Competition between ferroelasticity and ferroelectricity

In the crystals of the KH₂PO₄ family, the properties of ferroelasticity and ferroelectricity strongly interact. It is for example possible to change by application of an electric field the orientation of the phase front between high temperature and low temperature phases, and to observe the rotation of reticular planes during an electric hysteresis cycle drawing. These phenomena, as the existence of a quasi constant domain width illustrate the particular competition between ferroelasticity and ferroelectricity in these compounds.     

The effect of some transition metal oxides on the physical properties of K0.5Na0.5Nb0.95Ta0.05O3 ceramics

Abstract

Both K_0.5Na_0.5Nb_0.95Ta_0.05O₃ (KNNTO) and (K_0.5Na_0.5Nb_0.95Ta_0.05O₃)_0.99-M_0.01, M = Co₃O₄ and Mn₂O₃ (M/KNNTO) Ferromagnetic behaviour was observed for some M/KNNTO compounds. The hardness and compressive strength of all investigated samples are given. Comparisons with similar materials are discussed. Ceramics were synthesised using a solid-state reaction method. X-ray diffraction patterns of all samples revealed that the crystal structure is orthorhombic. Field-emission scanning electron microscopy was performed. Polarisation hysteresis curves indicated a disruption of ferroelectric order with the addition of M into KNNTO ceramics. The dielectric properties of the investigated ceramics have been studied as a function of frequency and temperature.     

Multiferroic properties of sol-gel derived Bi5Fe1-xCoxTi3O15 thin films

Co-doped Bi 5 FeTi 3 O 15 thin films (BFCT-x,Bi 5 Fe 1-x Co x Ti 3 O 15) were prepared using a sol-gel technique.XRD patterns confirm their single phase Aurivillius structure,and the corresponding powder Rietveld analysis indicates the change of space group around x=0.12.The magnetic hysteresis loops are obtained and ferromagnetism is therefore confirmed in BFCT-x thin films.The remanent magnetization (M r) first increases and reaches the maximum value of 0.42 emu/cm 3 at x=0.12 due to the possible Fe 3+-O-Co 3+ ferromagnetic coupling.When x=0.25,the M r increases again because of the dominant Fe 3+-O-Co 3+ ferromagnetic coupling.The remanent polarization (2P r) of BFCT-0.25 was measured to be as high as 62 μC/cm 2,a 75% increase when compared with the non-doped BFCT-0 films.The 2P r remains almost unchanged after being subjected to 5.2 × 10 9 read/write cycles.Greatly enhanced ferroelectric properties are considered to be associated with decreased leakage current density.     

热收缩化合物——负热膨胀性及成因

文章综述了负热膨胀化合物、负热膨胀机理与应用等方面的进展.负热膨胀是最近十多年来新兴的研究领域,目前已经发现较多化合物具有负热膨胀性能,它们广泛分布在类似ZrW2O8开放式框架结构化合物、磁性合金、反钙钛矿结构的Mn3AX、PbTiO3基铁电化合物、纳米颗粒等领域.在负热膨胀机理研究方面,原子热振动机理研究相对充分,成功地解释了一部分框架式结构化合物负热膨胀机理;然而,较多负热膨胀起源与非振动机理相关,如：物质磁性、铁电性、电子作用、纳米尺寸效应等.文章最后从实际应用角度出发对未来负热膨胀材料研究进行了展望.     

About the microstructure of PCVD prepared crystal mats of statistical oligo-vinylidene-fluoride-trifluoroethylene in relation to other fluorinated polymers

The room temperature phase of the physical/chemical vapor-deposited statistical co-oligomer VDF/TrFE(70/30) has been characterized by different experimental methods such as small-angle x-ray scattering, wide-angle x-ray diffraction, size-exclusion chromatography, infrared absorption and optical refractometry. The characterization of the elastic properties was carried out using high-performance Brillouin spectroscopy in connection with special scattering geometries. The co-oligomer VDF/TrFE(70/30) was obtained by cracking the parent statistical copolymer and subsequently vapor depositing the shortened chains on highly oriented PTFE substrates (PIA-technique). The room temperature phase of the resulting oriented waxy crystal mats of VDF/TrFE(70/30) is predominantly ferroelectric. The physical properties are very different to those of similarly PIA-prepared n-alkanes, perfluoroalkanes, and blockfluoroalkanes. The microstructure of VDF/TrFE(70/30) is interpreted in terms of partially crystalline nano-sized structures giving rise to a marked freezing process below room temperature. © 1995 John wiley & Sons, Inc.     

