Neutron Diffraction Study of the Irreversible R-M_A-M_C Phase Transition in Single Crystal Pb[(Zn_1/3Nb_2/3)_1-xTi_x]O₃

Abstract

Single crystals of the relaxor PZN-xPT display an enormously strong piezoelectric character. Recent x-ray scattering studies have revealed novel electric-field induced phase transitions in PZN-8%PT. As-grown crystals exhibit a rhombohedral structure that, under application of an electric field oriented along [001], transforms into a monoclinic (M_A) phase, and then irreversibly to another monoclinic (M_C) phase with increasing field strength. Since the latter phase change is very unusual, its transition sequence has been investigated by using triple-axis neutron scattering techniques so that the “skin effect” observed by x-ray scattering can be avoided, and the entire crystal bulk is probed. Contour maps of the elastic scattering have been mapped out in each phase in the (H0L) zone with high q-resolution. Increasing the field strength within the M_C phase induces a sharp c-axis jump around 15 kV/cm. This jump was observed easily with x-rays in previous studies, but it was not observed in 5 different crystals examined with neutrons. A subsequent high-energy x-ray study of the same crystals showed that the c-axis jump is distributed within the crystal volume, thereby washing out the jump. The observed R-M_A-M_C transformational path is in perfect accord with very recent first principles calculations by Bellaiche, Garcia, and Vanderbilt in the PZT system.