Incorporation of H+ into Mg-doped LiNbO3

Solid State Communications, Vol. 59, No. 7, pp. 499-501, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.

INCORPORATION OF H ÷ INTO Mg-DOPED LiNbO3 Ma.J. de Rosendo, L. Arizmendi, J.M. Cabrera and F. Agull6-L6pez Departamento de Optica y Estructura de la Materia e Institute de Ffsica del Estado S6lido, Universidad Aut6noma de Madrid, Cantoblanco. 28049 Madrid, Spain

(Received 4April 1986 by S. Amelinckx) Infrared spectra of OH- ions have been obtained from Mg-doped LiNbO3 crystals. The spectra, which are remarkedly different from those for pure crystals, exhibit two main well separated bands quite different for both samples, indicating the occurrence of two slightly different OH- bonds. From their polarization behaviour it is concluded that these bonds approximately point towards the center of the neighboring oxygen octahedra. Vacuum annealing treatments make the bands disappear at slightly different temperatures (about 700°), further supporting the assumption of two different sites. 1. INTRODUCTION LiNbO3 IS A DIELECTRIC crystal presenting important potential applications to optical storage, phaseconjugation and integrated optics devices. In particular, efficient wave-guiding properties of LiNbO3 can be obtained after suitable treatments such as Ti-doping or Li out-diffusion. Unfortunately, the usefulness of LiNbO3 waveguides is limited by laser damage, i.e. the lightinduced change of refractive index or photorefractive (PR) effect [I]. The same effect is responsible for the optical storage and phase conjugation capabilities of the material [2]. Hydrogen appears to have an important role in the stabilization or fixing of the photorefractive damage [3, 4]. Studies of the incorporation of H ÷ into LiNbO3 have been carried out including its diffusion behaviour [3, 5]. The location of H ÷ in the lattice is not definitely known, although polarization measurements indicate that the formed OH- bonds lie in the basal plane, i.e. perpendicularly to the trigonal C axis [6]. The situation appears complex since the OH- i.r. stretching bond shows some structure which seems to depend on the material [7]. On the other hand, Mg-doping [8] in high concentrations (about 5%), as well as OH--doping [9, 10], induces a marked inhibition of the optical damage, and opens the way to the preparation of light-resistant wave guides and integrated optics devices. It is assumed that Mg occupies a Li÷-substitutional site [11], and this is expected to have relevant implications on crystal stoichiometry. Therefore, it appears interesting to study the behaviour of H ÷ in Mg-doped LiNbOa. Although some spectra have been given as a marginal information in a recent paper [8], no systematic work 499

on spectra, polarization behaviour and effect of reducing treatments has so far been performed. To cover these topics is the purpose of the present paper. 2. EXPERIMENTAL METHODS Pure and Mg-doped LiNbOa crystals were pulled from the melt under a pressure of 1.2 atm. of pure oxygen. As starting powder Johnson Mattey Grade I chemicals were used with the congruent composition. Mg-doping was achieved by adding to the melt a 5% mol. concentration of MgO powder (Merck G.R.). A 0.1%mol. concentration of F%O3 (Merck G.R.) was also added to the melt for other experiments not discussed in this paper. The effect of this last doping, fifty times smaller than Mg-doping, is though to play a non-relevant role in the experiments described in the present paper. From the as-grown boules, 7 x 10 x 2 mm 3 sized plates were cut with their large faces parallel to the ferroelectric trigonal axis (C-axis). Optical fmishing was achieved by polishing them with diamond paste (1 lam grain size). Hydrogenation of the crystals was performed by maintaining them under a continuous H20 vapor flow at 900°C for two hours. H20 vapor was flowing during heating and cooling processes whose rates were about 50°C h -1 . To reduce the samples at a given temperature, they were annealed in a vacuum of approximately 10 -2 torr for two hours. 3. RESULTS

3.1. Spectra Figure 1A shows the infrared (i.r.) spectrum corresponding to a nominally pure LiNbO3 crystal grown

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INCORPORATION OF H + INTO Mg-DOPED LiNbO3

Vol. 59, No. 7

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Fig. l. Infrared absorption band of OH- in LiNbO3. A: Pure sample. B: Mg-doped sample. with the congruent composition (Li/Nb = 0.946). It has been treated as described in the experimental section in order to incorporate H ÷ ions. Figure 1B shows the spectra for the Mg-doped crystal after the same H20 vapor treatment. In both cases, crystals were always well oxidized before the hydrogenation treatment. The spectrum for the pure crystal shows a structured band apparently made up of two strongly overlapped components having similar height. This is in accordance with previous work [6]. For the Mg-doped sample, two well separatedbands peaked at 3537 cm -z and 3507 cm -1 are observed. The two bands appear at higher energy that that for the pure crystal. The high energy band is

complex and presents a shoulder on the low energy side of the main peak. This should is more visible after some reduction of the sample has been achieved. For pure LiNbO3 the OH- band has been shown to be completely polarized perpendicularly to the C axis. The polarization behaviour for the two OH- bands appearing in Mg-doped crystals is illustrated in Fig. 2. The intensities of the bands have been plotted as a function of the angle a formed by the electric vector of the incident linearly polarized light and the C axis. These data show that the optical spectra of the Mg-doped crystal are quite peculiar and indicate that the OHband is lying at an angle close to 45 ° with the C axis. This point will be more quantitatively elaborated in the discussion.

