GdCa₄O(BO₃)₃:Nd single crystals were grown by Czochralski method. The structure of GdCOB is monoclinic and non centrosymmetric with space group Cm. The lattice parameters are: a = 0.80957 nm, b = 1.60186 nm, c = 0.35588 nm, β = 101.26ş.

The unique combination of good mechanical properties, good nonlinear and spectroscopic properties make this crystal an excellent candidate for solid state lasers. The Nd doped GdCa₄O(BO₃)₃ crystals give the possibility of realization of laser action and second harmonic generation in the same crystal. Crystal lattice defects introduce strains into the crystal lattice, which change its optical properties hence for optical applications the crystals of high structural quality are demanded.

The aim of our investigation was to detect and characterize defects in sample GdCOB doped of 4% neodymium (Nd), cut perpendicularly to the growth axis b. The crystal was investigated by means of conventional X-ray Lang projection topography in transmission and back reflection geometry.

With respect to the formerly studied undoped crystals the investigated crystal GdCOB: of 4% Nd showed more other defects. A set of dislocation loops generated around the precipitate particle by punching was observed. The short white or black contrasts of shape depending on the direction of the diffraction vector g were detected. The white or black contrasts in the form of concentric rings surrounding the core, are very well seen in all topographs of GdCOB:Nd crystal. These contrasts are probably due to the non uniform incorporation of dopant atoms, forming the striations. They depend on the diffraction vector g direction. The striations with small period were modulated by changes of the diffracted beam intensity with greater period. On the striations some precipitates or inclusions were observed. The central core region is free of any fringe contrasts.

The very long straight dislocations characteristic for undoped crystals are not observed but shorter dislocations inclined to the crystal surface were seen. They have the similar nature as the long dislocations: the Burgers vector is along a axis – [100]. The line directions are approximately along an intersection of the (100) plane with the crystal surface, so they exhibit strong edge component.

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