Ferroelectric Phase Transition Point Research Articles

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Raman and Hyper Raman opalescence on the soft mode in barium sodium niobate crystals

Abstract A strong increase in the spectral intensity of Raman scattering (Raman opalescence effect) at the polar soft mode, responsible for the instability of the crystalline lattice, near ferroelectric phase transition points in the barium sodium niobate (BSN) crystal was found. Raman opalescence is explained as a consequence of a sharp increase in the amplitude of the transverse A1(Z; TO) polar optical soft mode near the temperature of the ferroelectric phase transition in BSN (Tc = 830 K). Experimental results were obtained using a new experimental technique, based on the construction of isofrequency temperature dependences, when registering the spectral intensity of Raman scattering at fixed frequencies of the spectrometer, near the frequency of the exciting line, for certain polarization geometry. The conditions for observing the effects of Hyper Raman opalescence near the phase transition temperature in the BSN are established.

Raman scattering from the effective soft mode in lead germanate crystal

AbstractThe Raman opalescence of an effective soft mode in crystalline lead germanate is investigated. Recording of Raman scattering intensity was in the x (zz)y geometry corresponding to the soft mode, responsible for ferroelectric phase transition point at 451 K. The Raman spectra were measured at fixed temperatures T (I(ω′, T); T = const) and also at fixed wavenumbers ω′ of Raman spectrometer (I(ω′, T); ω′ = ω0 – Ω = const) with slow temperature changing. The strong increase of Raman scattering intensity (Raman opalescence) has been observed at temperatures of phase transition region. The appearance of central peak scattering is attributed to the strong interaction of the effective soft mode with a low‐wavenumber nonfundamental oscillator.

Effects Associated with Confined Geometry in Nanocomposites Based on Mesoporous 2D-SBA-15 and 3D-SBA-15 Matrices Containing Sodium Nitrite Nanoparticles

Temperature dependences of the ferroelectric order parameter for nanostructured sodium nitrite under heating and cooling were obtained by analyzing the temperature evolution of diffraction spectra of neutron scattering by composites obtained by introducing sodium nitrite into pores of mesoporous 2D-SBA-15 (the average pore diameter is 69(4) A) and 3D-SBA-15 (94(5) A) matrices. It was demonstrated that the phase transition to the paraelectric phase in the process of heating occurs at TC = 433 ± 1 K in both nanocomposite materials. It was found that these sizes decrease at approaching to the ferroelectric phase transition point on heating. Temperature hysteresis (~15–20 K) in the temperature dependence of the order parameter between the heating and cooling regimes was revealed.

Raman Scattering on the Effective Soft Mode for Lithium Niobate Crystals

The evolution of Raman spectra in a wide range of temperatures that includes the ferroelectric transition point in lithium niobate single crystals is studied for polarization geometry X(ZZ)Y. In this geometry, the soft mode responsible for the phase transition distinguished by 1A1(TO)-type symmetry should appear in the spectra. Experimental studies show that the 1A1(TO) mode interacts resonantly with nonfundamental modes in the low-frequency region of the spectrum. Near the ferroelectric phase transition point, an isofrequency opalescence effect is observed that consists of an abrupt increase in Raman signal intensity at fixed frequencies near the excitation line.

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A2 type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.

On the mechanism of the increasing phase transition point in ferroactive nanocomposites

The numerical analysis of a nonlinear equation set has revealed a possible increase in the ferroelectric phase transition point at the presence of the intermediate layer at the ferroelectric–dielectric interface. The order parameter distribution in the ferroelectric particles, as well as the effect of the intermediate layer thickness on the Curie temperature, has been studied.

Temperature–Pressure Phase Diagram for Rb2ZnCl4 Crystals

We have studied the effect of uniaxial stress along the principal crystallophysical directions on the temperature variations in the birefringence Δn i and the refractive indices ni for Rb2ZnCl4 crystals in the region of phase transitions from the paraelectric phase to the incommensurate phase and then to the commensurate ferroelectric phase. We have established that Δn i and n i are rather sensitive to the action of uniaxial stresses. We have observed a significant pressure-induced shift of the paraelectric – incommensurate – ferroelectric phase transition points in different temperature regions, depending on the direction of the uniaxial compression.

Ferroelectric order parameter in the two-particle cluster system near phase transition point: collective variables method

A new approach in the investigation of the order parameter behaviour near ferroelectric phase transition point is suggested. The short range and dipole interactions between particles are taken into account. The logarithmic corrections and effective critical exponents are calculated and discussed.

