The annealing effect on the structural perfection of Fe-doped LiNbO 3 single crystals has been studied by high-resolution X-ray diffractometry (HRXRD), X-ray topography (XRT) and Fourier transform infrared (FT-IR) spectroscopy. The single crystals, prepared by mixing Li 2 CO 3 and Nb 2 O 5 powders in the molar ratio 48.6:51.4 with 0.05 mol% of iron at 1415 (1) K, were grown by the Czochralski (CZ) method along the [001] direction in air and poled during crystal growth by the application of a DC field. Two low-angle (tilt angle $1 arc minute) structural grain boundaries were observed in as-grown specimens. FT-IR spectra revealed that these crystals contain OH À and CO 3 2À ionic defects. Grain boundaries and CO 3 2À ionic defects were successfully removed, while the concentration of OH À ions was considerably reduced by post-growth thermal annealing at elevated temperatures.

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Crystalline perfection, piezoelectric response and optical transparency of in‐situ poled Fe–LiNbO3 single crystals was studied. Very low angle grain boundaries and the variations in the piezoelectric charge constant d33 were observed in the as‐grown crystals. Grain boundaries were successfully removed at higher annealing temperatures but the d33 value was decreased. Low crystalline perfection and d33 were observed after poling the annealed specimen. These parameters were improved by low temperature annealing followed by very slow cooling. FTIR spectra revealed that OH– and CO32– ionic defects were present in the as‐grown crystals. The OH– ion concentration was reduced, CO32– ions were removed and optical quality was improved after annealing at higher temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)