Displacive Ferroelectric Phase Transition Research Articles

Dynamical electric and magnetic responses in the Bi0.85Nd0.15FeO3 ceramic with morphotropic phase boundary

A series of experimental and theoretical studies have demonstrated that Bi1-xNdxFeO3 ceramics with morphotropic phase boundaries (MPBs) exhibit a very complex phase-transition process including the displacive ferroelectric phase transition and reorientation of antiferromagnetic alignments [Chem. Mater.23, 2166 (2011); Adv. Funct. Mater.25, 552 (2015)]. In this work, such complex phase-transition process in the Bi0.85Nd0.15FeO3 ceramic is detected by electric and magnetic responses as the function of frequency. First, the phase transition from orthorhombic Pbam phase (O2) to orthorhombic Pbnm phase (O1) is found to be accompanied by the change in the local strain induced by MPBs. Second, the electric response at O2→O1 transition with the disappearance of antiferroelectric order showed the relaxation and reversible rotation of the ferroelectric order coupled with the local strain. The moderate active energy of such coupling (∼2.7 eV) facilitates the possibility of recovering the deformation of Bi1-xNdxFeO3 ceramic by an external electric field. Third, the weak magnetic response is frequency-independent, and the reorientation of the antiferromagnetic order is also reversible. Finally, the rotation of ferroelectric/magnetic order connected to MPBs in the ceramic is discussed.

Making EuO multiferroic by epitaxial strain engineering

Multiferroics are materials exhibiting the coexistence of ferroelectricity and ideally ferromagnetism. Unfortunately, most known magnetoelectric multiferroics combine ferroelectricity with antiferromagnetism or with weak ferromagnetism. Here, following previous theoretical predictions, we provide clear experimental indications that ferroelectricity can be induced by epitaxial tensile strain in the ferromagnetic simple binary oxide EuO. We investigate the ferroelectric phase transition using infrared reflectance spectroscopy, finding that the frequency of the soft optical phonon reduces with increasing tensile strain and decreasing temperature. We observe such a soft mode anomaly at 100 K in (EuO)2/(BaO)2 superlattices grown epitaxially on (LaAlO3)0.29-(SrAl1/2Ta1/2O3)0.71 substrates, which is a typical signature for a displacive ferroelectric phase transition. The EuO in this superlattice is nominally subjected to 6.4% biaxial tensile strain, i.e., 50% more than believed needed from previously published calculations. We interpret our results with new first-principles density functional calculations using a hybrid functional, which provides a better quantitative agreement with experiment than the previously used local-density approximation and generalized gradient approximation functionals.

Dielectric Properties of 0.9Ag0.9Li0.1NbO3 – 0.1Bi0.5K0.5TiO3 Ceramics

New ferrielectric 0.9Ag0.9Li0.1NbO3–0.1Bi0.5K0.5TiO3 ceramics have been obtained. At room temperature ceramics exhibit perovskite structure, which is close to rombohedral-tetragonal phase boundary. The dielectric properties of ceramics were investigated in wide frequency range from 20 Hz to 1 GHz at temperatures 200 K–750 K. 0.9Ag0.9Li0.1NbO3–0.1Bi0.5K0.5TiO3 ceramics exhibits a first order mixed order-disorder and displacive ferroelectric phase transition at TC = 430 K. The phase transition dynamics is caused by relaxation soft mode at frequencies below 1 GHz and soft phonon mode at higher frequencies. In ferroelectric phase an additional dielectric dispersion is observed, which could be attributed to dipolar glass relaxation.

