Yu Sugak, D Sugak, Ya Zhydachevjjkii - Peculiarities of coloration processes in linbo3fe crystals under high temperature reducingoxidizing annealing - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія фіз. 2009. Bun. 43. С. 172-178

VISNYKLVIV UNIV. Ser. Physics. 2009. Is. 43. P. 172-178

PACS number(s): 61.72.-y; 61.72.Ji; 78.40.-q;

PECULIARITIES OF COLORATION PROCESSES IN LiNbO3:Fe CRYSTALS UNDER HIGH TEMPERATURE

REDUCING/OXIDIZING ANNEALING

Yu. Sugak1, D. Sugak1'2, Ya. Zhydachevjjkii1, O. Buryy1, I. Solskii2, S. Ubizskii1, K.-D. Becker3, A. Borger3

1 Lviv Polytechnic National University 12 Bandera, Lviv 79013, Ukraine 2 Institute of Materials, Scientific Research Company "Carat" 202 Stryjska, Lviv 79031, Ukraine 3 Institute of Physical and Theoretical Chemistry, Technical University Braunschweig D-38106 Braunschweig, Germany e-mail: crystal@polynet.com.ua

The work presents experimental results of an in-situ investigation of optical absorption of LiNbO3:Fe during reducing (95%Ar+5%H2) and oxidizing (O2) high-temperature treatments in the temperature range from 20 to 800oC. The absorption spectra measured in-situ at high temperatures in reducing/oxidizing atmospheres as well as the kinetics recorded at fixed wavelength during rapid replacement of gas atmospheres have been analyzed. The origin of the changes in optical absorption caused by the reducing/oxidizing treatments is discussed in terms of hydrogen and oxygen ion diffusion and the point defect structure of the material.

Key words: lithium niobate; thermochemical treatment; reduction and oxidation kinetics; optical spectroscopy; point defects.

As it is known, the changes of optical properties of lithium niobate single crystals under influences of external electro-magnetic fields and temperatures are caused by processes in the subsystem of point defects in the crystal structure [1, 2]. Particularly, the photorefractive properties of LiNbO3 depend on the presence of polyvalent ions, most of all by Fe ions [2]. These ions are always present in the crystal as an uncontrolled impurity. Besides, LiNbO3:Fe crystals are grown specially as a material for holographic recording [2, 3].

Iron ions in LiNbO3 are observed in two charge states: 2+ and 3+ [3] and the effectiveness of optical information recording depend on Fe2+/Fe3+ ratio. This fact caused a great interest to recharge processes of Fe ions under the influence of external factors, particularly, reducing/oxidizing annealing. In most cases, the changes in optical absorption that take place in LiNbO3:Fe crystals upon high temperature treatments [3-9] have been studied at room temperature after a certain cooling procedure. We know only two works [10, 11] where in-situ investigations of optical absorption were carried out for Fe-doped LiNbO3 crystals in the spectral region of absorption of the Fe ions, however in a narrow temperature range. Authors of [10] studied the absorption changes at the

© Sugak Yu., Sugak D., Zhydachevskii Ya. et al., 2009

spectral range of 0,4-2,5 цгп during heating the crystal up to 150oC, whereas in [11] the changes in the absorption band of Fe2+ with maximum at 0,5 цгп was studied at the temperatures up to 350oC. At the same time in [12] we have investigated the changes of optical properties of nominally pure LiNbO3 crystals directly during high-temperature (T ~ 20-800oC) treatment in different atmospheres.

Here we present the results of the in-situ investigations of optical properties of congruent LiNbO3:Fe crystals during reducing/oxidizing annealing. The analysis of the obtained experimental data is based on the comparison with the ones obtained previously [12] for nominally pure congruent LiNbO3, that allows to determine the role of Fe-ions in the reducing/oxidizing annealing of LiNbO3:Fe crystals.

LiNbO3:Fe crystals were grown by the Czochralski technique in air from platinum crucibles. Concentration of Fe ions in charge was 0,2 at.%. Samples for experiments were prepared as 7-cut polished plates with 0,8 mm thickness. The optical in-situ experiments have been performed using a specially designed high-temperature furnace placed in a Perkin-Elmer Lambda 900 spectrophotometer with spectral range 0,2-3 цгп (50 000-3 333 cm-1). The furnace allows to heat samples from room temperature up to about 1000oC in a certain gas atmosphere. The temperature controller ensures a linear heating program with the maximal rate of 5 K/min as well as the temperature stabilization at a desired temperature. The construction of the furnace allows rapid (~1 min) switching of gas atmospheres in the furnace and the registration of the subsequent reduction/oxidation (redox) kinetics at a certain wavelength. The absorption spectra were registered in the reducing (5%H2+95%Ar) and oxidizing (pure O2) atmospheres. Detailed scheme of the experiment is described in Ref. [12]. The absorption spectra of the crystals at T>400oC were corrected on the value of the heat radiation of the furnace.

