  Diffraction gratings with large angular dispersion rates are central to
obtaining high spectral resolution in grating spectrometers operating over a
broad spectral range from infrared to soft-x-ray domains. The greatest
challenge is of course to achieve large dispersion rates in the
short-wavelength x-ray domain. Here we show that crystals in non-coplanar
asymmetric x-ray Bragg diffraction can function as high-reflectance soft-x-ray
diffraction gratings with dispersion rates that are at least two orders of
magnitude larger than those that are possible with state-of-the-art man-made
gratings. This opens new opportunities to design and implement soft x-ray
resonant inelastic scattering (RIXS) spectrometers with spectral resolutions
that are up to two orders of magnitude higher than what is currently possible,
to further advance a very dynamic field of RIXS spectroscopy, and to make it
competitive with inelastic neutron scattering. We present examples of
large-dispersion-rate crystal diffraction gratings operating near the 930-eV
L$_3$ absorption edge in Cu and of the 2.838-keV L$_3$ edge in Ru.
