Nuclear Instruments and Methods in Physics Research B 326 (2014) 303–306

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

Accumulation features of radiation-induced hydrogen defects in LiF

http://dx.doi.org/10.1016/j.nimb.2013.10.0320168-583X/� 2014 Published by Elsevier B.V.

⇑ Corresponding author. Tel.: +995 558 54 28 36; fax: +995 322 39 14 94.E-mail address: zairaak@yahoo.com (Z.G. Akhvlediani).

Z.G. Akhvlediani ⇑, I.G. AkhvledianiE. Andronikashvili Institute of Physics, Tbilisi, Georgia

a r t i c l e i n f o

Article history:Received 27 June 2013Received in revised form 14 September 2013Accepted 2 October 2013Available online 6 February 2014

Keywords:Neutron irradiationLithium fluorideHydroxyl impurityHydrogen defectsInfrared spectroscopyEPR

a b s t r a c t

Infrared absorption and electron paramagnetic resonance spectra have been investigated for LiF:OH crys-tals irradiated in a nuclear reactor at low and normal reactor temperatures. The influence of irradiationtemperature on accumulation of interstitial hydrogen ions H�i (U1-centers) was studied. It was observedthat these centers were predominantly accumulated under the low-temperature irradiation. The maxi-mum frequency of local vibrations of H�i ions in LiF depended on the irradiation dose. The shift of vibra-tion frequency of U1-centers to low energy with the increase in the irradiation dose was studied andanalyzed. Negative hydrogen ions [H�] in anionic sites (U-centers) were registered in irradiated LiF:OHcrystals without their preliminary thermal treatment after the neutron irradiation both at reactor andlow temperatures. Accumulation of hydrogen atoms (U2-centers) in LiF for different irradiation fluxeswas studied by the thermal treatment. After the irradiation of LiF:OH at 20 K the H0 atoms were observedwithout annealing of the crystal.

� 2014 Published by Elsevier B.V.

1. Introduction

Due to their transparency in a wide spectral region and a simplecrystalline structure, the lithium fluoride crystals serve as modelobjects in solid state physics. The presence of the structural sensi-tive properties caused their wide use in such fields of technology asdosimetry, memories, quantum electronics, etc.

If the crystals grow in the air, they always contain the hydroxylions which are difficult to remove [1,2]. This impurity being in-serted in the anionic sub-lattice causes many processes occurringin the crystal. Under the effect of the rigid radiation hydroxyl radi-olysis takes place and hence, isolated oxygen and hydrogen com-ponents are formed.

The hydrogen impurity in alkali halides creates several types ofdefects depending on the charge state and arrangement. The inves-tigation of these defects in the matrix of irradiated LiF:OH is impor-tant because they, being formed in the result of the radiolysis ofhydroxyl and owing to the presence of Li6 in the host crystal, areexposed to multiple radiation disturbances.

Due to small masses, the hydrogen defects cause the appear-ance of high-frequency local oscillations in transparency regionof crystal [3–5]. In some cases the local oscillators are rather sen-sitive ‘‘probes’’ and by using them we can obtain information notonly on the dynamics of defect lattice, but also on the distributionof impurities in crystal, on formation of complexes, etc.

In alkali fluorides the hydrogen defects are little studied. In thispaper the hydrogen ions and atomic defects were studied in detailsafter neutron irradiation of LiF crystals containing OH� impurityions.

2. Experimental methods

LiF crystals with hydroxyl impurity in a ‘‘free’’ state and incomplexes with bivalent metals [2] were investigated. The sampleswith dimensions of 5�7 � 10 � 30 mm3 were irradiated with ther-mal neutrons at the interval of 3 � 1014–3 � 1017 n/cm2 in a fissionreactor. The irradiation was carried out in a low-temperature loopand in the channels of IRT-2000 reactor (Tbilisi).

The IR transmission spectra were measured at roomtemperature over the range of wave numbers of 400–4000 cm�1

(wavelengths 25–2.5 l, respectively) with double-beam spectro-photometer Specord-75 IR (Carl Zeiss, Iena).

The EPR measurements were performed by means of spectrom-eter RE-1306 (Russia).

The lattice constant of studied samples was measured withdiffractometer DRON-2 (Russia).

3. Results and discussion

3.1. U1-centers

Negative hydrogen ions in interstitials H�i ions (U1-centers)exhibit the characteristic infrared absorption spectra. These

Fig. 2. Influence of irradiation temperature on IR absorption of LiF crystal.

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centers were thoroughly studied by Fritz for several alkali halides[4]. It was established that their IR absorption maxima exceed1.7–1.9 times the frequency of local oscillations of hydrogen ionssubstituted in anionic sites.

