  Ultrarelativistic charged particles are predicted to emit hard
electromagnetic radiation of undulator type while being channeled in a crystal
with periodically bent crystallographic planes. The recently proposed
crystalline undulator with the bending amplitude smaller than the distance
between the bent planes and the bending period shorter than the period of
channeling oscillations is far superior to what was proposed previously. In the
same time, it is more challenging from the technical point of view because its
bending period has to be in the sub-micron range. It is shown that a mixed
crystal of silicon-germanium with properly varying germanium fraction can have
the necessary bending parameters. Moreover, it is predicted to be stable
against misfit dislocations.
