SPONTANEOUS POLARIZATION AND THE DISTRIBUTION OF INTERNAL STRESS AND ELECTRIC FIELDS AROUND A TRIPLE POINT

In this article, distributions of internal stress and internal electric fields around a triple point of ferroelectric polycrystals generated by the spontaneous deformation and spontaneous polarization were investigated. It was found that when all three grains consist of a single domain, the internal stresses and the internal electric fields do not vanish. Though it may be determined according to the principle of energy, the spontaneous configuration will not be unique without involving other conditions due to the symmetry of the crystal structure.

The unique spontaneous polarization property and application of ferroelectric materials in photocatalysis

The unique spontaneous polarization property of ferroelectric material makes it to be a special catalyst in photocatalysis.The spontaneous polarization property can induce the formation of built-in electric field,which can improve the separation of photoelectrons and holes to affect photocatalytic performance.The internal electric field induced by spontaneous polarization can be influenced by multiple factors such as the morphology,the concentration of defect,the type of doped heteroatoms,as well as the composition of heterostructures.Besides,the preparation method,pretreating temperature,the strength of prepolarized external electric field of ferroelectric-based photocatalysts as well as the strength of external mechanical force or external magnetic field in photocatalytic reactions can influence the photocatalytic effectivity via influencing spontaneous polarization-induced internal electric field.Thus,it is urgently to unveil the mystery of structure-activity relationships for ferroelectric materials-based photocatalysts,which is usually uncertain.With this in mind,this review was provided for the role of various complex influencing factors on ferroelectric materials-based photocatalysis based on the latest advancement in the fields of new energy development,environmental remediation.In the beginning,the basic structure and properties of ferroelectric material are given.Then,popular synthesis methods of ferroelectric-based photocatalysts are summarized.After that,two main mechanisms of ferroelectric photocatalysis are discussed.The research progress of ferroelectric photocatalysis is then given emphatically according to the classification of photocatalytic reactions.Finally,the problems existing nowadays and the challenges facing in the future on the application of ferroelectric materials-based photocatalysts are outlined in the summary and outlook section.

Critical behavior of the spontaneous polarization and the dielectric susceptibility close to the cubic-tetragonal transition in BaTiO_(3)

Using Landau mean field model,the spontaneous polarization and the dielectric susceptibility are analyzed as functions of temperature and pressure close to the cubic-tetragonal(ferroelectric-paraelectric)transition in BaTiO_(3).From the analysis of the dielectric susceptibility and the spontaneous polarization,the critical exponents are deduced in the classical and quantum limits for BaTiO_(3).From the critical behavior of the dielectric susceptibility,the spontaneous polarization can be described for the ferro-electric–paraelectric(cubic to tetragonal)transition between 4 and 8 GPa at constant temperatures of 0 to 200K in BaTiO_(3) within the Landau mean field model given here.

SPONTANEOUS POLARIZATION IN UNCONVENTIONAL Ta_(2)O_(5)DIELECTRIC CERAMICS BY LASER SINTERING

Spontaneous polarization was found in Ta_(2)O_(5)dielectric ceramics prepared by laser sintering techmique.After heat corrosion,90°and 180°domains in the ceramics was observed by field emission scanning electron micrographs.The crystal structure of the laser-si ntered ceramics at RT was determined as monocinic phase,which indicates the lattice distortion existed in the ceramics.The variation of O-Ta-O bending vibration mode was deduced from the variation of phonon mode at the low wave-mumber(57 cm^(-1)and 250cm^(-1)).The enhancement of effective electric field for tantalum atoms due to the lattice distortion was determined by calculation of charge distribution cofficients.The distortion of crystal structure due to the phase transformation from higher symmetric orthorhombic phase to lower symmetric monoclinic phase expl ained spontaneous polarization in the laser-sintered Ta_(2)O_(5)ceramics.

On the Spontaneous Electric Field in a Ferroelectric Body

The incompatibility of the spontaneous polarization across the interface between two grains may induce internal stress and electric fields in a ferroelectric body. The built up of stresses and electric fields may seriously affect the functioning of ferroelectric materials. In this article, the effects of the incompatibility of the spontaneous polarization for various types of interfaces on the electric fields are discussed. Numerical results indicate that the incompatibility of the spontaneous polarization may result in the electric field being unbounded when the interface does not have a continuous normal.

