Experimental results on self-diffraction of periodically phase-modulated laser beams from a relief grating formed on a free surface of a cubic photorefractiveBi12TiO20crystal due to the piezoelectric effect are reported. Periodic intensity modulations of the reflected beams up to10−2due to energy exchange at this unshifted phase grating are measured. The modulation depends linearly on the fringe spacing and on the amplitude of an externally applied electric dc field. The observed self-diffraction process is independent of the light polarization. ©1998 American Institute of Physics.