Weiguo Qu

Compositional disorder, polar nanoregions and dipole dynamics in Pb(Mg1/3Nb2/3)O3-based relaxor ferroelectrics

Abstract The complex structure of relaxor ferroelectrics comprises polar nanoregions (PNRs) which appear upon cooling below the Burns temperature and quenched compositional (chemical) disorder. The relation between the polar nanostructure and compositionally ordered regions (CORs) often observed in relaxors has been the subject of extensive theoretical investigations; however, the experimental data, especially concerning Pb(B′1/3B″2/3)O3-type complex perovskite relaxors, are rather limited. In this paper, we analyse and discuss the results of our recent investigations of the morphology of CORs and the dynamics of PNRs in Pb(Mg1/3Nb2/3)O3 based solid solutions in which the degree of compositional disorder was varied by means of changing the composition and/or by means of high-temperature annealing. The samples were characterised using X-ray diffraction, transmission electron microscopy, piezoresponse force microscopy, Brillouin light scattering, dielectric spectroscopy, as well as by measuring pyroelectric effect and ferroelectric hysteresis loops. No influence of the size of CORs on the PNRs relaxation in the ergodic relaxor phase is found. Instead, the CORs size influences significantly the diffuseness of the transition from the field-induced ferroelectric phase to the ergodic relaxor state. The results are interpreted in the framework of a model suggesting the coexistence of static and dynamic PNRs in the ergodic relaxor phase.

Enhanced ordered structure and relaxor behaviour of 0.98Pb(Mg1/3Nb2/3)O3–0.02La(Mg2/3Nb1/3)O3 single crystals

High quality Pb(Mg1/3Nb2/3)O3 (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seeded solution growth method. At a doping level as low as 2 at.% of La, the 1:1 B-site cation (chemical) order was significantly enhanced as revealed by the presence of superlattice peaks in the x-ray diffraction pattern and comparatively large (~100 nm) chemically ordered regions (CORs) in the transmission electron microscopy dark field images. The average chemical order parameter was calculated (from the intensity of superlattice peaks) to be S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumably rhombohedral) phase. The effect of La doping on the dielectric relaxation was not substantial. Though the magnitude of the dielectric constant peak in PLMN was reduced by half as compared to pure PMN, the peak temperature and diffuseness as well as the shape of the dielectric relaxation spectra remained almost unchanged. The analysis of the temperature dependence of the characteristic time for the main relaxation process in terms of the Vogel–Fulcher law revealed the freezing of dipole dynamics at temperature Tf = 192 K, which was only slightly lower than the Tf = 213 K in PMN. To reconcile the structural and dielectric data we suggest that the rhombohedral phase forms inside CORs while the chemically disordered matrix remains cubic and is populated by dynamic polar nanoregions that give rise to the characteristic dielectric response.

Room temperature magnetoelectric multiferroism through cation ordering in complex perovskite solid solutions

A strategy for developing ferrimagnetism through B-site cation ordering in thermodynamically stable ferroelectric complex perovskites is described, with the aim of realizing both a spontaneous polarization and magnetization at room temperature. Specifically, solid solutions of the complex perovskites Pb(Fe2/3W1/3)O3 ,S r (Fe2/3W1/3)O3 ,a nd Ba (Fe2/3W1/3)O3 with BiFeO3 are considered. Preliminary data on the 0.75Pb(Fe2/3W1/3)O3–0.25BiFeO3 ceramic show promising results.

Electric field-induced phase transitions in ferroelectric oxides: An in situ TEM study (Invited)

High electric fields were delivered to specimens during imaging in the transmission electron microscopy (TEM) chamber to reveal details of electric field-induced phase transitions in ferroelectric oxides. These include the growth of polar nanodomains in a Sc-modified Pb(Mg1/3Nb2/3)O3 ceramic and the R3c to R3m phase transition in a 0.02% Nb-doped Pb(Zr0.95Ti0.05)O3 ceramic.

Long Rang Cation Order in Pb(Mg 1/3 Nb 2/3 )O 3 Complex Oxides

Long range cation order in the Pb(Mg_1/3Nb_2/3)O₃ compound was developed through only 1 at.% cation substitution with La³⁺, Sc³⁺ or W³⁺. However, the dielectric/ferroelectric behavior of the chemically ordered PMN is strongly dependent on the substituting species. Generally speaking, incorporating La³⁺ significantly suppresses, while introducing Sc³⁺ moderately enhances, the ferroelectricity in PMN. The situation with the substitution with W⁶⁺ is complex due to the antiferroelectric nature of Pb(Mg_1/3Nb_2/3)O₃.

In situ TEM Study of Electric Field-Induced Phenomena in Ferroelectric Ceramics

High electric fields were delivered to specimens during imaging in the transmission electron microscopy (TEM) chamber to reveal details of electric field-induced phenomena in ferroelectric oxides. These include the grain boundary cavitation in a PZT ceramic, the domain wall fracture in a Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) single crystal, the transformation of incommensurate modulations in Pb_0.99Nb_0.02[(Zr_1-xSn_x)_1-yTi_y]_0.98O₃ (PZST100x/100y/2) ceramics, and the growth of polar nanodomains in a Sc-modified PMN ceramic.