DIFFUSE-SCATTERING INVESTIGATIONS WITH A GRAPHITE DOUBLE MONOCHROMATOR FOR X-RAY SYNCHROTRON-RADIATION - YBA2CU3O6.5 AND KNBO3

A graphite double monochromator for X-ray synchrotron radiation is pre sented for specific applications. The main feature is a broad waveleng th band and a high collimation of the reflected beam. The relaxed reso lution in energy results in an intensity gain of about two orders of m agnitude in comparison with a high-resolution monochromator like silic on. The graphite monochromator is therefore well suited for the study of diffuse scattering. We have built and tested such a device, and hav e used it in combination with an image plate to record diffuse scatter ing distributions of single crystals with the Weissenberg and the so-c alled modified Laue technique. In the high T-c superconductor YBa2Cu3O 6.5, the four domains of the main Bragg reflections and the two domain s of the very weak diffuse superstructure reflections are clearly obse rved. In another application, we show the good agreement between the m easured and the calculated intensities of the diffuse planes in KNbO3.