  The dispersive behavior of materials with atomic charge numbers varing from
$Z = 4$ (beryllium, Be) to $Z = 82$ (lead, Pb) was investigated experimentally
and theoretically at $\gamma$-ray energies up to 2 MeV. The experiment was
performed at the double-crystal gamma spectrometer GAMS6 of the ILL in
Grenoble. The experimental results were compared with theoretical calculations
which account for all major elastic processes involved. Overall, we found a
good agreement between theory and experiment. We find that for the development
of refractive optics at $\gamma$-ray energies beyond those currently in use
high-Z materials become increasingly attractive compared to the beryllium
lens-stacks used at X-ray energies.
