Formation Of Spontaneous Polarization Research Articles

Trends of complete anion substitution on electronic, ferroelectric, and optoelectronic properties of BiFeX3 (X = O, S, Se, and Te)

BiFeO3 is one of the most important perovskite ferroelectric materials. It is known that cation substitution in BiFeO3 can greatly modulate its fundamental physical properties for various applications. However, the role of anion substitution in BiFeO3 is still largely unknown. In this article, using first-principles calculations, we have systematically investigated the trends of electronic, ferroelectric, and optoelectronic properties of BiFeX3 as a function of complete anion evolution. Interestingly, the calculated bandgaps of BiFeX3, in the favorable G-type antiferromagnetic configuration, decrease from 3.32 eV (X = O) to 1.27 eV (X = S) to 0.75 eV (X = Se) and to 0.27 eV (X = Te) due to the enhanced Fe d and X p orbital hybridization at the bottom of the conduction band of BiFeX3, providing an insight to explore narrow bandgap ferroelectrics. Meanwhile, it is found that the value of spontaneous polarization of BiFeX3 decreases along with the increase in dipole moment from X = O to X = Te. In addition, the ferroelectric reversal barrier of BiFeX3 increases from X = O to X = Te, indicating that an increased external field is required to flip the opposite ferroelectric phases. The role of covalency of Bi–X bonds is revealed in the formation of spontaneous polarization. Furthermore, in the visible-light energy range, the increased absorption coefficient has been observed for larger anions and significant optical anisotropy has been found in different energy ranges for different BiFeX3 systems. Our work provides a fundamental understanding on the electronic, ferroelectric, and optoelectronic properties of complete anion substitution in BiFeO3.

Ferroelectric Polarization in CH3NH3PbI3 Perovskite.

We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in the dark and under illumination. Perovskite crystals with three different sizes of 700, 400, and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photoinduced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of the electric field, in which larger crystals showed longer retention behavior compared to the smaller sized one. Additionally, we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of the ferroelectric material even though insignificant size dependency in electric polarization was observed.

A micromechanics-based model for temperature effect on piezoelectric and dielectric constants of ferroelectric crystals

A micromechanics-based thermodynamic model is applied to study the piezoelectric and dielectric constants of BaTiO3 crystals as a function of temperature upon the cooling from the Curie point. The formation of spontaneous polarization at the beginning of cooling provides dramatic change in symmetry of the system. This plays an important role for the sudden change in piezoelectric constant of d31 at the beginning of the cooling. In this article, by considering such a heterogeneous nature of the crystal at the beginning of the cooling due to spontaneous polarization, the micromechanics-based model is able to evaluate the piezoelectric coefficient d31. To verify this modified model, the dielectric constant k33 of a BaTiO3 crystal in terms of temperature is also calculated and compared with the existing micromechanics-based model in the literature [Y. Su and G. J. Weng, J. Mech. Phys. Solids 53, 2071 (2005)]. It shows agreeable and improved result in dielectric constant k33 of BaTiO3. Both dielectric and piezoelectric constants are also found to be consistent with the available experimental data.

Strain induced ferroelectricity in the SrZrO 3 / SrTiO 3 superlattice: First principles study

The lattice distortion strain induced ferroelectricity in SrZrO 3/ SrTiO 3 superlattice is studied using first principles density functional theory. Within the tetragonal symmetry P 4 mm , the lattice distortion from the lattice mismatch in the superlattice structure is determined through energy minimization. This kind of lattice distortion leads to the formation of spontaneous polarization in the superlattice, although neither SrZrO 3 nor SrTiO 3 is ferroelectric. From analysis of the relative displacements of the cations and anions, we have found that the SrZrO 3 cell may make a greater contribution to the polarization in the SrZrO 3/ SrTiO 3 superlattice than the SrTiO 3 cell. An extremely large polarization of 42.7 μC/cm 2 has been predicted.

Spontaneous Polarization in One-DimensionalPb(ZrTi)O3Nanowires

Formation of spontaneous polarization in one-dimensional (1D) structures is a key phenomenon that reveals collective behaviors in systems of reduced dimensions, but has remained unsolved for decades. Here we report ab initio studies on finite-temperature structural properties of infinite-length nanowires of Pb(Zr0.5Ti0.5)O3 solid solution. Whereas existing studies have ruled out the possibility of phase transition in 1D chains, our atomistic simulations demonstrate a different conclusion, characterized by the occurrence of a ferroelectric polarization and critical behaviors of dielectric and piezoelectric responses. The difference is accounted for by the use of depolarizing effects associated with finite thickness of wires. Our results suggest no fundamental constraint that limits the use of ferroelectric nanowires and nanotubes arising from the absence of spontaneous ordering.