Microstructure and dielectric properties of highly tunable Ba0.6Sr0.4TiO3/MgO/Al2O3/ZnO composite

The effect of co-doping ZnO, Al2O3 and MgO on the microstructures and dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the mixed-oxide route at 1400°C was investigated. And a comparison criterion for the BST-based tunable ceramics was proposed. The results show that a cubic perovskite phase Ba0.6Sr0.4TiO3 and a face-centered-cubic spinel phase Mg(Zn)Al2O4 were observed in BST/MgO/Al2O3/ZnO (BMAZ) composite ceramics. When Zn2+ content exceeds solid solubility limit in the BST, excess ZnO will react with Al2O3 to produce ZnAl2O4 phase. With the increase of ZnO, the paraelectric–ferroelectric phase transition points of BMAZ ceramics shift to the lower temperatures, and the maximum of dielectric constant increased first and then decreased. A higher tunability which amounts to 27.4% under 1.0kV/mm biasing was obtained for BMAZ ceramics doped with 1.0wt% ZnO.

Acoustic Mode Behavior in Ferroelectric Phase Transition of Li Doped ZnO Semiconductor and its Photoinduced Effect

Acoustic phonon behavior and photoexcited effect in Li-doped ZnO ceramics were investigated at around the ferroelectric phase transition point Tc = 350 K by Brillouin scattering using a Sandercock-type tandem Fabry-Perot interferometer. The temperature dependence of Brillouin scattering spectra was obtained from 300 K to 500 K through Tc. A bunch of longitudinal-acoustic (LA) phonons did not show any anomaly at around Tc accompanied to the ferroelectric phase transition, although the LA mode is coupled to the spontaneous polarization in the ferroelectric phase with point group of 6 mm. The result indicates that the mechanism of the ferroelectric phase transition is not an order-disorder type related to size mismatch of the doped Li cation. No photoinduced effect was observed in the LA phonon bunch around Tc under ultraviolet (UV) excitation with energies higher than the band gap energy, although the intensity of base line in Brillouin scattering spectrum increased under UV. The component of base line includes an emission spectrum and /or a central peak component, which might be related to an electronic ferroelectricity.

Photoinduced Effect in Quasi-Longrange Ferroelectric Fluctuation on Bismuth Layered Perovskites BaBi4Ti4O15

The ultraviolet (UV) photoexcited effect in the bismuth layered perovskite oxide BaBi4Ti4O15 was investigated by Brillouin scattering using a Sandercock-type tandem Fabry-Perot interferometer on six-pass stage with the finesse more than 100. The temperature dependences of Brillouin scattering spectra were obtained with and without UV photoirradiation with energies higher than the band gap energy from 300 K to 900 K. A central peak and acoustic phonon mode show anomalies from 600 K to 800 K under a UV photoirradiation. The results indicate that the UV photoirradiation affects the quasi-longrange ferroelectric fluctuation near the ferroelectric phase transition point.

Ferroelectric Phase Transition and Photoinduced Cooperative Phenomena in Bi-Layered Perovskite Pb2Bi4Ti5O18 Ceramics Studied by Brillouin Scattering

The ferroelectric phase transition mechanism and ultraviolet (UV) photoexcition effect have been investigated in the bismuth layered perovskite Pb2Bi4Ti5O18 by Brillouin scattering using a Sandercock-type tandem Fabry–Perot interferometer. The temperature dependences of Brillouin spectra both with and without UV irradiation were observed around phase transition points. The observed Brillouin spectra include a central peak component that shows an anomaly in the ferroelectric phase transition point TC=450 K. The central peak spectra show narrowing under UV irradiation. The correlation length of the ferroelectric dipole fluctuation should be enhanced with the UV irradiation effect below TC.

Characterisation of Phase Transition in Strontium Barium Niobate by Bond Method

The high quality Sr0.61Ba0.39Nb2O6 (SBN61) single crystals without any striations were obtained by Czochralski pulling method. The temperature dependence of the lattice parameters of the investigated crystals was studied using a precision X-ray Bond's method. The angular precision of the goniometer 1 arcsine and metric value of Cu K1 (λ = 1.54059292Å) allowed to obtain high precision and accuracy of lattice parameter measurements. The received results allowed to determine the Curie temperature and character of the phase transition in this material. The lattice parameter a monotonically increases and shows hardly visible inflection point at TC when increasing the temperature. The temperature dependence of the lattice parameter c displays quite distinct temperature behaviour. Contrastingly, it decreases with increasing the temperature experiencing an evident anomaly in the vicinity of the structural phase transition point. Obtained value of the ferroelectric phase transition point is TC = 346 ± 3K.

