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DEFECT RECOMBINATION AND
AGGREGATIONRadiation induced point defects in
CaF2, SrF2, BaF2 and MgF2
W. Buttler, H.-J. Stöckmann, F. Fujara, P. Heitjans, G. Kiese, H. Ackermann,
B. Bader, K. Dörr, H. Grupp, H. Lauter
To cite this version:
JOURNAL DE PHYSlQUE Colloyue C6, supplkment au no 7, Tome 41, Juillet 1980, page C6-381
DEFECT RECOMBINA TION AND AGGREGA TION.
Radiation induced point defects
in
CaF2, SrF2, BaF2 and MgF2
W. Buttler, H.-J. Stockmann, F. Fujara, P. Heitjans, G . Kiese, H. Ackermann, B. Bader, K. Dorr, H. Grupp and H. Lauter
Institut Laue-Langevln, 156X, 38042 Grenoble Cedex, France,
Phys~kalisches Institut der UniverslUt, 6900 Heidelberg, R.F.A. and Fachbereich Physik der UniversiUt, 3550 Marburg, R.F.A.
Rhumb. - Des monocristaux de plusieurs fluorures alcalino-terreux ont Cte irradiks a diffkrentes temperatures
avec des neutrons thermiques polarists. Le processus de capture de neutrons 19F(n, y),OF produit des ions 'OF deplaces par recul (energie de recul : plusieurs centaines d'eV) e t en m&me temps quelques dtfauts ponctuels dans leur voisinage. O n a etudie ces defauts par leur influence sur le spectre RMN du 'OF, detect6 par mesure de l'asy- metrie de la desintegration
P
des noyaux polarises 'OF. Dans CaF,, SrF2 et BaF, cubiques, un stade de recuit a ete observe dans la region de 60 a 100 K, qui correspond a une enthalpie d'activation de 0,2 eV environ. Un deuxikme stade se produit vers 220 K, ce qui correspond a 0,6 eV environ. Dans le MgF, tetragonal deux stades de recuit ont ete observbA
13 et 60 K correspondant respectivement a 0,037 et 0,17 eV.Abstract. - Single crystals of several alkaline earth fluorides were irradiated with thermal polarized neutrons a t various temperatures. The neutron capture process 19F(n, ?)'OF produces recoil displaced 'OF ions (recoil energy : several 100 eV) and simultaneously some nearby point defects. These defects were studied by their influence on the 2 0N M R ~ spectrum monitored by the asymmetric B-decay of the polarized 'OF nuclei. In the cubic crystals CaF,, SrF, and BaF, one annealing step was observed in the region of 60
...
100 K corresponding to an activation enthalpy of about 0.2 eV. A second step occurs near 220 K, corresponding t o about 0.6 eV. In tetragonal MgF2 two annealing steps were observed a t 13 and 60 K corresponding to 0.037 and 0.17 eV, respectively.1 . Experimental method. - Defects in alkaline electric field gradient vanishes at regular lattice sites earth fluorides have been the subject of many studies with undisturbed environment. In this case the 'OF [I-4 and references cited therein]. This experiment probe nuclei show pure Zeeman interaction with B reports on (n, y) induced point defects in such crystals and their NMR spectrum consists of one single line studied by the method of
fi
radiation detected NMR [5]. at the Larmor frequency a,.. Close defects locally Defect production and examination proceeds as disturb the cubic symmetry and four quadrupolarfollows : split NMR transitions around o, occur [6]. This can
be used to study annealing processes : The tempe-
1 9 Y'S
F
+
npd -' 20F;$I -+Polarized, thermal neutrons are captured by the 19F host nuclei in an external magnetic field B. The pola- rized, excited 'OF& nuclei decay in about 1 ps by
emission of several y's with a maximum recoil energy of 1.2 keV. The average recoil energy of several 100 eV due to these y's is sufficient to displace the
2 0 ~ , , ions by some lattice constants and to create
some nearby point defects. The polarized 'OF,, nuclei, representing our probes, decay by emission of f?- radiation (zp = 16 s). This /I- radiation shows an asymmetry proportional to the polarization P of the 'OFpd ensemble. Defect properties are measured by their influence on P and on the NMR spectrum of the 20Fpd nuclei (spin I = 2).