ChemInform Abstract: Polarity of Organic Supramolecular Materials: A Tunable Crystal Property

Abstract Basic principles are reviewed which provide new access to the synthesis of acentric molecular crystals along with an application to the design of intermolecular interactions and the crystallization of polar host–guest materials. Because of the process of crystal growth, the primary confinement for the alignment of dipolar molecules is given by a surface–nutrient interface and not by the bulk state as usually assumed for the prediction of crystal structures by computational methods. Subject to defined restrictions, spontaneous polarity formation in slowly growing crystals can be regarded as a simple isomerization reaction, ↓ ⇌↑(↓, ↑, orientations of the dipole moments in crystals). Following results of Monte Carlo simulations, performed for a surface layer (adlayer) on a nonrelaxing substrate layer, we conclude the following: (i) Although binding motifs (synthons) are important to induce 1D or 2D order into chains, ribbons, and planes, lateral interactions between such structural elements enter polarity formation by a much higher weight factor. (ii) Particular lateral interactions favoring ↓···↓ are not necessary to obtain polarity in some molecular crystals. Channel-type inclusion materials represent a solution to (ii). Experimental confirmation is provided by, e.g., a large number of polar inclusion compounds of perhydrotriphenylene (PHTP). It is shown that in general a combination of van der Waals interactions for a 2D confinement (alignment of molecular frames) and one of the most nonbonding recognition motifs (↓ vs ↑ orientation) can optimize polarity formation in host–guest lattices. In essence, we review that in some molecular crystals polarity is a tunable property, and that a supramolecular synthesis can produce a material and a property by parallel reactions.

Polarity of Organic Supramolecular Materials: A Tunable Crystal Property

Basic principles are reviewed which provide new access to the synthesis of acentric molecular crystals along with an application to the design of intermolecular interactions and the crystallization of polar host–guest materials. Because of the process of crystal growth, the primary confinement for the alignment of dipolar molecules is given by a surface–nutrient interface and not by the bulk state as usually assumed for the prediction of crystal structures by computational methods. Subject to defined restrictions, spontaneous polarity formation in slowly growing crystals can be regarded as a simple isomerization reaction, ↓ ⇌↑(↓, ↑, orientations of the dipole moments in crystals). Following results of Monte Carlo simulations, performed for a surface layer (adlayer) on a nonrelaxing substrate layer, we conclude the following: (i) Although binding motifs (synthons) are important to induce 1D or 2D order into chains, ribbons, and planes, lateral interactions between such structural elements enter polarity formation by a much higher weight factor. (ii) Particular lateral interactions favoring ↓···↓ are not necessary to obtain polarity in some molecular crystals. Channel-type inclusion materials represent a solution to (ii). Experimental confirmation is provided by, e.g., a large number of polar inclusion compounds of perhydrotriphenylene (PHTP). It is shown that in general a combination of van der Waals interactions for a 2D confinement (alignment of molecular frames) and one of the most nonbonding recognition motifs (↓ vs ↑ orientation) can optimize polarity formation in host–guest lattices. In essence, we review that in some molecular crystals polarity is a tunable property, and that a supramolecular synthesis can produce a material and a property by parallel reactions.

Spontaneous polarity formation in thin crystalline films of host–guest materials

Statistical mechanics calculations show that in thin crystalline films of channel-forming host–guest lattices asymmetric but achiral molecules undergo spontaneous polar order under conditions of thermal equilibrium. Depending on the state of the substrate, unipolar or bipolar layers are obtained. For less than about L=25 layers, a nearly homogeneous polarisation 〈 S x 〉 ( x=0, 1, …, L; x is the layer index) close to its maximum values normalised to 1 or −1 is possible. A comparison of the polarisation state under conditions of either thermal equilibrium or metastable states resulting from a Markov process of growth reveals similar functions of 〈 S x 〉 up to a sufficiently low L for both cases when assuming particular, but realistic, interaction energies. Using crystalline host–guest thin films opens up the possibility of obtaining polar order of guest molecules, which, when crystallised as pure compounds into layers or the bulk, would to a high probability form centrosymmetric structures.