The U1-centers in lithium fluoride crystals were observed forthe first time in [6,7]. The typical IR transmission (and absorption)spectra for neutron-irradiated LiF:OH crystal are shown in Fig. 1(aand b) with the peak at the wave number of 1935 cm�1 and withtwo sidebands at the 2550 and 1550 cm�1.

The accumulation of these centers depends both on the amountof OH� ions in origin crystals and on the irradiation conditions.Namely, the low-temperature irradiation is more efficient for for-mation the interstitial hydrogen ions. Indeed, in Fig. 2 it is shownthat the IR absorption band is more intensive after the irradiationof the LiF crystal at 90 K. We think that the reason is the weak col-oration of LiF crystals at low temperatures [8], and hence the ex-cess electrons are consumed for H�i formation.

We investigated LiF crystals in a wide interval of irradiationdoses from 3 � 1014 to 3 � 1017 n/cm2 and observed that, withthe increase in the irradiation dose, the peaks of IR absorption spec-tra of H�i ions shifted significantly to the low energy region (Fig. 3).

Fig. 1. IR transmission (a) and absorption (b) spectra of the LiF crystal irradiatedwith the D = 2 � 1017 n/cm2 dose at reactor temperature, thickness d = 6.94 mm.

Fig. 3. Irradiation dose dependence of the frequency shift of local vibrationsmaxima of U1-centers in LiF crystals.

At the low-temperature irradiation, the maximum frequencyshift (>250 cm�1) and the maximum relative change (0.16%) ofthe lattice constant were observed (Fig. 4).

Such a result could be explained by weakening of the interac-tion between the hydrogen ions and the surroundings caused bythe increase of lattice constant. However, on the other hand, weearlier studied the IR absorption spectra of H�i ions at differentmeasuring temperature (from 300 to 90 K) and observed that theshift of maximum frequency to high energy was only 40 cm�1

[9], while according to [10] the relative change of the lattice con-stant in LiF in this temperature interval is equal to 0.42%. There-fore, the comparison of these data allows us to suppose that theabove-mentioned result for irradiated LiF (Fig. 4) was caused notonly by the change in the lattice constant, but probably there alsowas another reason such as anharmonicity of local vibrations.

3.2. U-centers

The negative hydrogen ion, localized in the anionic site ofalkali-halides is the simplest substitution impurity defect(U-center). Electronic and vibration excitations of such a defectin the matrices of alkali halide compounds were investigatedexperimentally and theoretically [11–14]. But there are much less

Fig. 4. Dependence of the lattice constant and vibration frequency change ofU1-centers on the irradiation dose in LiF crystals.

Fig. 5. IR transmission spectra of irradiated LiF crystals with U-centers(Tirrad.-reactor t-re): (1) D = 2.25 � 1016 n/cm2, thickness d = 0.45 mm;(2) D = 8 � 1015 n/cm2, d = 0.65 mm.

Fig. 6. IR absorption spectrum of the irradiated LiF crystal with U-centers:D = 2.75 � 1015 n/cm2 d = 0.98 mm, Tirrad. = 90 K.

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data on the U-centers for crystals irradiated with neutrons [15,16].In [15] it was concluded that U-centers did not exist in neutron-irradiated LiF:OH crystals immediately after their irradiation andthat they were formed in the process of annealing from the precur-sor suggested to be U1-centers and afterwards from H0-centers.

Our experiments carried out for lithium fluoride crystals withdifferent content of the hydroxyl impurity and irradiated underdifferent conditions showed that U-centers could be observed evenwithout thermal treatment. In Fig. 5 is shown the initial transmis-sion spectrum, verifying the fact that the U-centers in LiF were reg-istered without annealing of the crystal in contrast to thestatement given in [15].

In Fig. 6 the band of the local mode of U-center and its high-frequency sideband are distinctly shown for irradiation tempera-ture of 90 K. The intensity of U-band is less at low-temperatureirradiation than at reactor temperature if the content of OH� ininitial crystals is the same. According to data given in [17,18],radiation heating of lithium fluoride crystals during the processof irradiation in the reactor depended on the thermal neutronflux, sample dimensions and cooling conditions. Taking intoaccount these data, we think that in our case the radiation-induced heating of LiF samples at reactor temperature shouldbe about 200 �C, and probably in our case it stimulates the bestaccumulation of U-centers.

The absorption was measured on thinner samples in the regionof IR absorption of the U-center relative to the reference (non-irra-diated) sample apparently of the same thickness. Such a method ofmeasurement is necessary, because the absorption of the LiF latticebelow 1300 cm�1 increases strongly and therefore the U-band at1013 cm�1 cannot be measured quantitatively in thick samples.

The linear dependence of U-center absorption on the content ofOH� in the initial sample is shown in Fig. 7.

