  Resonant x-ray scattering shows that the bulk structure of the twist-bend
liquid crystal phase, recently discovered in bent molecular dimers, has spatial
periodicity without electron density modulation, indicating a lattice-free
heliconical nematic precession of orientation that has helical glide symmetry.
In situ study of the bulk helix texture for the dimer CB7CB shows an
elastically-confined temperature-dependent minimum helix pitch, but a
remarkable elastic softness of pitch in response to dilative stresses.
Scattering from the helix is not detectable in the higher temperature nematic
phase.