Interband Optical Transitions due to Donor Bound Excitons in Wurtzite InGaN Strained Coupled Quantum Dots： Strong Built-in Electric Field Effects

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field （BEF） in the wurtzite InGaN strained coupled quantum dots （QDs）, the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF, and the structure parameters of the QD system.

The first-principles study of ferroelectric behaviours of PbTiO3/SrTiO3 and BaTiOn/SrTiO3 superlattices

We have performed the first-principles calculation to investigate the origins of ferroelectricities and different po- larization behaviours of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. The density of state （DOS） and electronic charge profiles show that there are strong hybridizations between atoms Ti and O and between atoms Pb and O which play very important roles in producing the ferroelectricities of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. Ow- ing to the decline of internal electric field in SrTiO3 （ST） layer, the tetragonality and polarizations of superlattices decrease with increasing the fraction of SrTiO3 in the superlattices. We find that the polarization of PbTiO3/SrTiO3 is largerthan that of BaTiO3/SrTiO3 at the same ratio of components, because the polarization mismatch between PbTiO3 and SrTiO3 is larger than that between BaTiO3 and SrTiO3. The polarization and tetragonality are en- hanced with respect to those of bulk tetragonal BaTiO3 in the superlattices BaTiO3/SrTiO3, while the polarization and tetragonality are reduced with respect to those of bulk tetragonal PbTiO3 in superlattices PbTiO3/SrTiO3.

Theoretical Study of La , Ca modified and Pure PbTiO_3 Nanocrystals

Quantum chemistry calculations of La , Ca modified and pure PbTiO\-3 nanocrystals were carried out by means of the cluster model in DFT DVM calculating program on which the regularity of the charge distribution was drawn. The analysis of the density of state(DOS) showed that when the cubic phase PbTiO 3 nanocrystal is changed into tetragonal phase, the interaction among Ti 3d , O 2p , and Pb 6p atomic orbitals causes the dipole moment and spontaneous polarization on the axis of the crystal, which resulted in the appearance of ferroelectric phase. At the same time, the dipole moment and the intensity of the spontaneous polarization were calculated as well.

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg Landau-Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interracial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectrie materials.

Semi-empirical estimation for enhancing negative thermal expansion in PbTiO_(3)-based perovskites

Generally,most materials expand when heated and contract when cooled,whereas negative thermal expansion(NTE)materials are very rare.As a typical NTE material,PbTiO_(3) and related compounds have drawn particular interest in recent years.The discovery of an enhanced NTE system in PbTiO_(3) is beneficial to deepen our understanding of its mechanism and regulate its properties.At present,the method of discriminating an enhanced NTE material based on PbTiO_(3) is not universal.Here,we propose a semi-empirical method through evaluating the average lattice distortion in related systems to estimate the relative coefficient of thermal expansion conveniently.The rationality of the method was verified by the analysis of the 0.6PbTiO_(3)-0.4Bi(Ga_(x)Fe_(1-x))O_(3) system.So far,all PbTiO_(3)-based compounds with enhanced NTE conform well to this method.This method provides the possibility to find more enhanced NTE PbTiO_(3)-based materials.

Parameter-Dependent Third-Order Susceptibility of In_xGa_(1-x)N/GaN Parabolic Quantum Dots

The electron states confined in wurtzite InxGa1-xN/GaN strained quantum dots （QDs） have been investigated in the effective -mass approximation by solving the Schrtdinger equation, in which parabolic confined potential and strong built-in electric field effect （due to the piezoelectricity and spontaneous polarization） have been taken into account. The real part Rex^（3）（0,0,ω） and the imaginary part Imx^（3）（0,0,ω） of the third-order susceptibil- ity describe quadratic electro-optic effects and electro-absorption process of the QDs respectively. And both of them have been calculated in directions parallel and vertical to z axis. Furthermore, the study shows Rex^（3）（0,0,ω） and Imx^（3）（0,0,ω） increase under resonant conditions with the QDs＇ radius and height increase, and the same results occur when the content increase. In addition, the resonant position shift to the lower energy region when the parabolic frequencies increase.