3.3. U2-centers

The last defect, which we are going to consider below is atomichydrogen in interstitials H0

i (U2-center) formed in irradiatedLiF:OH. The most intensive accumulation of atomic hydrogen takesplace at the annealing of powdered irradiated LiF samples. Anneal-ing temperature at which the highest quantity of atomic hydrogenis accumulated depends strongly on the irradiation conditions (thedose and the irradiation temperature, and the differential flux ofneutrons). In this case, the higher is the irradiation dose, the higher

is the temperature of maximum accumulation of H0 atoms (at thegiven flux).

In Fig. 8, it is shown that the irradiation of LiF at the reactortemperature allows revealing the specific character of accumula-tion of hydrogen atoms: at a relatively low differential flux(1.2 � 1012 n/cm2 s) and a total absorbed dose of 1017 n/cm2, theaccumulation curve has two peaks (600 and 650 K), while, at alarger flux (4 � 1012 n/cm2 s) and the same absorbed dose, a singlepeak (550 K) is observed.

Fig. 7. Dependence of the accumulation of U-centers on the amount of OH�

impurity in LiF irradiated at 40 K with the dose of D = 8.7 � 1015 n/cm2.

Fig. 8. Temperature dependence of the accumulation of hydrogen atoms in LiFirradiated at different irradiation fluxes.

306 Z.G. Akhvlediani, I.G. Akhvlediani / Nuclear Instruments and Methods in Physics Research B 326 (2014) 303–306

It should be emphasized that, in the crystals irradiated at 20 K,the EPR spectrum of atomic hydrogen is detected without

preliminary annealing and the quantity of H0 atoms increases untilthe temperature of maximum accumulation.

4. Conclusion

It was established that the maximum of local vibrationsfrequency of interstitial hydrogen H�i ions (U1-centers) inirradiated LiF:OH crystals depended on the irradiation conditions.It shifted to the low energy and the maximum shift of more than250 cm�1 was reached at the low-temperature irradiation. One ofthe reasons of this fact could be the increase of LiF lattice constantas a result of irradiation. The relative increase of the latticeconstant with irradiation of the studied samples was registered.

It was shown that the formation of substitution H� ions (U-cen-ters) in irradiated LiF:OH crystals was possible even without theirthermal treatment. The number of U-centers depended first of allon the content of OH� ions in the origin crystal.

Besides the above-mentioned OH�-decomposition products inLiF, the interstitial hydrogen atoms were formed. Their accumula-tion depended on the irradiation conditions and on the annealingtemperature of the samples. The temperature of maximum accu-mulation of the H0 atoms depended on the irradiation fluxes.

Acknowledgement

Authors thank Dr. G. Gegiadze for measurements of the latticeconstant of irradiated LiF samples.

References

[1] B.P. Aleksandrov, G.A. Andreev, Solid State Phys. (Russ.) 3 (1961) 2835.[2] T.G. Stoebe, J. Phys. Chem. Solids 28 (1967) 1375.[3] G. Schaefer, J. Phys. Chem. Solids 12 (1960) 233.[4] B. Fritz, J. Phys. Chem. Solids 23 (1962) 375.[5] R. Shamu, W. Hartmann, E. Yasaitis, Phys. Rev. 170 (1968) 822.[6] Z.G. Akhvlediani, N.G. Politov, Pis’ma v SHETF 10 (1969) 249.[7] Z.G. Akhvlediani, K.-J. Berg, G. Berg, Cryst. Latt. Def. 8 (1980) 167.[8] F. Luty, Halbleiter Probl. 1 (1961) 238.[9] Z.G. Akhvlediani, A.V. Mlokosevich, in: Electronic and Ionic Process in Solids,

vol. VI. Tbilisi, Metsniereba, 1973, p. 32.[10] Y. Pautamo, Ann. Acad. Sci. Fenn. Ser. A, VI Phys. 129 (1963) 7.[11] H. Dotsch, W. Gebhardt, Ch. Martus, Solid State Commun. 3 (1965) 297.[12] H. Dotsch, Phys. Status Solidi 31 (1969) 649.[13] H.N. Spector, S.S. Mitra, H.N. Schmeising, J. Chem. Phys. 46 (7) (1967) 2676.[14] M.R. Hayns, Phys. Rev. B 5 (1972) 697.[15] T. Kamikawa, Phys. Status Solidi B 68 (1975) 639.[16] Z.G. Akhvlediani, I.G. Akhvlediani, IOP Conf. Ser:Mater. Sci. Eng. 15 (01) (2010)

2079.[17] A.C. Greene, H.O. Hooper, P.J. Bray, J. Phys. Chem. Solids 27 (1966) 163.[18] A. Vitols, D. Kalninsh, A. Lyushina, K. Shvarts, Izvestia Akad. Nauk Latv. 5

(1967) 66.