摘要:

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 cases of ferroelectric microdomains of tetragonal (FEt) symmetry are considered. It is shown that a nano‐scale 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 (IPS) to the parent cubic lattice, and the value of the self‐adjusted parameters constitutes a mixture of the lattice constants of the parent and product phases. Experimental investigations of Pb(Mg1/3Nb2/3)O3‐PbTiO3(PMN‐PT) and Pb(Zn1/3Nb2/3)O3‐PbTiO3(PZN‐PT) single crystals confirm many of the predictions of this theory. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609941

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

It is now well established that the unique properties of relaxor ferroelectrics are due to the presence of polar nanoregions (PNR’s). We present recent results from Neutron and Raman scattering of single crystals of PZN, PZN‐xPT, and PMN. Both sets of measurements provide information on the condensation of the PNR’s and on their slow dynamics, directly through the Central Peak and, indirectly, through their coupling to transverse phonons. A comparative analysis of these results allows identification of three stages in the evolution of the PNR’s with decreasing temperature: a purely dynamic stage, a quasi‐static stage with reorientational motion and a frozen stage. A model is proposed, based on a prior study of KTN, which explains the special behavior of the transverse phonons (TO and TA) in terms of their mutual coupling through the rotations of the PNR’s. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609942

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The effects of Ti and Zr on the structure and ordering in the (1 −x)Pb(Sc2/3W1/3)O3‐ (x)PbTiO3(PSW‐PT) and (1 −x)Pb(Sc2/3W1/3)O3‐ (x)PbZrO3(PSW‐PZ) systems were studied using synchrotron x‐ray and neutron diffraction. Rietveld refinement was carried out to determine the average long‐range crystallographic structure and pair distribution function (PDF) analysis to probe the local displacements of the atoms. Forx< 0.25 the B‐cations form a 1:1 ordered doubled perovskite structure (space groupFm3¯m). The refined occupancies were consistent with the “random site model”, where the ordered structure consists of one B‐sublattice occupied by Sc and the other by a random mixture of the remaining cations. The B‐site order is reduced by incorporation of Zr, but highly stabilized by Ti with the degree of order in excess of 95&percent; forx⩽ 0.25. The results of PDF analysis show that on the local scale the Pb and O atoms are significantly displaced from their average lattice positions. The PDF curves were simulated by several models of simple Pb and O shifts. The short‐range PDF of PSW could be approximated by allowing Pb shifts along [100] and rotations of BO6octahedra around [101¯]. This model was inadequate for a longer distances (r> 4.25 Å) suggesting the real cation displacements are more complicated. Distortions of the local structure in the PSW‐PT system were modeled also by density functional theory calculations. The obtained magnitudes of local Pb displacements coincide with the temperature of paraelectric transitionT&Vegr;,max. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609943

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The results of X‐ray and dielectric studies of ordering effects in as‐grown (1‐x) PbSc1/2Ta1/2O3‐(x)PbSc1/2Nb1/2O3(PST‐PSN) and (1‐x)PbMg1/3Nb2/3O3‐(x)PbSc1/2Nb1/2O3(PMN‐PSN) single crystals are reported. Regularities of cation ordering during the crystal growth seem to be the same for solid solution crystals of both 1:1 and 1:2 ternary perovskites. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609944

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We report first principles studies of the lattice instabilities of as yet unsynthesized perovskites based on alloys in the (Th,Pb,Ba,Bi,Y)ScO3system. These systems are characterized by strong structural instabilies of the cubic perovskite lattice and A‐site driven ferroelectricity. Tetragonal ferroelectric structures in these systems are found to show rather large values of thec/aratio, which when present near a morphotropic phase boundary are thought to be associated with large piezoelectric actuation. The relationship between octahedral rotational and ferroelectric instabilities is discussed as are the conditions for obtaining a morphotropic phase boundary. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609945

