We show that bistability and metastability of In and Ga impurities in
CdF2 crystals can be efficiently used in writing volume holographic gr
atings. Phototransformation of these impurities from the localized to
the hydrogenic state leads to a large change of the local polarizabili
ty of tile host, and thus a significant change of the refractive index
. While CdF2:In can be used only at low temperatures (below 150 K), Cd
F2:Ga crystals are suitable for room temperature recording.