Solid State Communications, Vol. 52, No. 12, pp. 1029-1031, 1984. Printed in Great Britain 0038-1098/84 $3.00 + .00 Pergamon Press Ltd. STOICHIOMETRY DEPENDENCE OF THE OH- ABSORPTION BAND IN LiNbO3 CRYSTALS L. Kov~cs and V. Szalay Research Laboratory for Crystal Physics of the Hungarian Academy of Sciences, 1502 Budapest, POB 132, Hungary and R. Capelletti Institute of Physics, University of Parma, 43100 Parma, Italy (Received 7 May 1984 by R. Fieschi) Hydrogen in the form of OH- ions can be incorporated into LiNbO3 single crystals during the growth process. The infrared absorption band due to the OH- defects has been measured at room temperature and a considerable change of the band shape has been observed depending on the crystal composition. The structure of the band found in nearly stoi- chiometric crystals indicates three slightly different proton sites which can be explained on the basis of the LiNbO3 crystal structure. 1. INTRODUCTION IT WAS SHOWN some years ago that hydrogen plays an important role in the thermal fixing of holograms in LiNbO3 [1 ] and its introduction into the crystal may suppress the undesired outdiffusion of Li20 during the fabrication process of LiNbO3 : Ti waveguides [2]. The presence of hydrogen in LiNbO3 can be monitored by measuring the OH- absorption band near 3500 cm-1 as first reported by Smith et al. [3]. These authors found that hydrogen diffuses into the crystal during an electric field annealing procedure, but it can be present in the 'as grown' crystal as well. Herrington et al. [4] demonstrated that the absorption band caused by the stretching vibrations of OH- ions was completely polarized perpen- dicular to the crystallographic c-axis and had a structure consisting of two peaks, separated by approximately 13 cm- 1. The results were interpreted as an evidence for two slightly inequivalent OH- configurations where the O-H bonds are in the plane perpendicular to the c-axis. The crystals studied by Herrington et al. were grown from congruent and Nb enriched melts of LiNbO3 and both the structure and optical anisotropy of the band were found to be independent of stoichiometry. Bollmann and St6hr [5] have also studied the infrared absorption caused by OH- ions and found that the relative intensities of the two peaks depended on the Li/Nb ratio in the crystal. They attributed the peaks to OH- either substituting 02- ions or occupying interstitial sites. In order to obtain further informations on the incorporation of protons into LiNbO3 the OH- absorp- tion band has been carefully studied in a wide range of the Li/Nb ratio in the melt. In the present paper we describe the observed bands and give a new interpreta- tion of their structure. 2. EXPERIMENTAL Lithium niobate crystals were grown in air by a balance controlled Czochralski technique [6]. The crystals were pulled along the hexagonal c-axis and, in order to obtain single domain crystals, they were poled applying an electric field after the growth process. Starck specpure Nb2Os and Merck suprapure Li2CO3 were used as raw materials. The Li/Nb mole ratio in the melt was varied from 0.82 to 1.20, which corresponds to the ratio of about 0.90 to 0.99 in the crystal using the calibration of Carruthers et al. [7]. The real composition of the crystals was checked by the method described in the paper of F61dvfiri et al. [8] giving essentially similar results as found in [7]. The infrared absorption spectra were recorded by a JASCO DS-702G type spectro- photometer. In most cases unpolarized light was used propagating along the hexagonal axis of LiNbO3. 3. RESULTS AND DISCUSSION Figure 1 shows the infrared absorbance of crystals grown from various metal compositions. In addition to measured spectra (full lines) the results of a Gaussian decomposition procedure (dashed lines) are also pre- sented. The values of the peak frequencies, heights and half-widths of the observed bands are presented in Table 1. Lorentzian curves were also fitted to the spectra but better results than in the case of the Gaussian fit were only achieved in the case of nearly stoichiometric crystals. Anyway, the parameters obtained from both fittings differed very little from each other. It can be 1029