Studies of dielectric spectroscopy, dc conductivity, and polarization switching allowed the observation of phase transition from the superionic to the insulating state in flux-grownKTiOPO4(KTP) crystals at low temperature where the high mobility of potassium ions is suppressed. A low temperature method of fabrication of engineered periodic domain structures in superionic KTP and isomorphic crystals is proposed. It enabled us to tailor homogeneous domain gratings with various periods in the range 4–39 &mgr;m for quasiphase-matched nonlinear optical converters in KTP plates over whole area of 30×30 mm2. ©1998 American Institute of Physics.