DIELECTRIC STUDIES OP 60Co-IRRADIATED AND OXYGEN-ANNEALED RARE-EARTH DOPED CALCIUM FLUORIDE

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DIELECTRIC STUDIES OP 60Co-IRRADIATED AND

OXYGEN-ANNEALED RARE-EARTH DOPED

CALCIUM FLUORIDE

J. Fontanella, D. Bair, C. Andeen

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Dkcembre 1976, page C7-273

DIELECTRIC STUDIES

OF'

60C~-IRRADIATED

AND

OXYGEN-ANNEALED RARE-EARTH DOPED CALCIUM FLUORIDE

(")

J. FONTANELLA and D. BAIR

Physics Department, U. S. Naval Academy, Annapolis, Md. 21402, U. S. A. and

C . ANDEEN

Physics Department, Case Western Reserve University, Cleveland, Ohio 44106, U. S. A.

RksumB. - On a cuit dans de I'oxyg&ne sec des cristaux purs de fluorure de calcium dopes

a

l'erbium et a l'europium. De plus, plusieurs Bchantillons ont kt6 irradies par le rayonnement d'une source en 60C0. La constante dielectrique complexe pour ces cristaux a CtB determinee sur l'intervalle de tempkrature 5,5-400 K aux cinq frequences auditives 102, 102,5, 103, 10395 et 104 Hz. La cuisson

a l'oxygkne s'est rev6lCe avoir un effet marque sur le spectre dielectrique de chaque cristal. Les resultats ont kt6 utilises pour discuter l'origine possible des relaxations observees. De plus ce traite- ment cree dans les cristaux un mecanisme de conduction & activation thermique ayant une energie d'activation de 0,78 & 0,87 eV. Plusieurs relaxations causees par rayonnement ont kt6 observees B

basses temperatures dans les Bchantillons. Les energies d'activation sont determinees pour chaque relaxation.

Abstract. - Crystals of pure and europium and erbium doped calcium fluoride have been annealed in dry oxygen. In addition, several samples have been irradiated with radiation from a 60C0 source. The complex dielectric constant for these crystals has been determined over the temperature range 5.5-400 K at the five audio frequencies 102, 102.5,103, 103.5, and 104 Hz. Oxygen annealing is found to have a marked effect on the dielectric spectrum of each crystal. The results are used to discuss possible origins of the observed relaxations. In addition, this treatment introduces a thermally activated conduction mechanism into the crystals with an activation energy of 0.78- 0.87 eV. Several radiation induced relaxations were observed at low temperatures in the samples. Activation energies are determined for each relaxation.

1. Introduction. - The authors have recently carried out dielectric relaxation studies on erbium [I] and europium [2] doped calcium fluoride. In the former system, four strong and one weak relaxations were observed. I n the latter three principal dipolar relaxations were found along with several weak relaxations. In CaF, : Er, one of the relaxations was tentatively assigned to a rare-earth-oxygen complex. A similar peak appears in CaF, : Eu. In both crystals a relaxation is found which is attributed to a rare-earth- nearest neighbour interstitial fluorine complex. In order to help verify the assignments in CaF, : Er and arrive at conclusions for CaF, : Eu, samples were annealed in oxygen since it is known from EPR studies 131 that this treatment enhances oxygen com-

plexes at the expense of the rare-earth-nearest neighbour interstitial fluorine configuration. I n an attempt to gain further insight into the nature of defects in these systems, several samples were also irradiated (*) Work supported by the Naval Academy Research Council.

with radiation from a 60Co source. Some of the results of both sets of experiments are described in the present paper.

2. Experiment. - The samples of erbium and

europium doped calcium fluoride were obtained from Optovac, Inc. and the Harshaw Chemical Co., respectively and were 25.4 mm diameter disks less than 1.5 mm thick. The origin of the pure samples is unspe- cified.

Several of the samples were annealed at 1070 K in a quartz tube in which dry oxygen was flowing. The annealing times are listed in table I. After annealing, the samples were cloudy white and the most heavily treated sample was almost totally opaque. The surfaces were repolished before any measure- ments were performed.

