This work was sponsored by the Bundesministerium fiir Forschung und Technologie.
Reference
[l] ACKERMANN, H., DUBBERS, D., GRUPP, H., GRUPP, M.,
HEITJANS, P. and ST~CKMANN, H. J., P h p . Lett. 54A (1975) 399.
DEFECTS AFTER NEUTRON CAPTURE AND INTRINSIC FLUORINE SELF-DIFFUSION IN CaF2 C7-539
DISCUSSION
K. REUTER.
-
YOU only mentioned the quadrupoleinteraction frequency for the 'OF ground state in the
case of the F; -molecule. Do you also know the electric quadrupole moment of this state ?
M. GRUPP.
-
We have measured and publishedthe quadrupole moment of the 20F ground state. The result was
I
Q ('OF)I
= 0.064 (12) b. In the present experiment we determined sign and magnitude of the quadrupole interaction of a 20F-ion on a regular lattice site with a IgF-ion on an adjacent interstitial site.L. SLIFKIN.
-
HOW can the 20F tell the difference between intrinsic defects and the large number of extrinsic defects in the crystal ?diffusion where we dont't see it, we are led to the con- clusion that spin diffusion is needed for the relaxa- tion by extrinsic diffusion. The difference between intrinsic and extrinsic diffusion should be noted :
The former works via vacancies, the latter probably via interstitials. It should not surprise too much that this has consequences for the diffusional relaxation. This point, however, requires considerable more inves- tigation in order to be really sure what happens.
C. R.' A. CATLOW.
-
I was interested to observethe vacancy activation energy that you deduced from your annealing studies. This agrees with recent calculations that I performed which suggested that pronounced reductions of vacancy activation ener- gies would occur in the vicinity of interstitials.
M. GRUPP.
-
The main difference between relaxa- M. GRUPP.-
Intuitively we expected this loweringtion of and 'OF is the lack of spin diffusion due to of the activation energy to be reasonable, since an high dilution and different magnetic moment in the interstitial right next to a vacancy is unstable.
