On the origin of irreproducible behaviour of Na1−xLixNbO3 solid solutions

The irreproducibility of electrical properties of Na_0.88Li_0.12NbO₃ solid solution on thermal cycling reported by M.A.L. Nobre and S. Lanfredi is explained by gradual decomposition of the supersaturated solid solution below ca. 800 °C.     

The phase diagram of KNO3-KClO3

The binary phase diagram of KNO₃-KClO₃ is studied by means of differential scanning calorimetry (DSC) and high-temperature X-ray diffraction. The limited solid solutions, K(NO₃)_1−x(ClO₃)_x (0<x<0.20) and K(NO₃)_1−x(ClO₃)_x (0.90<x<1.0), were formed in the KNO₃-based solid solutions and KClO₃-based solid solutions phase, respectively. For KNO₃-based solid solutions, KNO₃ ferroelectric phase can be stable from 423 to 223 K as a result of substituting of NO₃ by ClO₃-radicals. The temperatures for solidus and liquidus have been determined based on limited solid solutions. Two models, Henrian solution and regular solution theory for KNO₃-based (α) phase and KClO₃-based (β) phase, respectively, are employed to reproduce solidus and liquidus of the phase diagram. The results are in good agreement with the DSC data. The thermodynamic properties for α and β solid solutions have been derived from an optimization procedure using the experimental data. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent.     

Effect of Li2CO3 addition on the structural,optical, ferroelectric,and electric-field-induced strain of lead-free BNKT-based ceramics

In this work, we reported the effect of Li₂CO₃ addition on the structural, optical, ferroelectric properties and electric-field-induced strain of Bi_0.5(Na,K)_0.5TiO₃ (BNKT) solid solution with CaZrO₃ ceramics. Both rhombohedral and tetragonal structures were distorted after adding Lithium (Li). The band gap values decreased from 2.91 to 2.69 eV for 5 mol% Li-addition. The maximum polarization and remanent polarization decreased from 49.66 μC/cm² to 27.11 μC/cm² and from 22.93 μC/cm² to 5.35 μC/cm² for un-doped and 5 mol% Li- addition BNKT ceramics, respectively. The maximum S_max/E_max value was 567 pm/V at 2 mol% Li₂CO₃ access. We expected this work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT materials.     

Mechanisms and origin of multiferroicity

Motivated by the potential applications of their intrinsic cross-coupling properties, the interest in multiferroic materials has constantly increased recently, leading to significant experimental and theoretical advances. From the theoretical point of view, recent progresses have allowed one to identify different mechanisms responsible for the appearance of ferroelectric polarization coexisting—and coupled—with magnetic properties. This chapter aims at reviewing the fundamental mechanisms devised so far, mainly in transition-metal oxides, which lie at the origin of multiferroicity.     

Modification of dielectric relaxations in sodium bismuth titanate with samarium doping

Na_0.5Bi_(0.5−x) Sm_xTiO₃ (NBST) ceramics with x=0.05, 0.1, and 0.15 are prepared through chemical route. The X-ray diffraction studies confirmed the formation of single phase. Dielectric measurements in the temperature region ranging from room temperature (∼30 °C) to 600 °C at different frequencies (10 kHz-1 MHz) showed anomalies at 130, 306, and 474 °C (at 10 kHz frequency) for x=0.05 sample. Other samples showed only two peaks. To establish the electrical nature of these relaxations, impedance measurements are done at different temperatures and frequencies. The relaxation time, obtained from both impedance and modulus data, is found to decrease with increase in temperature. The relaxations observed are of non-Debye type. Increase in samarium content increases the activation energy for relaxation.     

First-principles study on structural,electronic and ferroelectric properties of Bi2VO5.5 compound

A first-principles investigation of the origin of ferroelectricity in the Aurivillius compound Bi₂VO_5.5 is presented. Calculations with the density functional theory, in conjunction with the modern theory of polarization, allowed us to study the structural, electronic, and polar properties of two configurations built with oxygen vacancies, causing a charge imbalance and a subsequent displacement of the ions, generating two maximum polarizations, one of 14.75 μC/cm² and one of 4.31 μC/cm² along [011] direction. Electron localization function schemes were used to identify the asymmetry of charges in (001), (010) and (100) planes. The results obtained in this study establish that the origin of ferroelectricity is due to the displacement of the ions caused by oxygen vacancies and the asymmetric distribution of the isolated pair of Bi ions. On the other hand, the bandgap calculations and the results of P_s establish that Bi₂VO_5.5 is a candidate ferro-photovoltaic material.     

Helical unwinding in polymer-dispersed ferroelectric liquid crystals

We demonstrate that ferroelectric liquid crystals dispersed in a polymer matrix can form uniaxially aligned elliptical droplets. The alignment is controlled by mechanical shear during the polymerization of a UV-curable adhesive. The possibility of prealignment makes polymer-dispersed ferroelectric liquid crystals suitable for application in flexible electro-optical displays. In contrast to surface stabilized ferroelectric liquid crystal displays, the electro-optical effect in our system is due to the deformed-helix ferroelectric (DHF) effect.     

Frustration between syn- and anticlinicity in mixtures of chiral and non-chiral tilted smectic-C-type liquid crystals

We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC a* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and SmC a*, transition via the SmC_β* and SmC_γ* subphases, or by “escaping” the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC a* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected even in this case, suggesting that chirality --although necessary-- does not have quite the importance that has previously been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order.     

Pyroelectric nano-rods grown inside alumina nano-pores

This paper reports the formation and properties of nano-composite pyroelectric thin films. They consist of pyroelectric triglycine sulfate (TGS) single-crystal nano-rods grown inside a highly dense array of alumina pores (about 65 nm diameter and density of 1011 cm⁻²). The nucleation and growth of the TGS single crystals are obtained by precipitation from a supersaturated aqueous solution. Nucleation is preferred only at the bottom of the pores due to a tight control of temperature, composition and pore diameter. Growth of single crystals with preferred crystallographic orientation is obtained with the aid of an applied electric field. Various crystallographic orientations (1 0 0) (−1 1 0) (−1 2 0) are obtained separately as a single preferred orientation by changing the amplitude of the electric field during crystal growth. The films exhibit ferroelectric behavior.