The Free Energy of Displacive Ferroelectrics with an Order-Disorder Component and a Sequence of Phase Transitions in Oxide Perovskites

A microscopic electron-lattice theory of displacive ferroelectric phase transitions with an order-disorder component in ABO 3 oxide perovskites has been developed. The short-range electron-phonon interaction in the BO 6 clusters and the long-range electron-lattice p-d hybridization coupling in host perovskites are considered. The short-range electron-phonon coupling leads to the static off-center displacements of the Ti ions in [111] directions in BaTiO 3 and SrTi 18 O 3 (a quantum ferroelectric) above and below the ferroelectric phase transition temperature, which results in the order-disorder component of the phase transition. The displacive components of ferroelectric structural phase transitions in perovskite oxides are determined by the long-range electron-lattice interaction. In general, the obtained free energy has the form of Landau-Ginzburg-Devonshire free energy with the coefficients determined by the electron-phonon coupling constant and the electronic and phonon states.

Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals

The specific features of diffuse X-ray scattering in BaTiO3, KNbO3, and PbTiP3 perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO3. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO3 and KNbO3 crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO3, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO3, KNbO3, and PbTiO3 are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.

Displacive Ferroelectric Phase Transitions with the Order-Disorder Component in Oxide Perovskites and Photo-Induced Ferroelectricity

A microscopic electron-lattice theory of displacive ferroelectric phase transitions with an order-disorder component in the ABO3 type (A is alkaline, an earth alkaline ion, B is a transition metal ion) oxide perovskites has been developed further. The short-range electron-phonon interaction in BO 6 n −(n = 6,8) clusters and the long-range electron-lattice coupling in oxide perovskites are considered. The short-range electron-lattice coupling between electronic oxygen 2p and titanium 3d states leads to the static off-center displacements of the Ti ions in [111] directions (local ferroelectricity) above and below the ferroelectric phase transition temperature, which results in the order-disorder component of the phase transition. The displacive components of the ferroelectric phase transitions in BaTiO3 and SrTi18O3 are caused by the long-range electron-lattice p-d hybridization. The coupling of the displacive and order-disorder components is considered. The theory is supported by recent experimental data.

Probing the ferroelectric phase transition through Raman spectroscopy in Pb(Fe2∕3W1∕3)1∕2Ti1∕2O3 thin films

The present work investigates the evolution of micro-Raman spectra of (1−x)Pb(Fe2∕3W1∕3)O3−xPbTiO3 (PFWT) (x=0.50) thin films in the temperature range from 80to600K. Raman and dielectric data indicate that the crystal structure changes from tetragonal to cubic, i.e., a ferroelectric phase transition at 575K. The dielectric properties of PFWT thin films were studied in the temperature range of 80–600K over a wide range of frequencies. The slope of the reciprocal of the dielectric constant is 2:1, matched well with the simplest Landau free energy model, and it indicates a continuous second order displacive ferroelectric phase transition.

Electron-Phonon Mechanism of Mixed Displacive and Order-Disorder Phase Transitions in Oxide Perovskites and Photoferroelectricity

A microscopic electron-lattice theory of displacive ferroelectric phase transitions with an order-disorder component in ABO 3 oxide perovskites has been developed. The short-range electron-phonon coupling between electronic oxygen 2p and titanium 3d states leads to the static off-center displacements of Ti ions in [111] directions in BaTiO 3 and SrTi 18 O 3 above and below the phase transition temperature. The displacive components of ferroelectric phase transitions are caused by the long-range electron-lattice p-d hybridization. The theory is partly supported by the recent experimental data. The photoferroelectricity in SrTi 16 O 3 in the strong light field is predicted. At this the light field leads to the absence of the inversion centre.

Tungsten bronze type dielectrics in SrO-Sm2O3-TiO2-Nb2O5 system and their dielectric anomaly

The dielectric behavior and the features of the diffused phase transition of the SrpSm6−pTi8−pNb2+pO30(p=4,5) ceramics were investigated together with their structure. Both Sr5SmTi3Nb7O30 and Sr4Sm2Ti4Nb6O30 had tungsten bronze structure and weak ferroelectric nature. Though Sr5SmTi3Nb7O30 showed the diffused ferroelectrics phase transition, it was concluded that the transition was mainly the displacive ferroelectric phase transition. In Sr4Sm2Ti4Nb6O30, the Sr2+ ions and Sm3+ ions did not form an order structure and it showed a typical displacive ferroelectric phase transition. The dielectric anomaly was attributed to the different BO6 octahedral distortion.