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Using DFT calculations, we investigate the local structure and the distortion patterns of the perovskite AgNbO3to examine the feasibility of using silver on the perovskite A‐site in lead‐free piezoelectrics. Our calculations show that in a 5‐atom AgNbO3unit cell, silver atoms can off‐center by about 0.5 Å, forming short covalent bonds similar to the short Pb&sngbnd;O bonds in Pb‐based ferrroelectrics. In the more realistic 40‐atom supercell, Ag behaves similar to Pb in PbZrO3, forming short covalent Ag&sngbnd;O bonds through a combination of large octahedral rotations and small Ag off‐center distortions. Unlike PbZrO3, AgNbO3is not antiferroelectric due to the presence of large Nb off‐center distortions. In the AgNbO3‐PbTiO3solid solution, both A‐cations off‐center significantly and display a preference for distortion away from Nb atoms and toward Ti atoms. The interplay between this preference and maximization of local dipole alignment should lead to an MPB in the AgNbO3‐PbTiO3phase diagram. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609946

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Structural properties of BaTiO3quantum dots are studied using a first‐principles derived effective‐Hamiltonian approach. Truncated long‐range dipole‐dipole interaction and modification of the short‐range interaction due to the existence of vacuum are taken into account. Our calculations show that there are significant off‐center atomic displacements in these dots; the amplitudes of such displacements are comparable with those occuring in bulk BaTiO3. However, unlike in the bulk system, the net polarization in dots is found to be zero. Our results also show that the local displacements in the dots tend to flip across the distance of the entire dot, resulting in an unusual and complex pattern. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609947

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Lattice dynamics for several ordered supercells with composition PbMg1/3Nb2/3O3(PMN) were calculated with first‐principles frozen phonon methods. Nominal symmetries of the supercells studied are reduced by lattice instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and simulated infrared (IR) reflectance spectra are reported. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609948

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Dielectric permittivity and domain structures have been measured as functions of temperature, frequency and E‐field on a ⟨102⟩‐cut PMN‐31&percent;PT single crystal. By using relations of optical indicatrices and extinction, the unpoled PMN‐31&percent;PT crystal was evidenced to possess a rhombohedral phase at room temperature. As temperature increases, the extinction angles don’t exhibit apparent change until ∼350 K, indicating that the crystal keeps a mostly rhombohedral phase below ∼350 K. Above ∼350 K, the extinction angles begin to rotate through the monoclinic phase and then reach the tetragonal phase for most domains near 400 K. Near 410 K, the crystal transforms into the isotropic cubic phase. In addition, E‐field (along the [102] direction) dependent domain structures have also been observed at room temperature. The optical indicatrices of the domain matrix begin to rotate near E= 3.25 kV/cm through monoclinic polarizations. However, the crystal doesn’t exhibit total extinction even under an electric field of E= 37 kV/cm, indicating that a field‐induced single domain has not been reached. The domain structures show a strong hysteresis behavior during processes of increasing and decreasing E‐field. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609949

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A monoclinic phase is evidenced between rhombohedral and cubic phases in a ⟨111⟩‐cut single crystal PMN‐33&percent;PT from polarizing microscope observation of domain structures. Some rules for interpreting these observations for various cuts are reviewed, to illustrate how particular cuts are useful for distinguishing among particular types of phases and domains. Near 360 K the structure begins to distort from the rhombohedral toward the tetragonal phase through monoclinic domains (probablyMA‐type but perhapsMB‐type). However, the present ⟨111⟩‐cut crystal seems to disfavor the tetragonal phase and persists in the monoclinic phase up to T∼420 K, where the cubic phase begins to develop. Temperature‐dependent orientations of optical indicatrices of domains indicate polarization rotations within the monoclinic planes. In addition, a previous electric‐field‐cooled process enhances long‐range order in domain patterns observed upon heating. At room temperature, with increasing E‐field along [111], the crystal undergoes successive phase transformationsviapolarization rotations (perhapsMA‐type),i.e.rhombohedral → tetragonal (associated with 90°‐domain walls) → [111] rhombohedral. As E‐field decreases, very different domain patterns appear (as compared with those with increasing field) which indicate a strong thermal hysteresis relation between E‐field and strain. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1609950

出版商:AIP    

年代:1903

数据来源: AIP