Low temperature - irradiation effect on KC1:Eu(2+) crystals

Solid State Communications, Printed in Great Britain.

LOW

TEMPERATURE

Vol. 71, No. 10, pp. 783-787,

y-IRRADIATION

EFFECT

H. Opyrchak, K.D. Nierzewski Institute

ON

KCl:Eu(Z+)

25 April

1988; in revised

CRYSTALS

and ET.Macalik

of Low Temperature and Structure Research, Polish 50-950 Wroclaw, P.O.Box 937, Poland

(Received

0038-1098/89$3.00+.00 Pergamon Press plc

1989.

form 5 April

Academy of Sciences,

1989 by R. Fieschi)

The formation of color centers at LNT (measured after warming to RT) in r-irradiated KC1 crystals doped with Eu(2+) ions in a wide concentraeion range is studied by optical absorption, emission and the ITC method. No effect of post-irradiation warminq up to RT on Eu(2+)-dipole concentration is observed. The ITC measurements reveal the drastic decrease of concentration of isolated europium dipoles induced by LNT irradiation. Moreover, the relationship between F center and Eu(2+) dipoles is different than that predicted by the theory 181. The results obtained are explained by radiation induced aggregation of Eu(2+) dipoles into small clusters at LNT.

1.

2.

INTROIJUCTION

The coloration of alkali halide crystals induced by ionizing radiation at liquid nitrogen temperature (LNT) was intensively studied in the past 11-51. It was concluded that the secondary thermally activated processes involving primary halogen-interstitial centers determine the Fcenter production 161. In fact, mobile interstitials can recombine with F centers or become trapped at impurities or another lattice defects. Moreover, the interstitials can cluster yielding planar dislocation loops evidenced by the electron microscope observation 171. Cation impurities markedly influence the LNT coloration being responsible for the enhancement of coloration level and the change of F-center growth kinetics 11,2,51. The effect of monovalent as well as divalent dopant cations is qenerally explained in terms of marked contribution of these cations in trapping of mobile interstitials. The quantitative description of F-center growth kinetics based on these models has been recently reported by several authors 18-101. In our previous papers of RT and LNT coloration of KCl:Eu(2+) crystals Ill-131 the alternative model of color center formation based on new experimental facts has been proposed. It has been found that the decrease of impurity dipole concentration is caused by radiation-induced aggregation and not by interstitial trapping proposed by other authors 15,141. Moreover, on the basis of V,-like band behavior it has been shown that the above aggregation products are effectively trapping the primary interstitials being responsible for the increase of F-center concentration. Up to now the radiation-induced aggregation was not taken into account in the low-temperature coloration mechanism of Me(2+) doped alkali halide crystals 16). The present paper reports the results obtained for Eu(Z+) doped KC1 irradiated by r-rays at LNT.

EXPERIMENTAL

DETAILS

KC1 crystals doped with Eu(2+) were grown from prepurified material by the Bridgman method ( 14 1. The contents of the dopant ions in samples examined varied between 3 and 270 mole ppm as estimated from the absorption coefficients of euro ium-related pertinent high-energy band according to P81. Before irradiation, all samples were annealed at 773K for lh and quenched to RT on a copper plate. The effectiveness of quenchins procedure was estimated by comparing’ the concentration of Eu(2+)-cation vacancv diooles (c;,,) calculated from -the area under the ITC p& and total Eu(2+) concentration (c,,) obtained from the optical absorption measurements. As shown in Table I Eu(2+) concentration determined by both methods is practically the same for the samples with low Eu(2+) contents. Concentration

TABLEI of dopant in quenched samples

Ceu(ppm)

3.5

43

110

170

260

civ(ppm)

3.0

42

86

110

97

At higher dopant concentration (170,260ppm) Civ values are distinctly lower from the corresponding ceu values, indicating that this quenching was in that case not effective enough to prevent partial aggregation of the I-V dipoles even during such a rapid cooling. The quenched samples closed in polivinyl containers filled with dry helium gas were immersed in a special double dewar with the liquid nitrogen and irradiated by r-rays from Co60 (dose rate - lMR/h). After irradiation the containers were warmed up to RT in the dark. The absorption spectra were taken at RT in a M40 Specord spectrophotometer (spectral resolution 703

784

IRRADIATION

EFFECT

better than 5 cm-l at 20 000 cm-l). Additional low temperature (15K) measurements of absorption and emission spectra of the samples warmed to RT were performed in a cti-Spectrim microcooler. ITC measurements of the same samples were performed in the arrangement described in 1161 using polarazing voltage 1.5kV at 243K. In some cases the EPR spectra of samples irradiated at LNT were measured at the same temperature (without heating to RT) by using the conventional X-band reflection spectrometer. RESULTS

