Browsing by Author "Wang, Yu. U."

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    Adaptive ferroelectric states in systems with low domain wall energy: Tetragonal microdomains
    Jin, Y. M.; Wang, Yu. U.; Khachaturyan, Armen G.; Li, Jiefang; Viehland, Dwight D. (American Institute of Physics, 2003-09-01)
    Ferroelectric and ferroelastic phases with very low domain wall energies have been shown to form miniaturized microdomain structures. A theory of an adaptive ferroelectric phase has been developed to predict the microdomain-averaged crystal lattice parameters of this structurally inhomogeneous state. The theory is an extension of conventional martensite theory, applied to ferroelectric systems with very low domain wall energies. The case of ferroelectric microdomains of tetragonal symmetry is considered. It is shown for such a case that a nanoscale coherent mixture of microdomains can be interpreted as an adaptive ferroelectric phase, whose microdomain-averaged crystal lattice is monoclinic. The crystal lattice parameters of this monoclinic phase are self-adjusting parameters, which minimize the transformation stress. Self-adjustment is achieved by application of the invariant plane strain to the parent cubic lattice, and the value of the self-adjusted parameters is a linear superposition of the lattice constants of the parent and product phases. Experimental investigations of Pb(Mg1/3Nb2/3)O-3-PbTiO3 and Pb(Zn1/3Nb2/3)O-3-PbTiO3 single crystals confirm many of the predictions of this theory. (C) 2003 American Institute of Physics.
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    Bridging domain mechanism for phase coexistence in morphotropic phase boundary ferroelectrics
    Rao, W. F.; Wang, Yu. U. (AIP Publishing, 2007-04-01)
    Computer modeling and simulation reveals a bridging domain mechanism that explains the phase coexistence commonly observed around the morphotropic phase boundaries of ferroelectric solid solutions. The simulation takes into account the important role of coupled ferroelectric and ferroelastic domain microstructures in phase coexistence phenomenon. It shows that minor domains of metastable phase spontaneously coexist with and bridge major domains of stable phase to reduce total system free energy, including bulk free energy, domain wall energy, and long-range electrostatic and elastostatic energies. The existence of bridging domains also explains the enhanced piezoelectric response in the vicinity of morphotropic phase boundaries. (C) 2007 American Institute of Physics.
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    Conformal miniaturization of domains with low domain-wall energy: Monoclinic ferroelectric states near the morphotropic phase boundaries
    Jin, Y. M.; Wang, Yu. U.; Khachaturyan, Armen G.; Li, Jiefang; Viehland, Dwight D. (American Physical Society, 2003-11-07)
    A theory is developed for intermediate monoclinic (FEm) phases near morphotropic phase boundaries in ferroelectrics of complex oxides. It is based on the conformal miniaturization of stress-accommodating tetragonal domains under the condition of low domain-wall energy density. The microdomain-averaged lattice parameters are determined and attributed to the parameters of an adaptive monoclinic phase. The theory is applied to the temperature, electric field, and compositional dependent FEm lattice parameters. The predictions of the theory are rigidly obeyed over the entire FEm stability range.
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    Diffraction theory of nanotwin superlattices with low symmetry phase
    Wang, Yu. U. (American Physical Society, 2006-09-18)
    A nanotwin diffraction theory is developed. It predicts an adaptive diffraction phenomenon, where the Bragg reflection peaks are determined by coherent superposition of scattered waves from individual twin-related nanocrystals and adaptively shift along the twin peak splitting vectors in response to a change in the twin variant volume fraction. Application of this theory to tetragonal phase explains the intrinsic lattice parameter relationships of monoclinic M-C phase recently discovered in ferroelectric Pb[(Mg1/3Nb2/3)(1-x)Ti-x]O-3 and Pb[(Zn1/3Nb2/3)(1-x)Ti-x]O-3.
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    Diffraction theory of nanotwin superlattices with low symmetry phase: Application to rhombohedral nanotwins and monoclinic M-A and M-B phases
    Wang, Yu. U. (American Physical Society, 2007-07-17)
    The Brillouin zone-dependent conditions for coherent and adaptive diffractions are formulated. Adaptive diffraction phenomenon of nanotwins is analyzed. Extraordinary Bragg reflection peaks appear and adaptively shift along the conventional twin peak splitting vectors, whose positions are determined by lever rule according to twin variant volume fractions. Analysis of rhombohedral nanotwins shows that the nanotwin superlattices of rhombohedral phase with {001} and {110} twin planes diffract incident waves just like monoclinic M-A and M-B phases, respectively, whose lattice parameters are intrinsically related to that of rhombohedral phase. Crystallographic analysis of rhombohedral nanotwins by nanodomain averaging gives the same monoclinic M-A and M-B phases.
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    Direct high-resolution transmission electron microscopy observation of tetragonal nanotwins within the monoclinic M(C) phase of Pb(Mg(1/3)Nb(2/3))O(3)-0.35PbTiO(3) crystals
    Bhattacharyya, Somnath; Jinschek, J. R.; Cao, Hu; Wang, Yu. U.; Li, Jiefang; Viehland, Dwight D. (AIP Publishing, 2008-04-01)
    We report on the direct observation of tetragonal nanodomains within an average monoclinic M(C) phase of Pb(Mg(1/3)Nb(2/3))O(3)-0.35PbTiO(3) single crystals by high-resolution transmission electron microscopy. These nanodomains are geometrically arranged in alternating layers of twins. The findings are consistent with the fundamental underlying assumptions of the ferroelectric adaptive phase theory.