2. Measurements.
-
2.1 CaF,, SrF, AND BaF, (FLUORITE STRUCTURE). - In these cubic fluorides theP ( o f f res.
Plw,)
t
Fig. I .
-
Polarization P (cc /?-decay asymmetry) of 'OF in a cubic crystal (e.g. CaF,) versus radiofrequency w . All nuclei without defect interactions are depolarized at o,. Thus the ratio P(w)/P(off res.) measures the number of defect disturbed 1°F probes. At low temperatures, if all 'OF nuclei see neighbouring defects, this ratio is 1 (i.e. no N M R signal). It approaches 0 at high temperatures (i.e. maximum N M R signal), if all defects are annealed.C6-382 W . BUTTLEK el al. rature dependence of the quantity P(w,)/P(off res.),
as explained in figure 1, shows an annealing step at temperatures where the mean lifetime of the defects z,(T) is equal to 7~ = 16 s (Fig. 2). One step is found at about 80 K for CaF2, SrF2 and BaF,. The annealing of this defect. its identification a s a I9F interstitial and its quadrupolar coupling constant of - 2.0 MHz
have already been reported in a former paper for the case of CaF, [6]. A second step is found at about 220 K though with lower significance than the 80 K step. The nature of the corresponding defect has not yet been clarified. Possibly it is the one observed by Kreische et al. with y angular correlation of 19F* [7], which was ascribed by the authors to the formation of a V,- or a H-centre.
We assume an Arrhenius behaviour
for the annealing times 7,. The 220 K step can be explained by eq. (1) with activation enthalpies given in table I. The 80 K step, however, is far too broad (eq. (1) with r, % 10-l2 . .. 10- l 4 s would correspond
to a step width of only about 10 K). This leads us to the assumption that we have a distribution of activa- tion enthalpies resulting in a broadened step rather than one well-defined activation enthalpy. This might be explained by different defect configurations, e.g. Frenkel pairs with varying distances. Assuming a Lorentzian distribution for the activation enthalpies and .rO = 10-l3 s, the experimental results are well described (full line in figure 2). The mean activation enthalpies obtained by this method are listed in table I. The full width of this distribution is about 0.1 eV for all three fluorides. A variation of To within
some orders of magnitude does not significantly affect the results.
2.2 TETRAGONAL MgF2 (RUTILE STRUCTURE). -
For MgF2 the above reported method for measuring annealing steps is not applicable, since MgF, is a noncubic crystal with a quadrupolar split 2 0 NMR ~
spectrum [8], in which additional defect induced splittings would have been difficult to separate. For MgF, a different method was applied : At certain B values energy conserving flip-flop processes between the quadrupolar split 'OF and the purely Zeeman split 19F (I = 112) occur (nuclear cross relaxation [9])
and result in a resonance-like decrease of the 'OF polarization (Fig. 3a). Within these dips cross relaxa-
0 100 200
T
/ K Fig. 3. - a) Poiarization of 20F nuclei in MgF, versus B at roomtemperature The d ~ p s are due to cross relaxation processes with
Fig. 2. - Relative number of 'OF nuclel w ~ t h neighbour~ng (n, y ) "F host nuclei The residual asyrnmctry in the labelled dip at
defects as a function of temperature at B = 0.5 T. Steps occur ~f the B = 0.1225 T is a measure of the number of 'OF nuclei without
defect annealing time s,(T) is equal to the p-lifetlme sp = 16 s. nearby defects. b) 'OF polarization at B = 0.1225 T as a function
Two annealing steps are observed corresponding to the anneal~ng of temperature. As in figure 2 the drawn ratio measures the propor-
RADIATION INDUCED POINT DEFECTS IN CaF2, SrF2, BaF2 AND blgF, C6-383
tion depolarizes only 2 0nuclei without neighbouring ~
defects (analogously to the Larmor frequency irradia- I .O tion in the cubic crystals). The temperature depen-
dence of the residual asymmetry in the dip at B = 0.1225 T (indicated by the arrow in figure 3a)
again shows two annealing steps (Fig. 3h). With
2
E 0.5zo = 10-l3 s and assuming the Arrhenius beha- a
\ viour one gets activation enthalpies of 0.037 (7) a
and 0.17 (3) eV for the 13 K and the 60 K step, res- pectively (full line in figure 3b).
