Temperature dependent dielectric constant measurements have been performed on <001> oriented 0.7 Pb(Mg1/3Nb2/3)O3–0.3 PbTiO3 crystals. These investigations have revealed an irreversible secondary transformation between a normal ferroelectric state and a relaxor ferroelectric state with increasing temperature. The results demonstrate that the anisotropy of the high performance piezocrystal state is only metastably locked in under application of field. Clearly, local random fields play a crucial role upon the ferroelectric phase stability and properties of <001> oriented piezocrystals. © 2001 American Institute of Physics.