Measurements of strain and the optical indices in the ferroelectric Ba0.4Sr0.6Nb2O6: Polarization effects

Improvements in piezoelectric and energy harvesting properties with a slight change in depolarization temperature in modified BNKT ceramics by a simple technique

For many BNT-based ceramics, an attempt to increase the piezoelectric properties usually results in a decrease in depolarization temperature (T _d). This trend limits the applications of the materials. Many previous experiments have used different methods to enhance the piezoelectric properties and improve the T _d characteristic. In this study, we demonstrated a simple technique (thermal annealing) to enhance the piezoelectric properties with a very slight decrease in T _d by ∼2 °C for a modified BNKT ceramic (BNKT doped with ZnO). Other phase transition characteristic temperatures of the studied ceramics were also slightly changed. The optimum dielectric (ε _r = 651, tan δ = 0.0503, T _F-R = 167.38 °C, T _m = 305.41 °C, ε _max = 5551, T _B = 367.15 °C, T _d = 155.98 °C, and γ = 1.43), ferroelectric (P _max = 41.28 μC cm^-2, P _r = 35.85 μC cm^-2, E _c = 42.60 kV cm^-1 and R _sq = 1.42), piezoelectric (d ₃₃ = 198 pC N^-1, k _p = 0.598, and g ₃₃ = 34.35 × 10^-3 Vm N^-1), and energy harvesting (FoM = 6.80 pm² N^-1) were obtained for the 8 h annealed ceramic. Furthermore, higher energy harvesting properties (which were 32% higher than that of the unannealed ceramic) were obtained after employing this technique.

Mode-controllable waveguide fabricated by laser-induced phase transition in KTN

We report the fabrication of a hexagonal cladding waveguide by femtosecond laser direct writing (FLDW) in a potassium tantalate niobate (KTN) crystal with a large electric-optical effect. Confocal micro-Raman results show the laser-induced phase transition occurs in the filament areas during the waveguide fabrication. The small filaments can strongly confine the polar nanoregions especially in its ferroelectric state to enhance the waveguide birefringence, enabling excellent polarization maintaining features for both TE and TM-polarized light propagations. The temperature-dependent phase transition allows for an active control of waveguide polarization modes as well as a switchable polarization-maintaining feature.

Thermooptical evidence of carrier-stabilized ferroelectricity in ultrathin electrodeless films

Ferroelectric films may lose polarization as their thicknesses decrease to a few nanometers because of the depolarizing field that opposes the polarization therein. The depolarizing field is minimized when electrons or ions in the electrodes or the surface/interface layers screen the polarization charge or when peculiar domain configuration is formed. Here, we demonstrate ferroelectric phase transitions using thermooptical studies in ∼5-nm-thick epitaxial Pb_0.5Sr_0.5TiO₃ films grown on different insulating substrates. By comparing theoretical modeling and experimental observations, we show that ferroelectricity is stabilized through redistribution of charge carriers (electrons or holes) inside ultrathin films. The related high-density of screening carriers is confined within a few-nanometers-thick layer in the vicinity of the insulator, thus resembling a two-dimensional carrier gas.

Abnormal optical anisotropy in correlated disorder KTa1-xNbxO3:Cu with refractive index gradient

In this report, an abnormal optical anisotropy in KTa1-xNbxO3:Cu (Cu:KTN) crystals with refractive index gradient is presented. Contrary to general regulation in a cross-polarization setup, the transmitted intensity of both TE (horizontally polarized) and TM (vertically polarized) lasers aligned with the basic crystallographic directions can be modulated quasiperiodically. The mechanism is supposed to be based on the polarization induced by the temperature gradient and the refractive index gradient. Meanwhile, the correlated disorder property of the crystals in the range of the freezing temperature (Tf) and the intermediate temperature (T *) also plays an important role. With the results verified both theoretically and experimentally, we believe this work is not only beneficial for the development of the theory associated with the correlated disorder structures in relaxor ferroelectrics, but also significant for the exploitation of numerous optical functional devices.

Large activation energy in aged Mn-doped Sr0.4Ba0.6Nb2O6 ferroelectric ceramics

Large activation energy and a different migration path of oxygen vacancy diffusion in Mn-doped SBN ferroelectric ceramics are revealed.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a "perfect" NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Nontrivial behavior of T(c) at the small electric field in BaTiO3 crystals, detected by means of acoustic emission

Cubic-tetragonal phase transitions of [100]-oriented BaTiO3 crystals have been studied by means of an acoustic emission (AE) method, under an external electric field. The Curie temperature (Tc) reveals a nontrivial behavior at the small field, namely, it initially decreases with enhancement of the field, attains a minimum at the threshold field Eth = 0.256 kV cm(-1), accompanied by the AE count rate N[dot] ≈ 9 s(-1), and then starts increasing with a rate of 1.5 K m kV(-1) as the field is enhanced. Such nontrivial behavior of Tc is explained as due to the presence of polar nanoregions, recently revealed in BaTiO3 crystals (Ko et al 2011 Phys. Rev. B 84 094123).

Reducing the positional modulation of NbO6-octahedra in Sr x Ba1–x Nb2O6 by increasing the barium content: A single crystal neutron diffraction study at ambient temperature for x = 0.61 and x = 0.34

We report on the influence of the barium content on the modulation amplitude in Sr x Ba1–x Nb2O6 compounds by comparing Sr0.61Ba0.39Nb2O6 (SBN61) and Sr0.34Ba0.66Nb2O6 (SBN34). Our single crystal neutron diffraction results demonstrate that the amplitude of the positional modulation of the NbO6 octahedra is reduced with increasing barium content, indicating that the origin of the modulation is the partial occupation of the pentagonal channels by Sr and Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced by the smaller Sr atoms, which leads to a larger deformation of the surrounding lattice and hence to a larger modulation amplitude. The more homogeneous the filling of these channels with one atomic type (Ba) the lower the modulation amplitude. Our results also show that the structure can be described with a two-dimensional incommensurate harmonic modulation. No second order modulation has been observed, both by single crystal diffraction measurements and q-scans. The positional modulation of the Nb atoms is much smaller than that of the oxygen atoms, such that the modulation can be seen as a rotational modulation of almost rigid NbO6-octahedra.

Thermal expansion, polarization and phase diagrams of Ba(1-y)Bi(2y/3)Ti(1-x)Zr(x)O(3) and Ba(1-y)La(y)Ti(1-y/4)O(3) compounds

The thermal expansion properties of the ceramic compositions Ba(1-y)La(y)Ti(1-y/4)O(3) (y = 0.0, 0.026, 0.036, 0.054) and Ba(1-y)Bi(2y/3)Ti(1-x)Zr(x)O(3) (y = 0.10; x = 0.0, 0.04, 0.05, 0.10, 0.15) were determined in the temperature range 120-700 K. We report the temperature-dependent measurements of the strain, thermal expansion coefficient and the magnitude of root mean square polarization. The results obtained are discussed together with the data on the structure and dielectric properties.