Phenomena in Polarized Disperse Systems Caused by Exchange-Correlation Interaction between Particles

We have studied the self-consistent states of nano- and micro-particle polarized powders and structures consisting of parallel particle chains and have determined conditions under which the static dielectric permittivity of a disperse system is negative. It has been shown that in such system an electric current runs without ohmic losses. We present the arguments for the physics of spontaneous emergence of the electric field in disperse systems made up of electrically neutral particles. It has been determined the influence the phase boundaries of a disperse system has on the origin of spontaneous polarization state. The structures consisting of parallel chains of dielectric particles can exhibit spontaneous polarization. In this case the properties of the spherical structure are similar to those of the ball lightning. It has been established correspondence of the obtained theoretical results with the experimental data available in the literature.

Construction of spontaneously polarized ceramic via synergistic mechanical activation-Biomimetic mineralization for activating air and water

Spontaneously polarized crystals with intrinsic electric dipole moment have attracted immense interest as excellent functional materials for extensive applications.It is of great significance to engineer sustainable spontaneously polarized materials with fascinating characteristics and performance for activating air and water.Herein,a novel strategy based on the synergy of mechanical activation(MA)and biomimetic mineralization(BM)was created to construct spontaneously polarized ceramic.MA induced the structural damage of clay and promoted the dissolution of ions and the release of free proteins,contributing to the formation of silicate precursor in BM process.After high temperature firing,the silicate precursor in clay was converted to form KCa_(3)AlCa_(3)Si_(4)O_(16)(hexagonal crystal system,L^(6)symmetry type,and P63 space group)in the resulting spontaneously polarized ceramic.The non-centrosymmetric structure of KCa_(3)AlCa_(3)Si4O_(16)and the high intrinsic electric dipole moments contributed by K(1)polyhedrons resulted in high spontaneous polarization(0.2322μC/cm^(2))and far-infrared emissivity(0.951)of spontaneously polarized ceramic.In air,spontaneously polarized ceramic can activate H_(2)O and O_(2)molecules to form negative air ions owing to surface electric field.In water,spontaneously polarized ceramic can disaggregate large water clusters to form small water clusters ascribed to surface electric field and far-infrared emission;water pH can be regulated from weak acidity to approximate neutrality via the capture of electrons by H+ions to produce releasable hydrogen gas.This work provides great promise for rational design and synthesis of spontaneously polarized materials for functional applications.

Polarization properties and crystal structures of ferroelectric(Ba,Ca)TiO_(3) single crystals

We have investigated the spontaneous polarization(P_(s))of Ba_(1-x)Ca_(x)TiO_(3)(BCT)by polarization hysteresis measurements using single crystals and by density functional theory(DFT)calculations.Single crystals of BCT(x=0:07)were grown by a top-seeded solution growth(TSSG)method.The polarization measurements show that the crystals(x¼0:07)have a Ps of 26.0μC/cm^(2),which is slightly small compared with BaTiO_(3)(27.3μC/cm^(2)).Our DFT calculations based on a supercell approach show that Ca atoms are markedly displaced cooperatively with the adjacent Ti atoms along the Ps direction.It is suggested that the CaTiO_(3)-like octahedral rotation is constructed in the BCT supercell around the Ca atoms,which is the origin of the smaller Ps observed for the BCT crystals.

Amplified spontaneous emission, optical waveguide and polarized emission based on 2,5-diaminoterephthalates

A series of 2,5-diaminoterephthalates with a simple structure were synthesized through one-step reaction, and their bar-shaped single crystals with a large size and a smooth surface have been obtained via the solvent-evaporation method. These crystals exhibit bright emission with fluorescence quantum yields higher than 0.2. They display the waveguide property, and low optical loss coefficients for waveguide have been determined for the crystal of one compound. In addition, the crystal can cause linear polarization of the light emitted from it, with a high polarization contrast of 0.70. Most importantly, these crystals can realize amplified spontaneous emission （ASE）, including the red ASE, with appreciable energy thresholds of 72-198 kW/cm^2 and high gain coefficients, which suggests the potential of these crystals for the application in organic solid-state lasers.

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe3O4： A first-principles study

As a single-phase multiferroic material, Fe3O4 exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe3O4and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature mon- oclinic Fe3O4 was investigated by first-principles calculations. The pseudo-centrosymmetric FeB42- FEB43 pair has a different valence state. The noncentrosymmetric charge distribution results in fer- roelectric polarization. The initial ferroelectric polarization direction is in the -x and -z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the FeB42-FeB43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe3O4. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.