Other samples were irradiated a t room temperature with radiation from a 60Co source then cooled slowly to liquid helium temperatures over a period of about four days with periodic interruptions of about a half-

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C7-274 J. FONTANELLA, D. B AIR AND C. ANDEEN

1 000 Hz Dielectric Constants for Various Samples

of <( Pure )) and Rare-Earth Doped CaF, Oxygen anneal time or Co(60) Crystal dose - - pure CaF2 24 Hr. CaFz : Er 0.3 % 48 Hr. Capz : Eu 1.26 % 96 Hr. Capz : Er 0.03 % 5.8 x 105 R. - 0.1 % 2.6 x 105 R. - 0.3 % 5 x 105 R. - 1.0 % 2.6 x 105 R. CaF2 : Eu 0.55 % 2.6 x 10s R. -

-

5.3 x 105 R. & I 8' Before After - - 6.798 4 6.799 9 6.462 1 6.462 5 6.988 2 6.966 5 6.646 1 6.557 3 6.871 6 6.879 6 6.479 8 6.475 3 6.860 5 6.466 6 6.914 8 6.502 6.987 8 6.645 7.278 9 7.03 1 6.879 8 6.485 5 6.878 3 6.485 3

hour during which time isothermal data was taken. The erbium doped samples appeared green after irradiation and the europium doped samples were grayish. The first measurement in most cases was to determine the real part of the dielectric constant, &', at

1 000 Hz, 300 K, and 1 atmosphere to an accuracy

of about 0.01

%.

This was accomplished using an electrodeless technique, the method of substitution (two-fluid technique) and apparatus described previously by the authors [4]. The results of these mea- surements before (and after oxygen annealing) treatment are listed in table I.

Next, electrodes were evaporated onto the faces of the samples in a standard three-terminal configuration. The capacitance, C, and conductance divided by the frequency, G / u , were determined at temperatures between 5.5 and 400 K in most cases and the five audio frequencies lo2, 102.5, lo3, lo3.', and 104 Hz using apparatus described elsewhere [I]. In the present work, however, this data was converted to dielectric cons- tants in the following manner. First, a set of 300 K capacitance values was obtained (in most cases directly measured) and Ejoo at frequencies other than lo3 Hz was calculated using the value at lo3 Hz determined by the method of substitution and the assumption that the relative change in dielectric constant with frequency is equal to the relative change in capacitance. The real part of the dielectric constant at tern- peratures other than 300 K was then determined from :

where a, is the isobaric linear thermal expansion coef- ficient of the samples. For lack of data on rare-earth

doped material, a, is taken to be equal to that for pure calcium fluoride

151.

At all temperatures, the imagi- nary part of the dielectric constant, E", was then calculated from :

In the present paper, the dipoles are assumed to be described by an Arrhenius equation of the form :

where z is the relaxation time, z, is the reciprocal frequency factor, E is the activation energy, and k is Boltzmann's constant. Making the approximation that the temperature at which the maximum in E" occurs, Tmax, is where the relaxation time is equal to the reciprocal of the applied frequency, E and z0 may be determined from a best fit of the equation :

In (z) = In (7,)

+

E/kTmax (4) where five points are used, one for each frequency. In the present paper, the position of the maximum for each plot was determined graphically and consequently the results for the activation parameters are neither as precise nor accurate as those reported previously by the authors [I, 21.

3. Results and discussion. - 3.1 OXYGEN ANNEALING.

-

a) CaF, : Eu. - The results for E" vs. tempe-

rature for an untreated sample of CaF, : Eu are shown by the solid curve in figure 1. This sample is the 1.26

%

crystal studied previously by the authors [2]. Three principal relaxations are observed. The peaks at about 200 K and 350 K were assigned to nearest neighbour (Type I dipole [6]

R,

relaxation [7]. 0.4 eV activation energy) and next nearest neighbour (Type I1 dipole [6]

R,,

relaxation [8]. 0.7 eV activation energy) interstitial fluorines, respectively, charge compensating for a trivalent rare-earth. The peak at about 275 K is similar to a peak observed in CaF, : Er which has been associated with an oxygen complex. In addition, there are a number of unidentified weaker relaxations at low temperatures. They are not considered in the present part of the experiment.

