Magnetic field influence on ferroelectric phase transitions in BaTiO3-type systems

The effect of an external magnetic field on ferroelectric characteristics of BaTiO₃-type crystals is discussed and estimated in the framework of the vibronic theory of ferroelectricity.     

Dielectric response in ferroelectric BaTiO3

The far-infrared optical and dielectric properties of ferroelectric perovskite titanate powder BaTiO₃ are reported. The terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low frequency dielectric response of BaTiO₃ is closely related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near at 180 cm⁻¹, which is verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon mode for ferroelectric materials. Combining terahertz TDS with Raman spectra, the overall low frequency optical phonon response of BaTiO₃ is presented in an extended spectral range from 6.7 to 1200 cm⁻¹.     

四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下，利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示，当Ti原子沿c轴位移0012nm时，四方相BaTiO3体系能量最低，其自发极化强度为0261C/m2，该结果与实验数据相符合；同时表明，O原子的2p轨道和Ti原子的3d轨道的杂 化是BaTiO3晶体出现铁电性的重要原因. 关键词： 四方相BaTiO3 铁电性 位移 态密度     

Electrostatics of the ferroelectric phases of BaTiO3

Extensive theoretical calculations are reported here on the electrostatics of the three ferroelectric phases of BaTiO₃ using the static polarizable point-ion model. For each structure the following quantities have been calculated: monopole and dipole fields at ionic sites, field gradients, spontaneous polarization, high frequency dielectric constant and the Coulomb energy terms. The parameters of the model have been fitted on the basis of the experimental information on the tetragonal structure and then treated as constants for all the studied structures. The use of the experimental positional parameters of the orthorhombic structure gives such exceptional behaviour for all the calculated quantities that a redetermination of the structural parameters for this phase is suggested. Numerical study of the changes in the Coulomb energy as a function of the macroscopic depolarization potential demonstrated that the most stable state is achieved with the value zero for the potential.     

Influence of ferroelectric polarization on the energetics of the reaction of 2-fluoroethanol on BaTiO3

The influence of ferroelectric polarization on the reaction of 2-fluoroethanol on ferroelectric BaTiO₃ thin films was characterized. 2-Fluoroethanol was found to absorb dissociatively on the BaTiO₃ surface to form an alkoxide that reacts upon heating to produce acetaldehyde, ethylene, and adsorbed fluorine atoms which remain on the surface. Temperature programmed desorption (TPD) studies showed that the activation energy for the production of the acetaldehyde product was a function the orientation of the ferroelectric dipoles with an ∼4 kJ mol⁻¹ higher value on the c+ termination relative to the c− termination.     

Effect of hydrostatic pressure on the phase transition of ferroelectric PbTiO3

The effect of hydrostatic pressure on the dielectric constant of the Nb-doped lead titanate ceramics was measured up to 60 kbar at room temperature. From the previously observed pressure dependence of lattice parameters and the present results, it is concluded that the tetragonality decreases linearly with a slope of ?7.6×10^?4/kbar and that the pressure dependence of the tetragonal-cubic transition temperature is ?8.4K/kbar.     

First-principle study of strain-driven phase transition in incipient ferroelectric SrTiO3

First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO₃. The P4mm-to-P4/mmm-to-Amm2 phase transitions in SrTiO₃ with the epitaxial strain changing from compressive to expansive have been identified by analysis of the soft-mode eigendisplacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2 phase transitions are −0.72% and 0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behaviors of the A₁ and B₂ modes in the Amm2 phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies.     

The first-principles study of ferroelectric behaviours of PbTiO3/SrTiO3 and BaTiOn/SrTiO3 superlattices

We have performed the first-principles calculation to investigate the origins of ferroelectricities and different po- larization behaviours of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. The density of state （DOS） and electronic charge profiles show that there are strong hybridizations between atoms Ti and O and between atoms Pb and O which play very important roles in producing the ferroelectricities of superlattices BaTiO3/SrTiO3 and PbTiO3/SrTiO3. Ow- ing to the decline of internal electric field in SrTiO3 （ST） layer, the tetragonality and polarizations of superlattices decrease with increasing the fraction of SrTiO3 in the superlattices. We find that the polarization of PbTiO3/SrTiO3 is largerthan that of BaTiO3/SrTiO3 at the same ratio of components, because the polarization mismatch between PbTiO3 and SrTiO3 is larger than that between BaTiO3 and SrTiO3. The polarization and tetragonality are en- hanced with respect to those of bulk tetragonal BaTiO3 in the superlattices BaTiO3/SrTiO3, while the polarization and tetragonality are reduced with respect to those of bulk tetragonal PbTiO3 in superlattices PbTiO3/SrTiO3.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Effect of ferroelectric polarization on the adsorption and reaction of ethanol on BaTiO3

The effect of ferroelectric polarization on the adsorption and reaction of ethanol on BaTiO₃ was studied using temperature-programmed desorption (TPD). Consistent with previous studies for methanol, the reactive sticking coefficient for ethanol on the surfaces of the ferroelectric, tetragonal phase of BaTiO₃ was found to be dependent on the orientation of the ferroelectric domains with a higher sticking coefficient on the c⁺ surface relative to the c⁻ surface. TPD peak shapes and the relative product yields were also found to be polarization dependent suggesting that ferroelectric polarization may affect the intrinsic reactivity of the surface.     