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    Domain wall broadening mechanism for domain size effect of enhanced piezoelectricity in crystallographically engineered ferroelectric single crystals
    Rao, W. F.; Wang, Yu. U. (AIP Publishing, 2007-01-01)
    Computer modeling and simulation reveal a domain wall broadening mechanism that explains the domain size effect of enhanced piezoelectric properties in domain engineered ferroelectric single crystals. The simulation shows that, under electric field applied along the nonpolar axis of single crystal without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. This mechanism plays a significant role in the vicinity of interferroelectric transition temperature and morphotropic phase boundary, where energy difference between stable and metastable phases is small. Engineered domain configuration fully exploits this domain wall broadening mechanism. (c) 2007 American Institute of Physics.
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    Electromechanical behavior of 001 -textured Pb(Mg1/3Nb2/3)O-3-PbTiO3 ceramics
    Yan, Yongke; Wang, Yu. U.; Priya, Shashank (AIP Publishing, 2012-05-01)
    [001]-textured Pb(Mg1/3Nb2/3)O-3-PbTiO3 (PMN-PT) ceramics were synthesized by using templated grain growth method. Significantly high [001] texture degree corresponding to 0.98 Lotgering factor was achieved at 1 vol. % BaTiO3 template. Electromechanical properties for [001]-textured PMN-PT ceramics with 1 vol. % BaTiO3 were found to be d(33) = 1000 pC/N, d(31) = 371 pC/N, epsilon(r) 2591, and tan delta = similar to 0.6%. Elastoelectric composite based modeling results showed that higher volume fraction of template reduces the overall dielectric constant and thus has adverse effect on the piezoelectric response. Clamping effect was modeled by deriving the changes in free energy as a function of applied electric field and microstructural boundary condition. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4712563]
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    Grain size effect of phase coexistence around morphotropic phase boundary in ferroelectric polycrystalline ceramics
    Rao, W. F.; Wang, Yu. U. (AIP Publishing, 2008-03-01)
    Computer modeling and simulation reveals a grain size- and composition-dependent behavior of phase coexistence around the morphotropic phase boundaries in polycrystals of ferroelectric solid solutions. It shows that the width of phase coexistence composition range increases with decreasing grain sizes; phase-coexisting domain microstructures effectively reduce elastostatic, electrostatic, and domain wall energies; and grain boundaries impose internal mechanical and electric boundary conditions, which affect the phase-coexisting domain microstructures in the grains and give rise to the grain size effect of phase coexistence. (c) 2008 American Institute of Physics.
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    Microstructures of coherent phase decomposition near morphotropic phase boundary in lead zirconate titanate
    Rao, W. F.; Wang, Yu. U. (AIP Publishing, 2007-07-01)
    (c) 2007 American Institute of Physics. Microstructures of coherent phase decomposition near morphotropic phase boundary in lead zirconate titanate are investigated by using computer modeling and simulation. The model is based on the recently proposed incoherent equilibrium phase diagram [G. A. Rossetti, Jr. , Appl. Phys. Lett. 88, 072912 (2006)] and takes into account the coherency strain energy and electrostatic energy. It reveals characteristic multidomain microstructures, where nanoscale lamellar domains of tetragonal and rhombohedral phases coexist with well-defined crystallographic orientation relationships and produce coherent diffraction effects. The simulated microstructures provide important information for interpretation of diffraction data and identification of phases near morphotropic phase boundary. (c) 2007 American Institute of Physics.
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    Three intrinsic relationships of lattice parameters between intermediate monoclinic M-C and tetragonal phases in ferroelectric Pb (Mg1/3Nb2/3)(1-x)Ti-x O-3 and Pb (Zn1/3Nb2/3)(1-x)Ti-x O-3 near morphotropic phase boundaries
    Wang, Yu. U. (American Physical Society, 2006-01-25)
    Systematic analysis of extensive experimental data confirms the theoretical prediction of three intrinsic relationships of lattice parameters between the recently discovered intermediate monoclinic M-C phase and the conventional tetragonal phase in ferroelectric Pb[(Mg1/3Nb2/3)(1-x)Ti-x]O-3 and Pb[(Zn1/3Nb2/3)(1-x)Ti-x]O-3 near the morphotropic phase boundaries. These intrinsic relationships of lattice parameters are fulfilled by experimental data reported in the literature for different temperatures, compositions, and electric fields. They present quantitative evidence that the intermediate monoclinic M-C phase is a mixed state of nanometer-sized twin-related domains of the conventional ferroelectric tetragonal phase. The analysis supports the concept recently proposed by Khachaturyan and co-workers [Phys. Rev. Lett. 91, 197601 (2003)] that the intermediate monoclinic M-C phase is adaptive ferroelectric and ferroelastic phase, which is homogeneous only on the macroscale while inhomogeneous on the nanoscale. Due to the small domain size and small ferroelastic strain, the conventional diffraction measurement does not resolve the lattice of individual nanodomains rather instead only perceives the average diffraction effect of nanotwins, yielding the experimentally observed monoclinic symmetry. The result indicates that the electric-field-induced domain-wall movement plays an essential role in the ultrahigh electromechanical responses of Pb[(Mg1/3Nb2/3)(1-x)Ti-x]O-3 and Pb[(Zn1/3Nb2/3)(1-x)Ti-x]O-3, and the high-density domain walls associated with the nanotwins have a significant contribution to the peculiar material properties near the morphotropic phase boundaries.