3.2. Effect of reduction treatments Pure as well as Mg-doped samples have been subjected to successive isothermal and isochronal treatments in

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INCORPORATION OF H + INTO Mg-DOPED LiNbOa

vacuum at increasingly higher temperatures. The effect on the height of the OH= bands is shown in Fig. 3. With increasing temperature, the OH- bands remain constant up to 450°C, and then decrease rapidly and practically disappear at 700°C. The bleaching temperatures for the pure and doped crystal are similar. 4. DISCUSSION The infrared spectra and polarization behaviour of OH- anions in heavily Mg-doped LiNbOa are markedly different from those found for pure crystals. In particular the two well separate bands shown in Fig. 1B clearly indicate the occurrence of two different sites for the proton. In fact, no band is observed at the 3480cm -1 wavenumber exhibited by OH- in pure crystals. Therefore a non negligible change of the crystal field seen by the OH- is present with respect to pure crystals. Assuming that Mg is not forming aggregates within the LiNbO3 lattice, a plausible explanation for this change is to consider some kind of OH--Mg+2 association for most protons. Because of the positive effective charge of both entities, protons could be located in a Mg-next-nearest position rather than in a nearest one. Less likely, heavy Mg-doping might be responsible for a change of crystal field of all crystal sites. From the polarization behaviour of the spectra (Fig. 2) an angle of about 45 ° between the OH- bonding and the C axis is inferred for Mg-doped crystals. This value is to be compared with the 90 ° angle observed in pure crystals. Taking into account the trigonal symmetry of the C axis, this angle can be obtained in a more quantitative way from Fig. 2 as follows. Let us call 0 and ¢ the angles formed by an OH- bond with +X and +Z (C axis) coordinates directions, and a the angle between the light electric field and +Z axis. Contributions to actual spectra will come form the projections onto the electric field of the three equivalent OH-, i.e. the value of 0, 0 + 120 ° , and 0 + 2 4 0 ° . After adequate algebraic manipulations it is found that the height of the band is proportional to the expression: P = cos2 ¢" cos: a + sin2 ~ • sin2 ~. By fitting this expression to the experimental data shown in Fig. 2 a value of ~ = 58 ° is obtained for both bands. In this Figure continuous lines represent the

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calculated dependence. This value is quite close to the angles formed by the C axis with the directions connecting an oxygen ion with the centers of the surrounding octahedra. Because of charge compensation arguments, structural vacancy octahedra would be more favoured. To further clarify this point more detailed calculations based on a point charge ions model are under way. On the other hand, the different annealing dependences for both bands shown in Fig. 3 further support the assumption made above about two different sites for protons, in spite of showing the same polarization behaviour. It is not reliable to obtain activation energies for the two OH- bands from Fig. 3, in order to compare with the optical frequencies. However, both thermal and optical data appear to be in good cualitative accordance since the band with higher vibrational frequency (peaked at 3537 cm -1 ) anneals at higher temperature.

Acknowledgements - Authors would like to thank illuminating discussions with Prof. Serratosa and collaborators (Institute de Fisico-Quimica Mineral, C.S.I.C.) during the initial stage of the work, as well as instrumental facilities. On the other hand, partial support by C.A.I.C. y T. is grateful acknowledged. REFERENCES 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Most of these topics are well covered in Processing of Guided Wave Optoelectronic Materials, (Edited by R.L. Holman & D.M. Smith), Proceedings of SPIE, The International Society for Optical Engineering, Vol. 460, Los Angeles (1984). For a review see for example: P. Gunter, Phys. Rep. 93, 201 (1982). R. GonzMez, Y. Chen, K.L. Tsang & G.P. Summers, AppL Phys. Lett. 41(8), 739 (1982). H. Vormann, G. Weber, S. Kapphan & E. Kratzig, Solid State Commun. 40,543 (1981). J.L. Ketchum, K.L. Sweeney, L.E. Halliburton & A.F. Armington, Phys. Lett. 94A, 450 (1983). J. Herrington, B. Dischler, A. Rauber & J. Schneider, SolidState Commun. 12,351 (1973). L. Kovhcs, V. Szalay & R. Capelletti, Solid State Commun. 52, 1029 (1984). D.R. Bryan, R. Gerson & H.E. Tomaschke, Appl. Phys. Lett. 44(9), 847 (1984). R.G. Smith, D.B. Fraser, R.T. Denton & T.C. Rich, J. AppL Phys. 39, 4600 (1968). J.L. Jackel, D.H. Olsen & A.M. Glass, J. Appl. Phys. 52, 4855 (1981). K. Nassau Ferroelectricity, p. 264, Elsevier, Amsterdam (1967).