The results of the oxygen annealing are shown by the dashed portion of the curve in figure 1. A strong new peak is apparent at about 250 K. It is likely that the new peak is due to oxygen. The activation para- meters, E = 0.50 eV and zo = 3 x 10-l4 s, in fact,

are quite close to E = 0.49 eV and 7, = 4 x 10-l4 s

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DIELECTRIC STUDIES OF RARE EARTH DOPED CaFz C7-275

FIG. 1.

-

e" vs. temperature at 1 000 Hz for a 1.26 % sample of CaFz : Eu. The solid curve represents the as-received sample and the dashed curve represents the crystal after an anneal in dry

oxygen at 1 070 K for 96 hours.

The 275 K peak, on the other hand, has not grown, if it exists at all after annealing. While these results do not rule out the possibility that the 275 K peak is due to oxygen, it certainly would make this assignment questionable.

Another important feature of figure 1 is the enhanced high temperature conductivity of the sample. That this is a thermally activated process is seen in figure 2 which is a plot of In (8") vs. 1/T. It is seen that the five

A best fit of the data gave E = 0.87 eV and 8: = 8.8 x 10". These results may be converted to conductivities since the dielectric conductivity is given by :

0 = O E o &I' (6)

where 8, is the permittivity of free space.

Finally, it is noted that the present experiment has not revealed the existence of Type I1 dipoles after annealing. More data will be necessary to determine the status of the high temperature relaxation after oxygen anneal.

b) CaF, : Er. - The dielectric spectrum for an untreated sample of 0.3

%

CaF, : Er is given by the solid curve of figure 3. This is the crystal studied pre- viously by the authors [I]. In addition to 200, 260, and 350 K peaks similar to those observed in unan- nealed CaF, : Eu, two strong low temperature relaxations are also observed. The lowest temperature peak is probably cluster associated since it occurs only for rare-earth concentrations greater than 0.03

%

and it's strength increases monotonically up to the largest concentration studied, 3.0

%.

The 80 K peak (R;I relaxation, [7], 0.15 eV activation energy), on the other hand, varies with concentration similar to the 200 K peak reaching a maximum at about 0.03

%.

Though there have been several suggestions as to the origin of this peak [I, 7, 81, the evidence appears to be inconclu- sive at the present time.

04 - _FIG.

3. - E" VS. temperature at 1000 Hz for a 0.3 % sample

I I I of CaF2 : Er. The solid curve represents the as-received sample

3 0 3 1 3 2 3.3 _{and the dashed curve represents the crystal after an anneal in dry}

T - I ( K-1) oxygen at 1070 K for 48 hours. The low temperature dotted

line represents a 0.3 % sample after a dose of 5 x 105 R. FIG. 2. - Semi log plot of E" vs. 1/T at 1 000 Hz for the 1.26 % from a 60C0 source.

sample of CaF2 : Eu after oxygen anneal.

The effect of oxygen annealing on this crystal is highest temperature datum points (which are outside given by the dashed portion of figure 3. The effects the range of figure 1) can be quite well represented by are not quite so pronounced as for CaF, Eu which is

an equation of the form : consistent with the fact that the sample of CaF, :Er

was annealed only half as long as the sample of CaF, :

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C7-276 J. FONTANELLA, D. BAIR AND C . ANDEEN the peak at about 260 K grows rather than disappears.

The new activation energy is 0.54 eV and the reciprocal frequency factor is 4.1 x 10-15 s which agree rather well with the values 0.534 eV and 8.1 x lO-I5 s reported previously by the authors [I] for this peak. (To repeat, the activation parameters determined in the present work are not as accurate as those determined previously. In particular, only a rough estimate of the effect of the high temperature conductivity on the position of the peak maxima was made.) These results, then, help confirm the previous assignment [I] of the 260 K peak to an oxygen complex. Consequently, this peak appears to have a different origin than the similar peak observed in CaF, : Eu. This is consistent w i ~ h preliminary high pressure work by the authors in which they have found significantly different activation volumes for these two complexes.