3. 3.1

AND

The LNT irradiation concentration

DISCUSSION

effect

on I-V

dipoles

Ther-irradiation induced changes of the I-V dipoles concentration were monitored by ITC technique requirinq heating of the irradiated at LNT samples up to RT. The-effect of warming has been checked for two different amounts of Eu(2+) ions (43 and 260 ppm) and two radiation doses by EPR measurements performed at LNT after irradiation (without heating up to RT) and after postirradiation heating up to RT. The comparison of the data obtained showed that position and intensities of Eu(2+)-dipole related lines did not change due to that post-irradiation treatment. that the changes of Hence, one should conclude Eu(2+)-dipoles measured by the ITC method are induced byg-irradiation at LNT only. In samples with lower Eu(2+) contents in which impurity ions are in the form of isolated dipoles (see Table I) the dipoles concentration rapidly decreases from the very beginning of irradiation reaching after - 20h a constant values 1A). In heavily doped samples (170 and (Fig. 260ppm) in which a fraction of the dopant ions is present in the form of some aggregates, the dipoles concentration initially increases reaching a maximum and then decreases down to a constant value (Fiq.18). One should expect that the increase (recovery of isolated Eu(2+)dipoles is due to ?T-rav induced dispersion of Eu(2+) aggregates present in these samples. The radiation-induced decrease of Eu(2+) dipoles can be interpreted in terms of three different prochange of Eu(2+) ions, the incesses : ;alency terstitial traooina bv Eu-diooles or the radiation-induced aggregation. As’will be shown later there are no spectroscopic evidences for valency change as well as for interstitial trapping by isolated dipoles. For this reason the decay of Eu(2+) dipoles is proposed to be interpreted in terms of radiation-induced aggregation process. On assuming the dimers or trimers to be the aggregation products, the usual procedure based on chemical reaction rate theory 117,181 was used to analyse the experimental data and to determine the type of aggregation kinetics. The best fitting of experimental data has been obtained for Unger-Perlman model 1181 assuming second order reaction (dimer formation) in which the back reaction was taken into account (cp. Fig.2).

ON KCl:Eu(Z+)

center

In order

Vol. 71, No. 10

treatment the concentration of the color centers left in samples was by about 30% lower than that originally reached at LNT (e.g. in a sample containing 80ppm Eu(2+) and irradiated at LNT during 1.5h, the measurements at LNT revealed 24ppm of F centers and after warming to RT 16.5ppm of F centers). As different from the F-center growth curves characteristic of RT irradiation 191, no F-center saturation was observed in samples irradiated at LNT (Fig.3) by similar T-ray doses. in samples with similar Eu(2+) conMoreover, tents F-center concentration reached after prolonged irradiation at LNT was higher than that reached after the same time of irradiation at RT. In spite of the thermal bleaching of color centers during warming up to RT, all F-center growth curves (see Fig.3) follow the same kinetic equation (characteristic of irradiation at LNT) do= AlI1 - exp(-alt)( + a2t, where al and a2 are the rate constants and A1 is the saturation value characteristic of the initial coloration stage 13,41. The solid lines in Fig.3 were

above “f”’

3.2 Color

CRYSTALS

L!ti

C,,lOI 1’

1

I

formation

to compare the results obtained by the optical measurements were the ITC method, performed also for the samples warmed to RT after previous irradiation at LNT. Owing to this

1.

$-(LNT)-induced changes of dipole The concentration in KC1 crystals with different Eu(2+) content: A0 -43ppm, B-X -170ppm, + - 260ppm. l -lloppm;

Vol. 71, No. 10

IRRADIATION

EFFECT

ON KCl:Eu(Z+)

4.

(t)-b/(ci It)-a)\ Plot of lnj(c. a-equr Y rbrium irradiation t!ze; concentration, b=aciv(0)/a-civ(0) 0 -43ppm, l -1lOppm.

2.

d;

crn~:!

1

vs. dipole ;

,+-

3.

lrradlatlon

time

Lhl

spectrum (taken (9Oh) irradiated contribution of -band and V2-

Eu(Z+)

absorption

and

emission

spectra

In Fig.6 the absorption and emission spectra of Eu(2+) in a sample irradiated at LNT during 40h and in a sample freshly quenched from higher In the absorption are compared. temperature, spectrum of irradiated sample the VP band (cf. Fig.4) is substracted. As shown in the Fig.6 the changes induced by the irradiation in Eu(2+) absorption and emission are rather small. No narrow lines characteristic to Eu(3+) ions have been found in the absorption and emission spectra

F-center growth curves for pure and europium doped KC1 crystals i irradiated at LNT. Points-experimenFull lines-computer tal values. calculated according to the relation X ,=A (1-exp(-a. t))+a2t; 0 -undoped . !3.5ppm, 0 -J3ppm, 0 -llOpom, + -260ppm.

calculated by assuming al=1.5x10-5 s-1 for the s-1 for the doped samples pure KCl, al=ZxlO-5 for all samples. It should and a2=2xlO- 4 cm-1s1 be noticed that similar parameters have been found for the F-center growth curves obtained for the samples both irradiated and measured at a LNT 131. Al value has been found to follow simple relation Al - (ceu)o.125. LNT irradiation induces essential changes in the UV absorption spectrum of Eu-doped KC1 crystals (Fig.4). In all doped and irradiated samples there is observed significant contribution of an absorption band - further referred to as Vz band - peaking at 225-230 nm. Moreover, this

The UV absorption at RT) of heavily The KC1:Eu sample. high-energy Eu(2+) band is shown.

band grows proportionally to that characteristic of F centers, as shown in Fig.5. Similar absorption bands and similar relationship have been earlier observed in alkali halide crystals doped with alkaline earth ions and irradiated at RT (191. It should be also noticed quite different behavior of Eu(Z+)-doped KC1 irradiated at RT. In that case, particularly in samples containing higher amounts of the dopant, Vz band has been hardly observed (111. 3.3.

