Defect formation of LiH at low temperature

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Defect formation of LiH at low temperature

M. Ikeya, T. Miki

To cite this version:

M. Ikeya, T. Miki. Defect formation of LiH at low temperature. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-312-C6-314. �10.1051/jphyscol:1980679�. �jpa-00220117�

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-312

Defect formation of LiH at low temperature

M. Ikeya (*) and T. Miki

II. Physikalisches Institut der Universitat Stuttgart, 7000 Sluttgart-Vaihingen Pfaflenwaldring 57 and Technical Coolcge, Yamaguchi University, Ube, Yamaguchi 755, Japan

Résumé. — La formation de défauts dans LiH à basse température a été étudiée par des mesures de RPE, d'ab- sorption optique de thermoluminescence (TL), de courant thermostimulé (TSC). Les larges signaux de RPE, qui sont attribués à des paires de défauts de Frenkel proches dans une interaction d'échange, disparaissent en s'accompagnant de TL mais pas de TSC. L'interprétation des résultats conduit à la recombinaison d'atome inter- stitiel d'hydrogène avec un centre F proche suivant la TL caractéristique ou à leur diffusion pour former des cavités d'hydrogène.

Abstract. — Defect formation of LiH at low temperature has been studied with the measurements of ESR, optical absorption, thermoluminescence (TL), thermally stimulated current. The broad ESR signals attributed to closed pairs of Frenkel defects in an exchange interaction disappear accompaning TL but not TSC. The results are inter- preted that interstitial atom of hydrogen recombine with nearby F center following the characteristic TL or dif- fuse to form hydrogen voids.

1. Introduction. — LiH is the simplest ionic crystal with NaCl type crystal structure consisting of Li+

and H^- with Is² electron configuration. A series of works has been made at Los Alamos Laboratory in early 1960. Some point defects analogous to those in alkali halides have been identified [1, 2]. The relaxed exciton similar to those in alkali halides, (H2 + e) type exciton seems to be probable from the luminescence properties of LiH [3]. However, no identifi- cation of HJ molecular ion, Vk center or H center has been made. The formation of di-interstitial, H2

molecule was anticipated from the growth of the ESR intensity of F centers in UV-irradiated LiH. Hydro- gen void formation was also reported from pulsed NMR studies. In this work we present the result of ESR study of LiH X-irradiated at 4 K and its thermal annealing together with the results of thermoluminescence (TL) and thermally stimulated current

(TSC) studies. A broad ESR spectrum attributed to the triplet splitting of F-H closed pairs has been detected by the X-irradiation at 4 K in contrast to F center formation by UV-irradiation.

2. Results and discussion. — Figure 1 shows the broad ESR derivate lines of LiH irradiated by X- rays at 4 K for the direction of the magnetic fields along (100) and (110). No clear signal for F centers is detected. The signals with the width of about 500 G were observed around 1 700 and 3 200 G in addition to the one at 3 100 G with the width of about 100 G.

Fig. 1 — ESR spectra of LiH X-rayed at 4 K for the direction of the magnetic field along (100) and (110). The UV-irradiation produces F centers. Position of the signal associated with F centers is indicated by the arrow.

The broad signal observed around zero field region for the magnetic field along (100) has angular depen- dence and was not detected for the field along (110).

It is interesting to note that UV-irradiation produces mainly the signal of F centers at the position indicated by an arrow with F and not the broad signal. In some crystal, ESR signal of Li colloids whose position is indicated by C closed to F center position was ori- ginally present and overlap the newly formed broad lines. The broad signal disappears by raising the temperature above 20 K following the thermoluminescence.

Figure 2 shows the thermoluminescence (TL) and thermally stimulated current of LiH X-irradiated at 10 K. The long lasting phosphorescence was observed after the termination of X-ray. The emission spectrum (*) Research Fellow of Alexander von Humbolt Foundation

BRD, at II. Physikalisches Institut der Universitat Stuttgart.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980679

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DEFECT FORMATION OF LiH AT LOW TEMPERATURE C6-3 13

Fig. 2. - Thermolumlnesccnce (TL) and thermally stimulated current (TSC) of X-rayed LiH at low temperature. The absence of the current at low temperature IS attributed to the migration of a neutral defect, H" to an F center givlng TL at low temperature.

UV-irradiation produce T L peaks and the enhancement of TSC around 90 K.

is roughly same as that of the exciton luminescence tentatively ascribed to the singlet and triplet transi- tion [3]. The T L curve has a peculiar shape of the linear rise and an exponential fall-off in contrast with the ordinary T L curve of the exponential rise and abrupt fall-off. The TSC was not observed below 90 K during the TL. Therefore, we consider that the mobile defect responsible for the T L at low temperature in electrically neutral in charge or the lurnines- cence is a tunnelling recombination. The TL and TSC above 100 K will be treated separately with the formation and destruction studies of F aggregate and hydrogen voids.

The observed ESR is too broad to make a clear analysis. The signals at 3 200 and 1 700 G with the width of 500 G can not be,attributed to the hyperfine coupling of a proton (500 G ) nor of H; molecular ion (H or V, center). The small lattice separation of Li-H- (2.04 A) aqd the extended wavefunction of nearby H - ions might broaden the spectrum of H;

center o r Ho center. However, the separation of 1 500 G of the broad signal can not be explained. The presence of the angular dependent broad line below 1 kG is difficult to explain by the hyperfine splitting by protons. The result that no F center signal is detected by X-irradiation in contrast with UV- irradiation leads us to a model of zero field triplet splitting ( S = 1 ) of closed pairs of Frenkel defects.