Another interesting feature of the spectrum is that the strength of the lowest temperature peak has been cut in half and the strength of the 80 Kpeak has increas- ed slightly. The former effect may be more evidence for assigning the lowest temperature peak to clusters since it is reasonable that clusters might be rather easily disturbed by oxygen annealing. One possibility suggested by the increase of the 80 K peak is that it may somehow be associated with oxygen. This possibility should be considered in the light of the several other explanations of this peak which already exist in the literature [I, 7, 81. The important feature of these results, however, is that the two low temperature peaks can be varied independently. This is different from the

FIG. 4. - 8'' vs. temperature at 1000 Hz for a pure sample of CaFz of unspecified origin. The solid curve represents the as-received sample and the dashed curve represents the crystal

after an anneal in dry oxygen at 1 070 K for 24 hours.

the peak at 200 K decreased and the high temperature conductivity was enhanced. In this case an Arrhenius plot of E" for the five highest temperatures was quite linear (similar to figure 2) and gave activation parameters of E = 0.84 eV and 8: = 3.9 x loi5. The

activation energy is quite close to that for CaF, : Eu probably implying that the conduction mechanisms are the same. No oxygen peak was noted which is not surprising since this sample was annealed for only a very short time, the nature of the trace impurity is unknown, and there is very little impurity in the first place.

results of Edgar and welsh for similar peaks in CaF, :

Gd. They found that the peaks disappeared together 3 . 2 EFFECTS. a) CaF2 : Eu. - in a vacuum anneal and quench. certainly further essentially identical 0.55

%

samples of CaF, : Eu were annealing studies of the low temperature peaks will be exposed to radiation from a 6 0 ~ o source for different

of interest. periods of time. No strong effects were noted and the

*nother interesting feature of figure is that once full dielectric spectrum remained essentially the same again the high temperature conductivity is enhanced. as the solid portion of figure 1. There was, however,

A plot for CaF, : Er similar to figure 2 showed that a a small effect in the low temperature region. That solely thermally activated region was being approach- data is plotted in figure where the solid curve repre- ed only at the highest temperatures (360 K for the

oxygen annealed samples). A fit of the three highest

0015 -

temperature points yielded E = 0.78 eV and E: = 4.8 x 1015. The activation energy is not far

0012

from that for the corresponding process in CaF, : Eu. The value should be closer yet once higher temperature

0009-

data is obtained.

c) Pure CaF,. - The dielectric spectrum for a sample of pure calcium fluoride is shown by the solid curve of figure 4. These results are typical of all of the pure calcium fluoride purchased by the authors over the past ten years. It is apparent that a trivalent trace impurity is present since the spectrum is characteris- tic of rare-earths in calcium fluoride. On the basis of the undistorted point ion model, the strength of the relaxation corresponds to a concentration of a little more than 4 ppm. Annealing this sample in oxygen resulted in the dashed portion of figure 4. Once again,

FIG. 5.

-

LOW temperature plot of e" vs. T at 1000 Hz for a 0.55 d/d sample of CaFz : Eu. The solid curve represents the as-received sample. The lower and upper dashed curves represent the sample after doses of 2.6 x 105 and 5.3 x 1 0 5 R.

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DIELECTRIC STUDIES OF RARE EARTH DOPED CaF2 0-277 senis the samples before irradiation. The plethora of

peaks is interesting in itself. One possible explanation, of course, is trace impurities. However, the authors have recently observed the same effect in samples of CaF, : Eu obtained from a different source and in CaF, : Pr. Consequently, these peaks may be of a more fundamental nature.

The upper dashed curve of figure 5 represents the most heavily irradiated sample and the lower is the sample irradiated for a shorter time. It is evident that the radiation has induced a peak at about 100 K.

