Highly efficient visible and UV light laser sources are of great importance for many applications. Visible emission can be obtained by frequency doubling in solid state lasers, using non-linear optical crystals. A particularly valuable recently developed non-linear optical crystals is GdCa₄O(BO₃)₃ (GdCOB), grown by Czochralski technique, which is a non hygroscopic, easy to polish and of high hardness. In order to achieve high efficiency of second harmonic generation the crystals of high structural quality are needed. Crystal lattice defects are the source of lattice strains, which change the crystal optical properties, mainly the refractive indices.
In the present work the GdCOB crystals are studied with the X-ray diffraction topography in reflection geometry. The conventional Lang topography and Synchrotron Radiation (SR) Plane Wave Reflection Topography were used.
The investigated samples were cut out perpendicularly to the growth axis at top, middle and end part of the crystal grown by Czochralski technique.
The topographs taken for symmetrical (0 16 0) reflection for different angle between the [100] crystallographic direction and the projection of the direction of primary beam on the crystal surface show the dependence of the diffraction contrast of the dislocations on this angle. The highest resolution was achieved in asymmetric reflection geometry, at the small angles of incidence of the primary beam.
The synchrotron radiation plane wave (SRPWT) topographs were recorded for different azimuths in different regions of the rocking curve. The characteristic changes of the dislocation contrast were observed in different topographs. Both plane wave and Lang topographs did not reveal any segregation fringes proving high homogeneity of the chemical composition of the examined crystal.

This work was partly supported by Polish Committee for Scientific research under the project
No T 11 B 054 22.

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