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:
M. Grupp, H. Ackermann, D. Dubbers, H. Grupp, P. Heitjans, et al.. DEFECTS AFTER
NEUTRON CAFTURE AND INTRINSIC FLUORINE SELF-DIFFUSION IN CaF2. Journal de Physique Colloques, 1976, 37 (C7), pp.C7-538-C7-539. <10.1051/jphyscol:19767123>.
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Submitted on 1 Jan 1976
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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 thermiques 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 investigated 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 neutrons. The 'OF nuclei act as probes, their polarization
- and thus any phenomenon destroying this polarization - being monitored via the asymmetry of
the P-decay of 'OF. AS a special feature of this experiment 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 neighbourhood of defects which can be divided in two
classes of defects showing different hyperfine interactions. 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 probabilities 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 between 650 K and 800 K via its influence on the spinlattice-relaxation time T l of 'OF (below 650 K the
phonon induced quadrupolar relaxation is the dominant 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 1l ] 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.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19767123
DEFECTS AFTER NEUTRON CAPTURE AND INTRINSIC FLUORINE SELF-DIFFUSION IN CaF2 C7-539
DISCUSSION
K. REUTER.
- YOUonly mentioned the quadrupole
interaction 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 published
the quadrupole moment of the 20Fground 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.
- HOWcan the 20Ftell the difference
between intrinsic defects and the large number of
extrinsic defects in the crystal ?
M. GRUPP.- The main difference between relaxation of
and 'OF is the lack of spin diffusion due to
high dilution and different magnetic moment in the
latter case. Since the 19Frelaxation people see extrinsic
diffusion where we dont't see it, we are led to the conclusion that spin diffusion is needed for the relaxation 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 investigation in order to be really sure what happens.
C. R.' A. CATLOW.
- I was interested to observe
the 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 energies would occur in the vicinity of interstitials.
M. GRUPP.- Intuitively we expected this lowering
of the activation energy to be reasonable, since an
interstitial right next to a vacancy is unstable.