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YB66, a complex boron-rich man-made crystal, has been singled out as a potential monochromator material to disperse synchrotron soft X-rays in the 1–2 keV region. Results of a series of systematic property characterizations pertinent for this application are presented in this paper. These include Laue diffraction patterns and high-precision lattice-constant determination, etch rate, stoichiometry, thermal expansion, soft X-ray reflectivity and rocking-curve measurements, thermal load effects on monochromator performance, nature of intrinsic positive glitches and their reduction. The 004 reflection of YB66 has a reflectance of ∼3% in this spectral region. The width of the rocking curve varies from 0.25 eV at 1.1 keV to 1.0 eV at 2 keV, which is a factor of two better than that of beryl(1010) in the same energy range, and enables measurements of high-resolution XANES spectra at the Mg, Al and Si K-edges. The thermal bump on the first crystal arising from the low thermal conductivity of YB66 causes an energy drift of a few eVs with storage-ring current and necessitates periodic energy calibration with metal foils. The positive glitches in the transmission function just above the Mg K-edge have substantially been reduced using an Si or SiC mirror which suppresses the sharp reflectivity increases associated with anomalous scattering for the YB66 006 reflection at the Y L3- and L2-edges. Continual operation over the past five years of a YB66 double-crystal monochromator installed on the JUMBO beamline at Stanford Synchrotron Radiation Laboratory (SSRL) indeed proves the long-term stability of this material in synchrotron radiation under ultrahigh vacuum conditions as indicated by the invariance in rocking-curve characteristics after being exposed to an accumulative power level of ∼3 × 108 J over this period of time.