The absorption spectra of LiNbO3:Fe crystal measured at different temperatures during heating in reducing atmosphere are shown in fig. 1. The absorption edge of the crystal at room temperature is located near 27500 cm-1, while in nominally pure LiNbO3 crystals it is observed in more high-energy region (near 35000 cm-1). This shift of the absorption edge to the low-energy region in LiNbO3:Fe crystals is evidently connected with the presence of Fe-ions that is confirmed by the literature data [7]. In the spectral region of 25000-5000 cm-1 the LiNbO3:Fe crystal is transparent. Increasing of the temperature leads to linear shift of the fundamental absorption edge of LiNbO3:Fe crystal to the low-energy spectra region (fig. 1).

The main peculiarity of LiNbO3:Fe crystals in comparison with nominally pure LiNbO3 during heating in reducing atmosphere consists in earlier beginning of optical absorption increase in the wide absorption band with a maximum near 20 000 cm-1. As it is seen from fig. 2, the optical absorption of LiNbO3:Fe starts to increase at the temperature about 300оС, whereas in pure LiNbO3 the changes of optical absorption begin at about 450оС [12]. At that the changes in pure LiNbO3 is initially observed at more low-energy absorption band with a maximum near 10 000 cm-1. The absorption band with a maximum near 20 000 cm-1 is observed in [3-4, 6, 8] and is connected with intervalence transition Fe2+ Nb5+.

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250      300      350      400      450 500

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Fig. 2. The changes of optical absorption at 20000 cm-1 during heating of LiNbO3:Fe crystal in a reducing atmosphere

During further heating of LiNbO3:Fe samples in reducing atmosphere as well as in case of nominally pure LiNbO3 crystal, the wide absorption band in the infrared spectral region arises at temperatures higher than 600oC (fig. 1). However, the maximum of this band is shifted to more low-energy region (8 000 cm-1) in comparison with pure LiNbO3. Further heating in reducing atmosphere leads to increasing of absorption in the visible spectral region - in the band located near 15 000 cm-1. This band is also shifted to low-energy region in comparison with similar absorption band in nominally pure LiNbO3 crystals, where it is located near 1 6000 cm-1.

Similarly to pure LiNbO3 crystals, the process of absorption centers formation both in IR and visible regions has got an activation character. However, it is impossible to determine parameters of the process of the absorption centers formation, because the absorption in the bands grows up continuously even at a constant temperature. Such a behavior of the absorption can be explained by the fact that formation of the absorption centers is originated from a diffusion process that takes place continuously.

Heating in oxidizing atmosphere does not lead to formation of absorption bands in visible and near IR- regions of the crystal spectra.

At the sufficiently high temperatures, the change of the annealing atmosphere from reducing to oxidizing one leads to bleaching of the absorption bands, i.e. the crystal becomes transparent again. A cycling of the atmospheres from oxidizing to reducing one (and vice versa) results in the formation and bleaching of the absorption bands, so the redox processes have got the reversible character.

a b Fig. 3. Time dependent absorbance change of a LiNbO3:Fe crysiai uuring reduction (a) and oxidation (b) registered at 14 000 cm-1 at different temperatures. Initial points are shifted for all curves to same value

The kinetics of the redox processes for the absorption bands 8000 cm-1 and 14000 cm-1 at different temperatures are shown in fig. 3-4. As it is seen from figs. 3, a, 4, a on the reduction kinetics one can observe some delay in the reduction process that becomes visible in saturation of the crystal coloration (even some bleaching of the crystal). After that the process of the crystal coloration becomes monotonous again. Obviously, such peculiarities of reduction kinetics are caused by the presence of Fe-ions in the crystal. At the same time any analogous peculiarities are not observed on the oxidation kinetics for both absorption bands (figs. 3, b, 4, b). This behavior of the reduction kinetics can not be explained unambiguously at the time, though it is possible to make certain assumptions. It can be assumed that reduction of Fe-ions takes place at the initial stages (Fe3+—Fe2+). At the same time the quantity of free electrons formed owing to the diffusion of oxygen from the crystal is too small. That's why re-trapping of the electrons by the Fe-ions from polarons takes place and it explains the delay (extremum) in the reduction process. After all the Fe-ions have been reduced, the reduction kinetics become monotonous again and are determined only by the polarons formation similar to nominally pure Ln [12].