A New Organic Nanoporous Architecture: Dumb-Bell-Shaped Molecules with Guests in Parallel Channels

A new type of dumb-bell-shaped host molecule (6-8) has been synthesised, of which 1,8-bis((1)-adamantyl)-1,3,5,7-octatetrayne (8 = BAOT) forms an open porous architecture when cocrystallised with a number of typical solvent molecules. Adamantyl substituents attached to a tetraalkyne spacer build up the walls of parallel channels wherein guest molecules are aligned. Surprisingly, the tetraalkyne unit is significantly bent. Desolvation experiments provide evidence for a reversible inclusion of guests. In the case of the inclusion of 2-butanone, a partial substitution by symmetrical and asymmetrical long-chain chromophores during crystallisation was possible. Stained crystals showed optical frequency doubling. The crystal structure analysis revealed a centric space group, although considerable translational and orientational disorder was present. Application of scanning pyroelectric microscopy revealed that the growth of inclusion compounds with 2-butanone produced polar ordering of guest molecules, which were aligned in two macro-domains of opposing polarity. The resulting orientation of the carbonyl dipoles is in agreement with the theoretical prediction of a Markov model of spontaneous polarity formation based on molecular recognition processes on growing crystal faces. The present case represents a new example of a property-driven supramolecular synthesis.

A theoretical base for optimising intermolecular interactions driving polarity formation in channel-type host–guest materials

Spontaneous polarity formation in channel-type host–guest materials is analysed by a Markov model of growth. Criteria for the strength of collinear intermolecular interaction energies E ij between acceptor–π–donor (A–π–D) guest molecules are discussed in order to optimise characteristic properties of the system. In the case of bulk growth, the sequence E AA≫ E AD≈ E DD is producing (i) high values of net polarity n net, (ii) a low number of attachments q to reach constant values of n net, and (iii) a low density of orientational defects η within polar chains. Similarly, the optimum sequence for the growth of oriented films on substrates featuring uniform interactions to guest molecules is: E AA> E DD≫ E AD.

VARIATION OF INTERCHAIN POTENTIAL DURING MOLECULAR CHAIN ROTATION IN POLY (VINYLIDENE FLUORIDE)

The variation of interchain potential in poly (vinylidene fluoride) polymer during chain rotation was calculated.In the case that center chain rotates while all other chains keep static as discussed by “six-site-potential” theory,the potential curves show four high potential barriers owing to the steric effect of fluorine atom from adjacent molecule,which is in contrast to the six-fold sysmmetry pre-dicted by the “six-site-potential” model.It was found that if all chains rotate synchronically, the barrier height will reduce dramatically.The result also indicates that the polarization reversal is ac-complished with the change of lattice constant.The finally obtained barrier height is about 7.5kJ/mol.In comparison with the thermal energy of chain rotation at room temperature (about2.5kJ/mol),the potential well is too shallow to form stable spontaneous polarization.It was specu-lated therefore,the formation of spontaneous polarization in these materials should include some ex-ternal factors.

Nanoporous and mesoporous organic structures: new openings for materials research

Organic solid state and molecular architectures providing nano- or mesoporous space for the inclusion of guest species open up new opportunities for the synthesis of materials showing designed chemical and physical properties. Porous materials are reviewed by taking into account recent progress in inorganic zeolites, metalloorganic and organic networks including carbon nanotubes and condensed cyclodextrins. Various properties requiring a parallel alignment of close-packed guests are addressed. As a particular example of physical property design, spontaneous polarity formation in channel-type inclusion compounds is discussed.

Interaction between electric field and liquid crystals with spontaneous polarization: Derivation of suitable free energy expression for a cell with an applied voltage

Abstract The interaction between electric fields and liquid crystals is reconsidered, taking into account spatially varying anisotropic dielectric constant and ferro- resp. flexoelectricity. The general static case as well as planar samples with applied constant voltage is treated, and explicit formulas for the free energy are given. Some results: The consequences of the simple fact that the curl of the electric field is zero have been overlooked in many articles. The Landau free energy expansion for the chiral smectic C phase have been developed by R.B. Meyer1 and Pikin-Indenbom2. In the Pikin-Indenbom theory the P2-term, controlling the size of the spontaneous polarization, has been given the coefficient ½ χ, where χ is the dielectric susceptibility. However, there are several independent sources to the dielectric susceptibility, and not all of them do contribute to the formation of spontaneous polarization. This indicates that the size of the spontaneous polarization and the size of the induced polariz...

Electric phenomena at phase transitions with unit cells multiplication

The phase transition linked in some way or other with electric polarization could be proper, antiferroelectric, improper and incommensurate. In the case of proper transition spontaneous polarization PS can be taken as a transition parameter. In the case of antiferroelectric transition the primary transition parameter is PS as well, the multiplication of initial unit cells take place but the multiplied unit cell does not have the resulting polarization. The characteristic of improper transitions is a formation of spontaneous polarization through the doubling of ''axial''(non polar) units.