This peak is found to have an activation energy of about 0.16 eV and z0 = 4.5 x 10-13 s. One of the samples was studied

a

year later and no traces of the radiation induced peak were found.

b) CaF, : Er. - Four samples of CaF, : Er were also exposed to the 6 0 ~ o source. Stronger effects were noted for these samples. The most interesting were the results for a 0.03

%

sample which is the one studied previously by the authors [I]. The results of the measu- rements are shown in figure 6 where the solid and dash-

FIG. 6. - E" VS. temperature at 1 000 Hz for a 0.03 sample of CaFz : Er. The solid curve represents the as-received sample and the dashed line represents the sample after a dose of

5.8 x 105 R. from a 60C0 source.

ed curves once again represent the before and after respectively. It is seen that the cluster associated relaxation is decreased and at least four new relaxations are induced. The two low temperature peaks have activation energies of about 0.017 eV and 0.028 eV and

reciprocal frequency factors of 2 x and

5 x 10-l3 s. The lower of the induced double peak has E = 0.086 eV and z0 = 8.7 x 10-14 s. The shoul- der was not fitted, however, as seen by the low temperature dotted line of figure 3, this peak grows at the expense of the lower peak as concentration increases. The analogous peak in the 0.3

%

sample was then ana- lyzed and activation parameters of E = 0.097 eV and z0 = 4 x 10-l3 s were found. In the 0.1

%,

0.3

%,

and 1.0

%

samples, however, the two lowest temperature relaxations were not observed. Only the

double peak was found. In addition, the strength of the double peak in the 0.1

%

sample was only slightly smaller than the 0.3

%

sample, but those in the 1.0

%

sample were less than half as large.

One possible explanation for these results is radiation disturbed clusters. Perhaps vacancies are intro- duced which act as channels through which ions may relax. This could explain the low activation energies. The absence of certain peaks at higher concentrations could be explained if the recombination time for those defects decreases rapidly as concentration increases. 4. Conclusions.

-

Oxygen annealing has been found to have a marked effect on the dielectric spectrum of CaF, : Eu and CaF, : Er. In both cases a relaxation with an activation energy of about 0.5 eV has grown at the expense of a 0.4 eV relaxation. These results help confirm the assignment of the former relaxation to an oxygen complex and the latter to the reorientation of a nearest neighbour interstitial fluorine. In CaF, : Er two low temperature peaks are found to vary independently, a 0.028 eV cluster associated relaxation decreasing with oxygen anneal and a 0.15 eV unidentified relaxation increasing. In addition, this treatment introduces a thermally activated conduction mechanism into both CaF, : Er and CaF, : Eu and a sample of pure CaF, with an activation energy of 0.78-0.87 eV.

Radiation from a 60Co source is also found to affect the dielectric spectrum of the rare-earth doped samples. In CaF, : Eu a peak is induced with an activation energy of 0.16 eV and a reciprocal frequency factor of 4.5 x 10-l3 s. Four peaks are induced in a 0.03

%

sample of CaF, : Er. Two low temperature peaks are found with activation energies of 0.017 and 0.028 eV

and reciprocal frequency factors 2 and 5

x

10-l3 s. In addition, a double peak is found with E = 0.086 and 0.097 eV and z0 = 0.87 and 4 x 10-13 s. Only the double peak is observed in higher concentration samples. For CaF, : Er, it is suggested that the low temperature peaks may be due to radiation disturbed clusters.

Acknowledgments.

-

The authors would like to thank Donald Schuele and Richard Hoffman of Case Western Reserve University and J. R. Miller 111 of the U. S. Army Metrology and Calibration Center, Redstone Arsenal, Alabama for their encourage- ment and support throughout the duration of the work. They would also like to express their gratitude to Earle C. Gregg of CWRU for arranging for them to use the 60Co source. In addition, they would like to express their appreciation to Fred Wasem of the U. S. Naval Academy for his invaluable administrative and technical assistance. Finally, they are indebted to Donald Treacy and Richard L. Johnson of USNA for daily discussions and support during the final stages of preparation of the manuscript.