Ferroelectric poly(vinylidene fluoride) PVDF films derived from the solutions with retainable water and controlled water loss

To obtain β‐phase dominant ferroelectric poly(vinylidene fluoride) (PVDF) homopolymer thin films on aluminum‐coated silicon substrates, the retaining and loss of water were manipulated by introducing several hydrated and hygroscopic chemicals in the precursor solutions, including aluminum nitrate nonahydrate, aluminum chloride hexahydrate, chromium nitrate nonahydrate, tetra‐n‐butylammonium chloride, and one hygroscopic but nonhydrated chemical, ammonium acetate. Their ability of retaining water during the thermal annealing of the films and the relationship between water retaining and the effects on promoting the β phase were investigated. The results showed an ideal scenario was that the added hydrated salts should be able to retain substantial amount of water during the PVDF crystallization to effectively promote the β phase but completely dehydrate or decompose at the further elevated annealing temperature in order to obtain β‐phase dominant PVDF film without substantially incorporating water and deteriorating the electrical properties. As one of the hydrated chemicals well satisfying the above requirements, Al(NO₃)₃·9H₂O, of different amounts was introduced to the PVDF precursor solutions and the optimal resulting β‐phase dominant ferroelectric PVDF thin films exhibited smooth morphology, low dielectric loss, high remnant polarization of 89 mC/m², and large effective piezoelectric coefficient d₃₃ of ?14.5 pm/V (under the clamping of the substrate). © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2410–2418, 2009     

Ferroelectric behavior and structural change of poly(trimethylene terephthalate) induced by external electric field

The ferroelectric behavior and structural change induced by the external electric field were observed for poly(trimethylene terephthalate) (PTT), which has nonpolar crystalline structure with carbonyl groups aligned antiparallel to each other. The displacement‐electric field hysteresis curves of PTT films showed hysteresis above 140 °C, which indicated the polarization reversal of PTT. The pyroelectric constant and thermally stimulated depolarization current suggest the depolarization of the polar structure of PTT induced by the external electric field above 150 °C. An endothermic peak caused by the depolarization was also observed in the differential scanning calorimetry for the PTT film poled at 30 MV/m. The change of the crystalline structure and the orientation of carbonyl groups induced by the electric filed were investigated by X‐ray diffraction patterns and infrared spectra. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1991–1996, 2009     

Ferroelectricity in hexagonal YFeO3 film at room temperature

In this paper we report the leakage current,ferroelectric and piezoelectric properties of the YFe O3film with hexagonal structure,which was fabricated on Si(111)substrate by a simple sol-gel method.The leakage current test shows good characteristics as the leakage current density is 5.4×10-6A/cm2under 5 V.The dominant leakage mechanism is found to be an Ohmic behavior at low electric field and space-charge-limited conduction at high electric field region.The P–E measurements show ferroelectric hysteresis loops with small remnant polarization and coercive field at room temperature.The distinct and switchable domain structures on the nanometer scale are observed by piezoresponse force microscopy,which testifies to the ferroelectricity of the YFe O3film further.     

E–mv coupling of vibrational overtone in organic conductors: Relationship to optical nonlinearities and ferroelectricity

A strong electron–phonon coupling effect, referred to as electron–molecular vibration (e–mv) coupling, induces characteristic vibronic signals in the vibrational spectra of organic conductors. This paper discusses a new spectroscopic signal induced by the e–mv coupling effect, and the physical implications of its emergence. This vibronic signal, attributable to an overtone of a molecular vibration, appears with an anti-resonance form in the infrared spectrum of some mixed-valency complexes when the compounds undergo a charge-ordering transition. Based on the results of cluster model calculations, the activation of the overtone signal is interpreted as an indication of the generation of anharmonicity in the electronic potential. This analysis suggests that the series of the complexes should possess potential nonlinear optical abilities, though these compounds have long been viewed as electric conductors having the opposite characteristics of dielectrics that show nonlinear optical properties.     

A Ferroelectric Metallomesogen Exhibiting Field-Induced Slow Magnetic Relaxation

Magnetoelectric (ME) materials exhibiting coupled electric and magnetic properties are of significant interest because of their potential use in memory storage devices, new sensors, or low-consumption devices. Herein, we report a new category of ME material that shows liquid crystal (LC), ferroelectric (FE), and field-induced single molecule magnet (SMM) behaviors. Co(II) complex incorporating alkyl chains of type [Co(3C₁₆-bzimpy)₂](BF₄)₂ ( 1 ; 3C₁₆-bzimpy=2,2’-(4-hexadecyloxy-2,6-diyl)bis(1-hexadecyl-1H-benzo[d]imidazole)) displayed a chiral smectic C mesophase in the temperature range 321 K–458 K, in which distinct FE behavior was observed, with a remnant polarization (88.3 nC cm⁻²). Complex 1 also exhibited field-induced slow magnetic relaxation behavior that reflects the large magnetic anisotropy of the Co(II) center. Furthermore, the dielectric property of 1 was able to be tuned by an external magnetic field occurring from both spin-lattice coupling and molecular orientational variation. Clearly, this multifunctional compound, combining LC, FE, and SMM properties, represents an entry to the development of a range of next-generation ME materials.