YB66: A new soft-X-ray monochromator for synchrotron radiation

YB66, a complex binary semiconducting compound with a cubic-in-crystal structure and a cell parameter of 23.44 A, has been singled out as a potentially useful soft-X-ray monochromator for dispersing synchrotron radiation. There is no intrinsic absorption by the constituent elements in the region 1-2 keV (the YL3-edge is at 2080 eV). Using the known structure factors for the (400) and (222) reflections having 2d values of 11.76 and 13.53 A respectively, their rocking curves have been calculated and are shown to be comparable to or better than that of beryl (10anti 10). In terms of vacuum compatibility, resistance to radiation damage, and thermal and mechanical stability, YB66 satisfies all the material requirements for use as a monochromator for synchrotron radiation in the soft-X-ray region. Recent experiments in growing single crystals of this material large enough to intercept 1 mrad of radiation are discussed. Rocking curve measurements, etch pit density, and X-ray white-beam topography are used to characterize the quality of these large crystals as a function of some critical growth parameters such as pulling rate and thermal gradient at the crystal-liquid interface. From this study, it is clear that future work should be directed toward the control and retention of convexity of the crystal-liquid interface during growth, a key factor to achieve high perfection in crystals. (orig.)