2 . 3 FAST DEFECT ANNEALING STUDIED BY THE REO- RIENTATION OF 'OF. - By the NMR measurements
annealing times in the order of r , = 16 s were deter- mined. The following procedure allows to study annealing behaviour in the ps time scale :
Figure 4a shows the P(B) dependence in CaF2 at 10 and 295 K. At low B fields, where the defect interactions are large compared to the Zeeman energy, the polarization of the 'OF nuclei is partly lost due to a coupling of the 2 0 spins to the defect fields. ~ If B
becomes large the defect interactions are decoupled and the maximum polarization P,,,,, is obtained. Cur-
ves as shown in figure 4a are called reorientation curves. If now P is measured not versus B but versus T at sufficiently low B (Fig. 46, c), we expect again a step in the region where the annealing time T , ( T )
is of the order of the precession period of the nuclei in the defect field. Contrary to figures 2 and 36 the curves in figure 4b, c approach 1 at high and not at low temperatures since reorientation in the defect fields is effective only at low temperatures where the defects live sufficiently long to allow a coupling of the nuclei to their fields. Again a broadened step is observed. If one assumes a Lorentzian distribution for the activation enthalpies as described in section 2.1, one gets values that agree with the ones obtained by the NMR measurements (Table I). This means that Table I. - Aclivation enthalpies in eV as obtained from measurements shown in figures 1 and 3 assuming
so
=
10-12 ... 1 0 - l 4 S.F I ~ . 4. -a) Polar~zation P of 'OF in CaFz as a function of B at 10 K
and 295 K. P decreases at low B by coupling of the 'OF nuclei to defect induced crystal fields. This coupllng 1s less effectwe at 295 K
than at 10 K because of faster annealing of the defects. b) and c)
P versus T at low B (0.02 T) for CaF, and SrF,. Annealing steps occur at temperatures where the annealing time T, is of the order of the precession period (- s) of the 'OF nuclear spins about the defect field.
Measured by CaF2 SrF2 BaF2
- .- - this annealing step and the one at about 80 K reflect
NMR (220 K step) 0.59 (6) 0.69 (7) 0.52 (20) annealing of the same defect in different time scales. NMR (80 K step) 0.18 (3) 0.24 (3) 0.18 (3) This work was sponsored by the Bundesministe- Reorientation 0.21 (4) 0.24 (5) rium fur Forschung und Technologie.
DISCUSSION
Question. - W. A. SIBLEY. Reply.
-
W. BUTTLER.W. BUTTLER et al.
'References [I] Crystals with the Fluorite Structure, Ed. W . Hayes (Oxford,
Clarendon Press) 1974.
[2] CATLOW, C. R. A., J . Phys. C . Solid State Physics 12 (1979) 969.
[31 CALL, P. J., HAYES, W., STOTT, J. P. and HUGHES, A. E., J. Phys. C . Solid State Physics 7 (1974) 2417.
[4] NORMAN, C. D. and HALLIBURTON, L. E., Phys. Rev. B 15 (1977) 5883.
[5] ACKERMANN, H., DUBBERS, D. and S T ~ K M A N N , H.-J., Advan- ces in Nuclear Quadrupole Resonance, Vol. 3, Ed. J. A. S. Smith (Heyden, London) 1978, pp. 1-66.
[6] S T ~ C K M A N N , H.-J., DUBBERS, D., GRUPP, M., GRUPP, H., ACKERMANN, H. and HEITJANS, P., 2. Phys. B 30 (1978)
19.
[7] KREISCHE, W . , MAAR, H.-U., NlEDRIG, H., REUTER, K. and ROTH, K., Hyperfine Interactions 4 (1978) 732. [8] STOCKMANN, H.-J., ACKERMANN, H., DUBBERS, D., GRUPP, M.
and HEITJANS, P., Z. Phys. 269 (1974) 47.
