  Next-generation, high-brilliance x-ray photon sources call for new x-ray
optics. Here we demonstrate the feasibility of using monolithic diamond
channel-cut crystals as high-heat-load, beam-multiplexing, narrow-band,
mechanically-stable x-ray monochromators with high-power x-ray beams at
cutting-edge, high-repetition-rate x-ray free-electron laser (XFEL) facilities.
The diamond channel-cut crystals fabricated and characterized in these studies
are designed as two-bounce Bragg reflection monochromators directing 14.4-keV
or 12.4-keV x-rays within a 15-meV-bandwidth to $^{57}$Fe or $^{45}$Sc nuclear
resonant scattering experiments, respectively. The crystal design allows
out-of-band x-rays within a $\simeq 1$-eV XFEL bandwidth to be transmitted with
minimal losses to alternative simultaneous experiments. Only $\lesssim 2$\% of
the incident $\simeq 100$-W x-ray beam is absorbed in a 50-$\mu$m-thick first
diamond crystal reflector, ensuring that the monochromator crystal is highly
stable. Other x-ray optics applications of diamond channel-cut crystals are
anticipated.