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C7-278 J. FONTANELLA, D. BAIR AND C. ANDEEN

References

[l] FONTANELLA, J. and ANDEEN, C., J. Phys. C : Solid State [6] KITTS, E. L., IKEYA, M. and CRAWFORD, J. H., Phys. Rev. B

Phys. 9 (1976) 1055. 8 (1973) 5840.

[2] ANDEEN, C. and FONTANELLA, J., J. Phys. & Chem. Solids, _{[7] FRANKLIN,}_A. _{D., CRISSMAN,}_J._M._and_YOUNG,_K._F.,

to be published. _{J. Phys.}_C_:_{Solid State Phys.}₈₍₁₉₇₅₎_1244.

[3] TWIDELL, J. W., J. Phys. & Chem. Solids 31 (1970) 299.

[4] ANDEEN, C., FONTANELLA, J. and SCHUELE, D., Rev. Sci. 18] EDGAR* A. and WELSH, K.9 J. Phys. : "lid State

Instrum. 41 (1970) 1573 : Phvs. Rev. B 2 (1970) . , 5068 : Phys. 8 (1975) L336.

J . Appl. phyS. 42 (1971) 2216." [9] KITTS, E. L. and CRAWFORD, J. H., Phys. Rev. Lett. 30

[5] BAILEY, A. C. and YATES, B., Proc. Phys. Soc. 91 (1967) 390. (1973) 443.

DISCUSSION F. BBNIERE. - From thermodynamics, one can

calculate the solubility of oxygen gas in CaF,. Also from thermodynamics, one can calculate the proba- bility of substituting 0 for F (which must be very low). The peak in the relaxation curve can show the pre- sence of oxygen dipoles or oxygen-vacancy pairs but is not a definite proof that anion vacancies have been formed.

J. FONTANELLA. - We infer that by very low Pr. BCniere means essentially zero. Though we do not know the details of the thermodynamic calculations, we wonder about this result particularly because of the complexities of the solid state. We realized that the dipolar relaxation phenomena which we displayed were not evidence for anion vacancies. Our evidence was the enhanced high temperature conductivity which the crystals exhibited after oxygen annealing. This evidence is, of course, not definite proof. In fact, our conductivity data are over a small temperature range and very low temperatures at that. Viewed in the context of the data presented by Levitskij et al., Chadwick, or Jacobs presented at this confertnce, what we see is probably not due to vacancy migra- tion. Hopefully, our data are of interest as the jirst (in temp.) thermally activated conduction process which occurs in oxygen doped CaF,.

J. H. CRAWFORD. - HOW persistent were the effects of irradiation on the dielectric relaxation spectrum ?

J. FONTANELLA. - We have no quantitative data

on this effect, however, the following comments apply :

1) The crystals sat at room temperature for about a day after the irradiations were performed before the actual cooling procedure was begun. The cooling pro-

cedure was then begun. The double peaks were observed on the third day after cooling was begun and the rest on the fourth. The double peaks were checked on warming up from liquid He temperatures on the fifth day and had not changed at all.

2) The CaF, : Eu samples were checked 1 year later and the effects had disappeared.

L. SLIFKIN. - Your value of 0.84-0.87 eV for the migration activation energy seems somewhat higher than that commonly believed.

D. L. JONES, J. FONTANELLA, C. ANDEEN.

-

Our conductivity data are over a small temperature range and very low temperatures at that. Viewed in the context of the data presented by Levitskji et al., Chadwick, and Jacobs at this conference, what we see is probably not due to vacancy migration. Hopefully, our conductivity data are of interest as the first (in temp.) thermally activated conduction process in oxygen doped CaF,.

P. W. M. JACOBS. - A migration energy of 0.87 eV seems high for the anion vacancy in calcium fluoride. Our conductivity data yield a value in the range 0.38-0.47 eV and this would seem to indicate a consi- derable degree of association and a binding energy about 0.8 eV.