Doping-induced ferroelectric phase transition in strontium titanate: Observation of birefringence and coherent phonons under ultraviolet illumination

The doping-induced ferroelectric phase transition in Ca-doped ${\text{SrTiO}}_{3}$ is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K. Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet (UV) illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.

Central Peak of Bismuth Layered Ferroelectrics Pb2Bi4Ti5O18 Studied by Brillouin Scattering

Brillouin scattering spectra have been observed in ceramic sample of Bi-Layered Perovskites Pb 2 Bi 4 Ti 5 O 18 using a Sandercock type tandem Fabry-Perot interferometer. A central peak was observed in the spectrum, which shows the anomalous increase of relaxation time with approaching to T 1 = 450 K. The temperature dependences of intensity and width in the central peak spectrum show anomaly at T 2 = 740 K in addition to T 1 . However, no anomalous behavior has been obtained at T c = 583 K reported as a ferroelectric phase transition point with a dielectric measurement by E. C. Subbarao.

Low-Frequency Raman Spectroscopy in Pb(Zn1/3Nb2/3)O3-PbTiO3 Mixed Crystals

The Raman spectra in (1−x)Pb(Zn 1/3 Nb 2/3 )O3 − xPbTiO 3 (PZN-xPT) mixed crystals were measured under several optical geometries below 400 cm − 1 . Raman modes in the low frequency region were assigned on the basis of our spectrum in the poled sample of PZN-20%PT. Low frequency Raman modes in PZN-PT (x ≤ 20%) were confirmed to be primarily caused by ferroelectric modes rather than 1:1 B-site ordering. It was found that the Raman scattering is sensitive for the inhomogeneous ferroelectric distortion or existence of the polar nano-region in the cubic phase of the PZN-type relaxor. Both the E(TO1) and A 1 (TO1) modes, corresponding to the soft modes in PbTiO 3 , do not almost depend on temperature near the ferroelectric phase transition point in PZN-PT (x ≤ 20%).

Dielectric properties of betaine phosphite films

Polycrystalline films of betaine phosphite are grown on single-crystal quartz and lithium niobate substrates atop interdigitated transducers. It is revealed using a light-polarizing microscope in the reflection mode that the films are made up of large (up to 1 mm in size) single-crystal blocks. The temperature dependences of the film capacitance measured at frequencies from 120 Hz to 1 MHz pass through a strong maximum at the ferroelectric phase transition point. The effect of a bias on the permittivity of the films is investigated. It is shown that the differences in the temperature dependences of the permittivity of single crystals and films originate from the influence of dynamic strains in the substrate on the film and from the presence of an effective depolarizing field.

Observation of coherent phonons in Ca-doped SrTiO3: Doping-induced ferroelectric phase transition and ultraviolet-illumination effect

Photo-induced effect in Ca-doped SrTiO3 was investigated by observing coherent phonons. Coherent phonons of soft modes were studied by using ultrafast polarization spectroscopy. Under an ultraviolet (UV) illumination, a shift of the ferroelectric phase transition point at 28 K toward the lower temperature side was observed, and a decrease of phonon frequencies after the UV illumination was found.

Polarized Raman Study in Pb(Zn1/3Nb2/3)O3-PbTiO3 Mixed Crystal

The Raman scattering experiment in several optical geometries below 400 cm−1 was carried out in the poled sample of the 0.8Pb(Zn 1/3 Nb 2/3 )O 3 -0.2PbTiO 3 (PZN-20%PT) crystal. Raman modes in the low frequency region were assigned. It was concluded that the low frequency Raman modes in PZN-20%PT are mainly due to a polar distortion of the paraelectric cubic Pm m structure. The Raman scattering was found to be sensitive to the inhomogeneous ferroelectric distortion or existence of the polar nano-region in the cubic phase of the PZN-type relaxor. Both the E(TO1) and A 1 (TO1) modes, corresponding to the soft modes in the tetragonal phase of PbTiO 3 , do not at all depend on temperature near the ferroelectric phase transition point in PZN-20%PT.