Growth and X-ray topographic characterization of κ-(BEDT-TTF)2Cu[N(CN)2]Br single crystals

Crystalline perfection in organic charge-transfer complex single crystals, κ-(BEDT-TTF)2Cu[N(CN)2]Br, was characterized by means of synchrotron X-ray topography. Single crystals of κ-(BEDT-TTF)2Cu[N(CN)2]Br were grown electrochemically using a relatively large glass cell in order to scale up the synthesis. The quality of the crystals was examined using white and monochromatic X-ray topography. There was no asterism of each spot in the Laue photographs, though the defect image in the spot was not resolved because of low-resolution power of the white beam topography. We also obtained monochromatic X-ray topographs with 1.0 Å X-rays and were able to observe line images of dislocations. Dislocation density of the specimen was approximately 7 x 103 cm-2 on an average in the whole area of crystal. However, dislocation lines were annihilated in the topograph taken after annealing at room temperature for 5 months. Furthermore, new lattice defects were generated in the crystal after similar room temperature annealing for 15 months. The change of defect structure was caused by X-ray irradiation because dislocations in the un-irradiated crystal were observed by the monochromatic X-ray topography even after 2 years annealing at room temperature. Point defects formed by the X-ray irradiation probably interacted with grown-in dislocations and secondary defects were formed in the specimen crystal, though a long time was needed for this reaction at room temperature.