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Fig. 4. Time dependent absorbance change of a LiNbO3:Fe crystal during reduction (a) and oxidation (b) registered at 8000 cm-1 at different temperatures. Initial points are shifted for all curves to same value

a

One more peculiarity of LiNbO3:Fe crystal in comparison with pure LiNbO3 one is observed cooling in reducing atmosphere. As it was shown in [12], when undoped LiNbO3 is cooled in reducing atmosphere to room temperature both from 600oC (when only the low-energy absorption band is present) or from 800oC (when the high-energy absorption band dominates), the absorption spectrum at room temperature is characterized only by the high-energy band. When LiNbO3:Fe crystal is heated up to 655oC in reducing atmosphere, the absorption of Fe2+-ions is so intensive that it is hard to talk about possible band realignment during cooling the crystal from this temperature. The spectrum of LiNbO3:Fe crystal is characterized by wide absorption band with the maximum at 20 000 cm-1 both at 655oC and at room temperature (fig. 5). Some decreasing of the absorption at the low-energy region can be caused either by the temperature shift of band maximum or by possible realignment of bands, as it is observed in nominally pure LiNbO3 [12].

22500    20000     17500     15000     12500     10000     7500 5000

Wavenumber, cm"1

Fig. 5. Optical absorption spectra of LiNbO3:Fe crystal registered during cooling from 600oC to room temperature in reducing atmosphere

Thus we have determined two main peculiarities in behavior of LN crystals, doped with Fe-ions in comparison with pure LiNbO3 during their high-temperature annealing.

Firstly, coloration processes of LiNbO3:Fe crystals during their high-temperature treatments in reducing atmosphere start in the visible spectral range at noticeably lower temperatures (300oC). Obviously, it is caused by recharging processes of iron ions Fe3+ - Fe2+.

Secondly, on reduction kinetics both in visible (to a lesser extent) and in infrared spectral region (to a greater extent) one can observe their significant deviation from monotony when annealing atmosphere is changed from oxidizing to reducing. At the initial stages the speed of band formation is decreasing, but after some time the dependencies becomes monotonous again that is explained by the redistribution of electrons from polarons to Fe-ions.

The work was partially supported by the Ukrainian Ministry of Education and Science (project # 0108U004774, Acronym M/53-2008).

1.   Kuzminov Yu. Electrooptical and non-linear optical lithium niobate crystal. M.: Nauka, 1987. 264 p. (in Russian).

2. Arizmendi L. Photonic applications of lithium niobate crystals // Phys. Stat. Sol. (a). 2004. Vol. 201. N 2. P. 253-283.

3. Clark M.G., DiSalvo F.J., Glass A.M., Peterson G.E. Electronic structure and optical index damage of iron-doped lithium niobate // J. Chem. Phys. 1973. Vol. 59. N 12. P. 6209-6219.

4. Staebler D.L., Phillips W. Fe-doped LiNbO3 for read-write applications // Appl. Opt. 1974. Vol. 13. N 4. P. 788-794.

5. Bukharaev A.A., Migachev S.A., Monakhov A.A. et al. Investigation of impurity iron ions in lithium niobate // Solid State Physics. 1976. Vol. 18. N 2. P. 602-605. (in Russian).

6. Shah R.R., Kim D.M., Rabson T.A., Tittel F.K. Characterization of iron-doped lithium niobate for holographic storage applications // J. Appl. Phys. 1976. Vol. 47. N 12. P. 5421-5431.

7. Akhmadullin I.Sh., Golenischev-Kutuzov V.A. et al. Optical spectra of Fe-ions in LiNbO3 // Solid State Physics. 1995. Vol. 37. N 2. С. 415-421. (in Russian).

8. Basun S.A., Evans D.R., Bunning T.J., Guha S. et al. Optical absorption spectroscopy of Fe2+ and Fe3+ ions in LiNbO3 // J. Appl. Phys. 2002. Vol. 92. N 12. P. 7051-7055.

