Polarization switching mechanisms and electromechanical properties of La-modified lead zirconate titanate ceramics
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The electromechanical properties of (Pbi-jLa^HZryTii-^Os [PLZT x/y/{\ - y)] have been investigated in the compositional range 0 < x < 0.10 for y = 0.65 (rhombohedral PLZT) and 0 < x < 0.18 for y = 0.40 (tetragonal PLZT). Both field-induced strains (e-E) associated with polarization switching and piezoelectric responses (d33) were studied. Transmission electron microscopy (TEM) and dielectric investigations were also performed. Room temperature TEM investigations revealed common trends in the domain structure with increasing La content for both PLZT x/65/35 and x/40/60, including a micron-sized domain structure, a subdomain tweed-like structure, and a nanopolar domain state. Changes in the field-induced strains and piezoelectric properties were then related to these microstructural trends. The dominant electromechanical coupling mechanism in the micron-sized domain state was found to be piezoelectricity. However, an electrostrictive coupling became apparent with the appearance of the subdomain tweed-like structures, and became stronger in the nanopolar domain state. It is believed that polarization switching can occur through 70° or 110° domains, the subdomain tweed-like structure, or nanopolar domains depending on La content.
I. INTRODUCTION La modification (x) of PbCZr^Tii-^Os (PLZT x/j/l ~~ y) results in the destruction of long-range ferroelectric order. Rather, a state characterized by the existence of nanopolar domains develops. This nanopolar domain state exhibits a frequency dispersion of the dielectric response and is commonly known as a relaxor ferroelectric.1 The degree of relaxor behavior in the PLZT x/65/35 and x/40/60 compositional series is known to increase with increasing La content. 23 It is believed4 that the coupling of ferroelectrically active oxygen octahedra is broken by the aliovalent La ions and resultant A-site vacancies. Burns and Dacol5 showed that a RMS (local) polarization exists in PLZT up to the transition temperature for pure PZT (623 K). Cross subsequently proposed that the orientations of these local polar moments were thermally reversible in a coherent manner,6 analogous to superparamagnetism. At lower temperatures the polarization fluctuations undergo a Vogel-Fulcher-like freezing into a long-lived metastable condition with glass-like characteristics.7 This glasslike state is characterized by a freezing temperature (Tf). The slowdown of the polarization fluctuations has been shown to give rise to large nonlinearities in the dielectric, elastic, and electroelastic proprieties. These nonlinearities are believed to be associated with an induced ordering of the nanopolar domain state and the destruction of the glass-like condition. 926 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 4, Apr 1995 Downloaded: 14 Mar 2015
Relaxors do not exhibit piezoelectricity in the absence of an applied electric field. However, a strong electrostrictive coupling of the polarization to the lattice is known to exist.8 Under application of a bias, piezoelectricity is induced.9 This response
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