We present the first experimental demonstration of quasi-permanent holographic recordings
in Bi_12TiO_20 crystals through what is to our knowledge a new electrophotochromic effect.
This arises from spatial redistribution of some centers that are different from photorefractive
centers. Light diffracted from this photochromic grating records a secondary photorefractive
hologram. Interaction between both gratings results in asymmetric polarization properties
and reversible electric field enhancement of diffracted beams.