The spin Hamiltonian of a closed pair may be written as

where I and 2 in the suffix indicate the paramagnetic F and presumably H type interstitial center. The ferro- magnetic exchange coupling is observed in electron and hole in semiconductor by Feher and radical pairs in organic crystal irradiated at low tempera-

ture. The exchange coupling is roughly deduced to a zero field splitting with DS: for same g values. The direction between them might be distributed. These results in the magnitude and axis distribution or the zero field splitting parameter D,j and contribute to the broadening of the spectrum. If the interstitial is H; molecular ion type, several angles between F-H axis and the-molecular axis would further enhance the broadening due to site symmetries. In our case, the F center signal is inhomogeneously broadened with nearby protons. The hyperfine and superhyper- fine splitting of the interstitial center would be thus masked by the broadening. The present width of 500 G is sufficient to mask the hyperfine splitting due to the protons. The extension of the wavefunction and the considerable overlap in LiH shown in the previous LCAO calculation of the clusters of L H would also broaden the spectrum beyond the accurate analysis. The average D value is

-

1 200 G and the axis is closed to (100).

If the local concentration of defects is high the interstitial would form di-interstitial, H2 molecule.

The F centers are thus formed by low temperature UV-irradiation. However the unform irradiation by X-rays at 4 K would predominantly produce closed pairs. The separation of F and H Frenkel pair occurs viu excited states of the H center in alkali halides because of the small repulsion energy along (1 10) direction for the excited state [7]. The absence of such state in H; molecular ion might hinder the crowdion type motion and favor the closed pairs in LiH at low temperature.

The T1 curve can be analyzed by the first order recombination of F-H pairs and the second order kinetics of the thermal separation of the closed pairs.

The result that no current was observed supports that the migration of the interstitial atom of hydrogen, whatever the form, is responsible for TI. In fact, we have observed the formation of ESR signal of M n + + under a large axial field in some crystals irradiated at 4 K by the thermal annealing. It is considered that impurity association with the hydrogen atom occurs similarly as the association of Mn" and radiation- induced defect was detected in NaCI. The background absorption is also enhanced by annealing presumably due to the light scattering by the formation of hydrogen voids [8]. These results support that the mobile species are hydrogen atoms. Thus, the broad ESR spectrum responsible for the T L will be associated with closed pairs of Frenkel defect in LiH.

Acknowledgment. - ESR studies at 4 K were made at University of Stuttgart by one of the authors (MI) during the stay as a research fellow of Alexander von Humbolt Foundation. He wishes thank to Prof. H.

Pick and Dr. L. Schwan for their help during his stay at the Institute.

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M. IKEYA AND T. MIKI

DISCUSSION Question. - D. Y . SMITH.

Have you observed a

fl

band associated with the F band in this system ? This would be good evidence for F centers. An even better test would be the obser- vation of a negative spin-orbit splitting in the MCD of the absorption you assign to the F center.

Reply. - M . IKEYA.

No we have not. There is some controversy on the F band assigned by Prezel in visible region. It might be associated with a simple colloid. F band should be located in UV region of LiH with a small lattice separation if the theory in alkali halides still hold.

The measurement of MCD might be fruitful as you suggest.

Reply. ^-M. IKEYA.

The aging of LiH irradiated at room temperature produces the broad absorption background due to the light scattering by voids. The broad absorption is annihilated by the thermal annealing with the destruction of colloids at high temperature. The growth or the precipitation of hydrogen voids proceeds gra- dually at room temperature. Our main interest in this study is to find the evidence of hydrogen motion by tunnelling, defection type defect. But the voids formation at 10 K does not proceed dominantly.

Question. ^-M. GEORGIEV.

Can you produce voids in your crystals by higher- temperaturc annealing following irradiation ?

Reply. ^-M. IKEYA.

Yes. We are studying the formation and destruc- Question. - A. B. LIDIARD. tion of colloids and voids by the thermal annealing Could you please say something of the conditions of the irradiated LiH. Thermoluminescence and (temperature, etc.) under which you observe the for- exoelectron emission measurements are also employed.

mation of voids and colloids in LiH ? We hope to publish the results separately.

References

[I] P R ~ Z E L , F. E. and RVSHIYG, C C . , J. Phys. Chem. Solrd\ [5] S o u t ~ s , P. C., BLAKL, T S , PENPRAZE, R. M. and CLINE, C.,

17 (1961) 232. J . Phys. Chem. Solids 30 (1969) 2649

[2] PRETZEL, F E. and P m , R. L., Phys. Rev. 127 (1962) 777 [6] MIKI, T. and I K ~ Y A , M., Phys. Lett. A 62 (1977) 356 [3] IKEYA, M., Solid Stale Commun 17 (1 975) 1235. [7] ITOH, N

.

^.I.P/ivsi(~ue Colloq. 37 (19-6) C7-27.

[4] ROWMAN, R. C. and LOCKER, D. R., Solid State Commun. [8] MIKI, T. and I K ~ Y A , M., J. Phys. C (1980).

11 (1972) 1489.