New perspectives of pure and lanthanum modified lead zirconate titanate crystalline solutions
Dai, Xunhu
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https://hdl.handle.net/2142/19256
Description
Title
New perspectives of pure and lanthanum modified lead zirconate titanate crystalline solutions
Author(s)
Dai, Xunhu
Issue Date
1995
Doctoral Committee Chair(s)
Viehland, Dwight D.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Two types of oxygen octahedral rotational systems (R- and M-type) were found in the rhombohedral phase field of lead zirconate titanate (PZT). A hierarchy of domain structure was observed in the high temperature rhombohedral ferroelectric (FE) state, where local octahedral tilt domains on the scale of $\sim$50 A existed within the normal FE micron-sized domains. Constriction of the polarization by the local oxygen octahedral tilting was found. Small amounts of La- and Nb-doping was found to relax the lattice, resulting in normal square dielectric hysteresis loops.
Incorporation of La into PZT (PLZT) was found to disrupt long-range FE order. Near the antiferroelectric (AFE-FE) phase boundary within the high-Zr phase region of PZT, an incommensurate antiferroelectric (AFE$\sb{\rm In}$) state was found to develop at the expense of the FE phase upon La-modification. In addition, evidence of a relaxor state with antipolar order was found in high La-content rhombohedral PLZT, which was characterized by relaxor-like dielectric behavior and double P-E loop features. For PZT compositions away from the AFE-FE phase boundary and far into the rhombohedral and tetragonal FE phase regions, La-modification was observed to transform the normal micron-sized FE domain structure into nanodomain state, which exhibited typical relaxor behavior. In the tetragonal phase field, spontaneous (thermally driven) relaxor-normal ferroelectric transformations were observed near crossover compositions between normal and relaxor states.
Finally, it was shown that an increase in the ease of polarization switching is essential for maximum electrically-induced strains in PLZT ceramics. For PLZT compositions close to the morphotropic phase boundary (MPB), this was realized by adjusting the base composition to be close to the boundary between tetragonal and rhombohedral states, where the polarization has maximum equivalent orientations by which to switch. For PLZT compositions locating in either the rhombohedral or tetragonal states, the ease of polarization switching was facilitated by La-doping until the point where the normal micron-sized ferroelectric domains began to break up into tweed-like subdomain structures. Polarization switching may then occur through the subdomain state, rather than by the creation and subsequent motion of normal micron-sized domain boundaries.
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