USEFUL TOOLS FOR HIGH T_c SINGLE CRYSTALS ON THE DIFFRACTOMETER: TESTING, DATA COLLECTION AND REFINEMENT OF CRYSTAL STRUCTURES. V.N.Molchanov, Institute of Crystallography, Moscow 117333, Russia

Several useful tools were accumulated especially for high-T_c single crystals during more than 7 years of their studies in the X-ray Lab. of the Institute of Crystallography. The set of these tools covers all stages of diffraction experiment, starting from sample preparation, and includes effective strategy for evaluation of systematic errors in the course of crystal structure refinements. The following short list includes some of these methods.

Two-dimensional [[omega]]/2[[theta]] angle maps of special type reflections (e.g. h00, 0k0, hh0 and 00l) were widely used for checking crystal quality, twinning and distribution of mosaic spread. The sensitivity of this procedure is so high that enables to detect 1:500 volume ratio of twin domains for Y-123 samples. Apart from that the pin hole collimator may be used to visualize the distribution of twin domains at different parts of small single crystals.

Intensity measurements of specially chosen strong and weak reflections at different [[psi]]-angles are very effective to check anisotropy of extinction for plate-like single crystals. Side faces of small single crystal cut from a big one are usually very rough for unambiguous indexing under a microscope. In this case rocking curves measured may be used also for improvement of absorption correction by fitting the best set of indices for side faces.

Typical refinement procedure includes subsequent statistical analysis of F_obs/F_calc and F_obs-F_calc values for each reflection from the reflection set along different variables, such as setting angles, index L for layered structures, extinction coefficient, mean path length, sin([[theta]])/[[lambda]] and others. In many cases such a distribution served as a criteria of systematic errors absence.

Examples of effective using these tools are discussed.