Phase transitions in lead scandoniobate PbSc0.5Nb0.5O3

Crystals of PbSc0.5Nb0.5O3 are investigated by x-ray diffraction and optical spectroscopy. It is found that the phase transformations in PbSc0.5Nb0.5O3 are governed by two mechanisms: displacive ferroelectric phase transitions and consistent rotations of oxygen octahedra.

Lead Titanate is not a Classic Case of a Displacive Ferroelectric Phase Transition

Recent titanium K edge XAFS results show that the ferroelectric to paraelectric phase transition at the Curie temperature (Tc), 763K, in lead titanate is an order-disorder transition. This contradicts the classic displacive model used to describe this transition. The tetragonal distortion of the perovskite unit cell introduces distinctive structure into the near-edge. This structure shows a weak, monotonically decreasing temperature dependence in the range 10 K to 900 K, and remains in the near-edge even above Tc where the displacive model would predict its substantial decrease. The Ti EXAFS also indicates that the tetragonal distortion measured by crystallographic techniques below Tc changes smoothly as the material is heated above Tc. This continuous variation in the local structure around the titanium atom across Tc shows that the ferroelectric transition is order-disorder and not the classic displacive model.

Evidence for coexistence of ferroelectricity and high-T c superconductivity in YBa 2Cu 3O 7-δ (δ < 0.15) from ultrasonic experiments

In order to determine the dielectric properties of the bound charges (soft dipoles) in superconducting YBa 2 Cu 3 O 7-δ (δ < 0.15) we have extended conventional ultrasound experiments to the determination of the dispersion and absorption of the sound-induced rf magnetic field. In contrast to the ultrasonic behaviour, the temperature dependence of the dispersion and absorption of the sound-induced rf magnetic field shows pronounced anomalies. It is shown that these experimental findings are highly indicative for displacive ferroelectric phase transitions appearing in the temperature range of 150 K < T < 180 K and in the vicinity of the superconducting transition temperature. The possible connection between ferroelectricity and high temperature superconductivity is also discussed.

Off-diagonal coherent potential approximation and their application to PSZT systems

The self-consistent phonon theory and the coherent potential approximation method are applied to investigate the displacive ferroelectric phase transitions occurring in the ternary solid solution. Especially, the substitutional disordered phonon system with both random diagonal and off-diagonal interactions has been dealt with and numerical computation has been made under different strengths of the atomic coupling. The results show that the interactions between different atoms greatly influence the properties of phase transitions in solid solutions and the anomalous relation between dielectric constant versus temperature can be explained. The new approach is shown to be able to offer a much more detailed description for the various solid solution systems.

Raman Scattering by Soft Plasmon-LO Phonon Modes in Ferroelectric Semiconductors

We present a theory of Raman scattering by the low-lying soft coupled plasmon-longitudinal optical (LO) phonon modes at low temperatures in semiconducting incipient ferroelectrics. The softening of this mode gives a dynamical intimation of the screened LO phonon instability induced by the displacive ferroelectric phase transition. We compare our calculations with Raman spectra from n -KTaO 3 observed by Uwe et al. , on the basis of the soft mode-like behavior of the coupled modes as well as of the simple energy band structure.

Ferroelectric semimetal-dielectric phase transition induced by the interband electron-phonon interaction

Abstract According to the vibronic theory the displacive ferroelectric phase transitions are caused by the interband electron-phonon interaction (see, for example Ref. 1). As a result of the vibronic interaction the phonon and electron spectra of the system are changed. The arising soft phonon-elect ron branch of elementary excitations determines the dielectrical properties of the crystal.2 The electron-phonon interaction induces a spontaneous lattice distortion (polarization) dependent on temperature, and an increase of the forbidden gap3 depending on the distortion.