60

785

CRYSTALS

OCF [cm-‘1

I

200-

5.

q -unhoped, n -3.5ppm, + -260ppm. IlOppm,

0 -43ppm,a

-

786

IRRADIATION

EFFECT ON KCl:Eu(2+)

CRYSTALS

Vol. 71, No. 10

CF

Ippml

20

7.

/ / LlO

6.

L20

The absorption spectra (taken in KCl: l-after of r-irradiation

630

alnml

(A) and emission (B) at 15K) of Eu(Z+) ions quenching, Z-after 40h at LNT.

taken in wide spectral range of 200 to 800 nm at spectroscopic investigation 15K. The earlier showed that the emission of thermally induced small aggregates was similar to that of isolated dipoles 120,211. Therefore, the Eu(Z+) related optical spectra found in investigated sample supports the conclusion of the kinetic studies (cp. Sec.3.1) that LNT irradiation brings about the formation of some small aggregates composed of a few original dipoles. 3.4 Discussion

and conclusion

The comparison of the results obtained by the ITC method and by the optical absorption measurements does not reveal an unique relationship between the number of F centers formed (CF) and the number of destroyed (lost) Eu(2+) dipoles ( n civ). For the samples in which after quenching civ was equal to ceu (total Eu(2+) concentration) there is observed in the initial stage a linear relationship between CF and nciv (Fig.7), according to which in 43ppm sample 0,; and in 1lOppm sample 0.14 F centers F centers. (as well as equivalentnumbers of interstitials) per one lost Eu(Z+) dipole are formed. In samples for which the quenching was not effective enough to keep all europium dipoles in the isolated form (Civfceu), radiation-induced initial recovery of the isolated dipoles (see Fig.16) renders Moreover the impossible comparison. similar experiments in which the LNT irradiated samples

LO

AC<,[ppml

The relation between the concentration of F centers Cc,) and europium dipoles destroyed ( d c. ) during the LNT irradiation; o-43~$n,.110ppm.

were optically bleached at RT show that the color centers creation and the decay of europium dipoles are caused by two different processes. The illumination of the colored sample by the Fband light brings about a significant drop under the F band and Vz band. In spite of color centers bleaching no recovery of isolated Eu(2+) dipoles was observed by ITC and EPRmeasurements, proving that they are not involved in the formation of V,-type centers.According to the above consideration, it is to conclude that, on contrary to previous suggestions 1101, the europium dipoles do not participate in the coloring process by interstitial trapping. Taking into account the results of absorption, emission and ITC measurements, the decrease of dipoles is proposed to be consequence of the association into small aggregates of few original dipoles in agreement with the result of kinetic analysis (cp. Sec. 3.1). Since the simple dipoles do not act as traps for interstitials, cation vacancies or europium aggregates would be expected to play this role. The problem of interstitial trapping is closely related with the formation of Vz centers present in KCl:Eu crvstals irradiated at LNT and virtually absent in similar samples irradiated at RT Ill\. According to 1221 in the crystals of the same origin large quantities of over-equilibrium vacancy pairs are present (e.g. 1019 per ccm in a 175ppm Eu(2+) crystal). In agreement with 1191 in alkaline earth doped crystals these over-equilibrium vacancies 1231 are responsible for the formation of Vz-type centers (enhanced formation of V2 centers is also observed in pure crystals quenched from higher temperatures or plastically deformed (191). The differences in the absorption bands in the UV range found for the crystals irradiated at RT or LNT can be explained by considering the trapping cross-section relations of the over-equilibrium vacancies and Eu(Z+) dipole aggregates. At room temperature the trapping cross-section for the interstitial centers are significantly larger in the case of the large Eu(2+) dipole aggregation products than in the case of the vacancies. Hence, at hi her concentration of Eu(2+) no VP band appears 9 111. In crystals irradiated at LNT the europium ion aggregates consist of a few dipoles (presumably of two dipoles) displaying very low trapping cross-section for the interstitials compared to that of the vacancies.

Vol. 71, No. 10

IRRADIATION EFFECTON I(Cl:Eu(2+) CRYSTALS

AcknowledgementsThe authorswish to express their thanks to Mrs. T. Morawska-Kowal for the growthof the crystals.They are indebtto Prof. J.Z. Dan for numerousdiscussionsand valuable commentsconcerningthis paper.Thanksare also

787

due to Dr. M. Czapelskifor providingthe computer program for calculatingF-center growth curves parameters,and to Mrs. 8. DobrowolskaNierzewskafor her care in makingthe figures.

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