9. Garsia-CabanesA., Dieguez E., Cabrera J.M., Agullo-Lopez F. Contributing bands to the optical absorption of reduced LiNbO3 // J. Phys.: Condens. Matter. 1989.

Vol. 1. P. 6453-6462.

10. Yukselici M.H., Allahverdi C., Tung A.V. Temperature shift of the Fe2+ absorption band in LiNbO3:Fe crystal // Phys. Stat. Sol. (B). 2004. Vol. 241. N 13. P. 3041­3046.

11. Panatopulos G., Luennemann M., Buse K., Psalitis D. Temperature dependence of absorption in photorefractive iron-doped lithium niobate crystals // J. Appl. Phys. 2002. Vol. 92. N 2. P. 793-796.

12. Sugak D., Zhydachevskii Ya., Sugak Yu., Buryy O. et al. In-situ investigation of optical absorption changes in LiNbO3 during reducing/oxidizing high-temperature treatments // J. Phys.: Condens. Matter. 2007. Vol. 19. 086211 (12 pp).

ОСОБЛИВОСТІ ПРОЦЕСІВ ЗАБАРВЛЕННЯ КРИСТАЛІВ LiNbO3:Fe ПІД ЧАС ВИСОКОТЕМПЕРАТУРНИХ ОКИСНО-ВІДНОВНИХ ОБРОБОК У

РЕЖИМІ IN SITU

Ю. Сугак1, Д. Сугак1,2, Я. Жидачевський1, О. Бурий1, І. Сольський2, С. Убізький1,2, К.-Д. Беккер3, А. Бьоргер3

'Національний університет "Львівська політехніка " вул. Бандери 12, 79013, Львів, Україна 2Науково-виробниче підприємство "Карат " вул. Стрийська 202, 79031 Львів, Україна 3Інститут теоретичної та фізичної хімії Технічного Університету Брауншвейга, Німеччина

У роботі досліджено вплив високотемпературних відпалів (20-800оС) в окиснювальній (O2) та відновлювальній (Ar + H2) атмосферах на оптичні властивості   кристалів   LiNbO3,   легованих   іонами   Fe   у   режимі   in situ.

Проаналізовано отримані спектри поглинання і кінетичні залежності. Зроблені припущення про можливу природу змін оптичного поглинання, спричинених високотемпературними окисно-відновними відпалами. Визначено особливості процесів забарвлення кристалів LiNbO3:Fe порівняно із номінально бездомішковими кристалами LiNbO3.

Ключові слова: ніобіт літію, термомеханічна обробка, оптична спектроскопія, точкові дефекти.

ОСОБЕННОСТИ ПРОЦЕССОВ ОКРАСКИ КРИСТАЛЛОВ LiNbO3:Fe ВО ВРЕМЯ ВЫСОКОТЕМПЕРАТУРНЫХ ОКИСЛИТЕЛЬНО-ВОССТАНАВЛИВАЮЩИХ ОБРАБОТОК В РЕЖИМЕ IN SITU

Ю. Сугак1, Д. Сугак1,2, Я. Жидачевский1, О. Бурий1, И. Сольский2 С. Убизский1,2, К.-Д. Беккер3, А. Бергер3

'Национальный университет "Львовская политехника " ул. Бандеры 12, 79013 Львов, Украина 2Научно-производственное предприятие " Карат"

ул. Стрыйская 202, 79031 Львов, Украина 3Институт теоретической и физической химии Технического Университета Брауншвейга, Германия

В работе проведены исследования влияния высокотемпературных отжигов (20-800оС) в окислительной (O2) и восстанавливающей (Ar + H2) атмосферах на оптические свойства кристаллов LiNbO3, легированных ионами Fe в режиме in situ. Проведен анализ полученных спектров поглощения и кинетических зависимостей. Сделаны предположения о возможной природе изменений оптического поглощения, вызванных высокотемпературными окислительно-восстанавливающими отжигами. Определены особенности процессов окраски кристаллов LiNbO3:Fe в сравнении с номинально беспримесными кристаллами

LiNbO3.

Ключевые слова: ниобат лития, термомеханическая обработка, оптическая спектроскопия, точечные дефекты.

Стаття надійшла до редколегії 04.06.2008 Прийнята до друку 25.03.2009

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Yu Sugak, D Sugak, Ya Zhydachevjjkii - Peculiarities of coloration processes in linbo3fe crystals under high temperature reducingoxidizing annealing