Infrared spectra of paraelectric and ferroelectric CsH2PO4

ir spectra are reported for polycrystalline CsH₂PO₄ above and below the transformation temperature, 153 K. Three absorption bands at 1760 cm^-1 2350 cm^-1 and 2800 cm^-1 are assigned to -OHO- vibrations. Whilst the 2350 cm^-1 band remains essentially unaltered, the 1760 cm^-1 band splits into multiplets and the 2800 cm^-1 band shifts to lower frequency on cooling below 153 K. A broad band centred at 3240 cm^-1 is found to exist in the ferroelectric phase and is associated with the -OHO- mode involving the H(2) proton ordered below the transition.     

Weak field magnetic transition in Tb3Fe5O12

A jump like magnetic transition is observed at relatively weak field in Tb₃Fe₅O₁₂ when the magnetic field is applied along the hard axis and no transition is observed with the field applied along the easy axis. We believe that this evolution is related to the ”Umbrella” type magnetic structure known to exist in Tb₃Fe₅O₁₂ at low temperatures.     

Influence of the magnetic transition on the optical properties of CrBr3

Reflectance and thermoreflectance spectra of the magnetic insulator CrBr₃ have been measured in the para- and ferromagnetic phases. Below the Curie temperature the structure at ? 7.30 eV splits into two components separated by ? 0.27 eV. This doublet is ascribed to an optical excitation involving the exchange split conduction band.     

Asymmetric reversible diode-like resistive switching behaviors in ferroelectric BaTiO3 thin films

In this work, the resistive switching behaviors of ferroelectrictric BaTiO3/La0.67Sr0.33MnO3 .heterostructures de- posited by pulsed laser deposition are investigated. The BaTiO3 films show both well-established P-E hysteresis loops, and asymmetric reversible diode-like resistive switching behaviors, involving no forming process. It is found that both the ON/OFF ratio and the stability of resistive switching are substantially dependent on operation voltage （Vmax）. At a Vmax of 15 V, a large ON/OFF resistance ratio above 1000 is obtained at a Vmax of 15 V, which is able to maintain stability up to 70-switching cycles. The above resistive switching behaviors can be understood by modulating interface Schottky barriers as demonstrated by I-V curve fitting.     

The phase transitions and ferroelectric behavior of dense nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering

Dense nanocrystalline BaTiO₃ ceramics with uniform grain sizes of 30 nm was obtained by pressure assisted sintering. The phase transitions were investigated by Raman scattering at temperatures ranging from −190 to 200 °C. With increasing temperature, similar to 3 μm BaTiO₃ normal ceramics, the successive phase transitions from rhombohedral to orthorhombic, orthorhombic to tetragonal, tetragonal to cubic were also observed in 30 nm BaTiO₃ ceramics. Especially, the coexistence of ferroelectric tetragonal and orthorhombic phases was found at room temperature. The ferroelectric behavior was further characterized by P-E hysteresis loop. The experimental results indicate that the critical grain size of the disappearance of ferroelectricity in nanocrystalline BaTiO₃ ceramics fabricated by pressure assisted sintering is below 30 nm.     

Study of the ferroelectric–paraelectric phase transition of NaH3(SeO3)2 single crystals by H and Na NMR

The ¹H and ²³Na spin–lattice and spin–spin relaxation times of NaH₃(SeO₃)₂ single crystals grown using the slow-evaporation method were measured as functions of temperature and frequency in the ferroelectric and paraelectric phases. The changes in the symmetry of the (SeO₃)²⁻ dimers as a result of the ferroelectric–paraelectric phase transition are associated with large changes in the spin–lattice and spin–spin relaxation times, and in the number of resonance lines. The large changes in the relaxation times at 195 K indicate that the H and Na ions are significantly affected by this transition. The change in the number of resonance lines for the ¹H and ²³Na nuclei means that the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions change at T_C. Therefore, the orientations of the (SeO₃)²⁻ dimers and the environments of the Na ions play important roles in the phase transitions. In conclusion, the ferroelectric–paraelectric phase transition of NaH₃(SeO₃)₂ is accompanied by changes in hydrogen-bond structure and distortions of the (SeO₃)²⁻ and Na⁺ ion lattices, which form a slightly distorted octahedron.     

The atomic and electronic structure of the TiO2- and BaO-terminated BaTiO3(0 0 1) surfaces in a paraelectric phase

Relaxation and rumpling of BaTiO₃(0 0 1) surface with two different terminations have been investigated from ab initio local density approximation calculations. Large displacements of ions deviated from their crystalline sites have been obtained. These kinds of displacements lead to the formation the surface rumpling, dipole moments and electric field in the near-surface region. Band structures, density of states, bond population and electronic density redistributions have been obtained. Considerable enhancement of Ti–O chemical bond covalency nearby the surface, especially for the TiO₂ termination surface, has been found.     

17O NQR study of the ferroelectric phase transition in PbHPO4

The temperature dependence of the ¹⁷O NQR spectra of the O-H---O bonded oxygens in PbHPO₄ have been studied using a proton-¹⁷O double resonance technique. The results show that the protons move between two off-centre sites above T_c and freeze into one of the possible equilibrium sites below T_c.     

Field-induced magnetic transition in CrCo3

The action of a magnetic field on a single crystal of the compound ErCo₃ has been investigated by both magnetization and neutron diffraction measurements. ErCo₃ is ferrimagnetic with an easy magnetization axis parallel to the c axis of the crystal. When the field is applied perpendicular to the c axis a transition occurs: the moment of one of the two Er sites remains nearly oriented along the c axis while this of the other Er site rotates through an angle of 55°. From the value of the threshold field, the difference between the anisotropy energies of the two sites has been evaluated.     

Positron annihilation in BaTiO3

The angular correlation of 2 γ-annihilation radiation has been measured in barium titanate single crystals. The effect on the angular correlation of the ferroelectricity in BaTiO₃ was not detected.