Neutron diffraction study of field-cooling effects on the relaxor ferroelectric Pb[(Zn1/3Nb2/3)0.92Ti0.08]O3

High-temperature (high-T) and high-electric-field (high-E) effects on Pb[(Zn1/3Nb2/3)0.92Ti0.08]O3 were studied comprehensively by neutron diffraction in the ranges 300≤T≤550 K and 0≤E≤15 kV/cm. We have focused on how phase transitions depend on preceding thermal and electrical sequences. In the field-cooling process (E parallel [001]≥0.5 kV/cm), a successive cubic (C)→tetragonal(T)→ monoclinic (MC) transition was observed. In the zero-field-cooling process, however, we have found that the system does not transform to the rhombohedral (R) phase as widely believed, but to an unidentified phase, which we call X. X gives a Bragg-peak profile similar to that expected for R, but the c axis is always slightly shorter than the a axis. As for field effects on the X phase, it transforms into the MC phase via another monoclinic phase (MA) as expected from a previous paper [Noheda et al., Phys. Rev. B 65, 224101 (2002)]. At a higher electric field, we confirmed the field-induced MC→T transition, which shows a gradual c-axis jump contrary to a sharp c-axis jump observed by strain and x-ray diffraction measurements