T. Ostapchuk

Infrared dielectric response of relaxor ferroelectrics

In the article we discuss the dielectric response of relaxor-type complex perovskites in the phonon-frequency region on the basis of a rich collection of experimental data accumulated in our laboratory by numerous infrared spectroscopy investigations over the past decade. The effect of the cation occupational disorder and of the nanoscopic inhomogeneous polarisation on the infrared response is considered in the framework of the factor-group symmetry analysis and the effective-medium approach. Polar mode assignment and comparison of the mode parameters of different materials is facilitated by systematic evaluation of mode-plasma frequencies. Whenever possible, the results are presented in form of tables convenient for further analysis or by comparison with results of other techniques, including ab-initio calculations. Infrared properties of other non-perovskite relaxors are briefly mentioned.

Room temperature surface piezoelectricity in SrTiO3 ceramics via piezoresponse force microscopy

SrTiO3 ceramics are investigated by piezoresponse force microscopy. Piezoelectric contrast is observed on polished surfaces in both vertical and lateral regimes and depends on the grain orientation varying in both sign (polarization direction) and amplitude. The observed contrast is attested to the surface piezoelectricity due to the flexoelectric effect (strain gradient-induced polarization) caused by the surface relaxation. The estimated flexoelectric coefficient is approximately one order of magnitude smaller as compared to that recently measured in SrTiO3 single crystals. The observed enhancement of piezoresponse signal at the grain boundaries is explained by the dipole moments associated with inhomogeneous distribution of oxygen vacancies.

Grain-boundary and crack effects on the dielectric response of high-permittivity films and ceramics

Grain-boundary and crack effects on the dielectric response of high-permittivity films and ceramics were analyzed using the generalized effective-medium approximation. It was demonstrated that the brick-wall and coated spheres models, which become equivalent for a small porosity, best describe the strong reduction of the dielectric response and hardening of the soft-mode frequency recently revealed in SrTiO3 films and PbZrO3 ceramics.

Anisotropic dielectric function in polar nanoregions of relaxor ferroelectrics

The Letter suggests treating the infrared reflectivity spectra of single crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally frozen polarization makes the dielectric function strongly anisotropic in the phonon frequency range and the random orientation of the polarization at nanoscopic scale requires taking into account the inhomogeneous depolarization field. Employing a simple effective medium approximation (the Bruggeman symmetrical formula) turns out to be sufficient for reproducing all principal features of room temperature reflectivity of Pb(Mg(1/3)Nb(2/3))O3. One of the reflectivity bands is identified as a geometrical resonance entirely related to the nanoscale polarization inhomogeneity. The approach provides a general guide for systematic determination of the polar mode frequencies split by the inhomogeneous polarization at the nanometer scale.

Multiple Soft-Mode Vibrations of Lead Zirconate

Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.

Dielectric Response of Soft Modes in Ferroelectric Thin Films

The concept of effective soft mode in thin films and its role in the effective dielectric response of high-permittivity thin films is introduced. Compared to bulk soft mode response, it may be strongly influenced by stresses from the substrate and dielectric inhomogeneities like interface layers, grain boundaries and porosity. The available techniques for the determination of the effective soft-mode response (far-infrared and time-domain THz spectroscopies) are discussed. The experiments on various ferroelectric (PbTiO 3, PZT, BaTiO 3, SrBi 2 Ta 2 O 9 ), incipient ferroelectric (SrTiO 3 ), doped incipient ferroelectric (SrTiO 3 :Ba), and relaxor ferroelectric (PLZT) films are briefly reviewed and compared with the results in bulk materials.

Infrared and Raman studies of the dead grain-boundary layers in SrTiO3 fine-grain ceramics

Infrared reflectivity, terahertz and Raman spectroscopy studies were carried out with two types of SrTiO3 ceramic of mean grain size 1500 and 150 nm, and the results were compared with single-crystal data. Strong ferroelectric soft-mode stiffening was detected in the infrared data upon the decrease of the grain size, fully compatible with the reduction of the low-frequency permittivity in the whole temperature range 300–10 K. Weaker stiffening of this mode observed in the Raman spectra is explained by the higher phonon wavevector active in the Raman response and by a different geometry of its propagation. Raman data also confirmed a smaller tetragonality below the structural phase transition as compared to single crystals. This is explained by a special topology of tetragonal domains along the grain boundaries, in which the tetragonal axes are oriented perpendicularly to them. Broad low-frequency and low-temperature features in the Raman spectra are assigned to the smeared first-order ferroelectric soft-mode response from the polarized regions (~3 nm thick) with reduced permittivity attached to the very narrow grain boundaries. Such an effect is caused by a local frozen polarization of mean value ~15 µC cm−2 tending perpendicular to the grain boundary. To account for the full permittivity size effect, the grain boundary itself is modelled by a very thin (~1 nm) dead layer with a strongly reduced local permittivity (~8). The microscopic origin of the narrow dead layer is apparently a strong oxygen deficit, as suggested from recent local experiments and first-principles calculations by other authors. Coupling between the E-components of the ferroelectric and structural soft-mode doublets observed in the Raman spectra is due to the propagation of the E-symmetry polaritons through the polar grain boundaries, whereas the non-coupled A-symmetry polaritons are localized inside the non-polar grain bulk. Since the structurally disordered grain boundaries are very narrow, taking up only a very small volume fraction, their effect on the relaxation of the phonon momentum conservation can be neglected.

FIR and near-millimetre dielectric response of SrTiO3, BaTiO3 and BST films and ceramics

Abstract The dielectric response of Ba 1− x Sr x TiO 3 ceramics and films ( x =0, 0.9, 1) was studied with a particular attention to the soft mode behaviour in the far IR and near-mm range and its comparison with the single crystal behaviour. To analyse the response quantitatively, we used commercial Fourier transform IR and monochromatic backward-wave-oscillator spectroscopy in both transmission and reflection modes and a rigorous evaluation. The soft-mode behaviour is extremely sensitive to the sample quality, particularly in films, and correlates with the lower low-frequency response. The most important factors for this behaviour in the polycrystalline films are grain boundaries and porosity and nano-cracks, which often appear in thicker polycrystalline films along some of the grain boundaries. The brick-wall model for grain boundaries and cracks discussed within the generalized effective medium approximation appears to be appropriate for describing the observed phenomena. In quasi-epitaxial films the macroscopic tensile stress, which influences the phase diagram very sensitively and induces ferroelectricity even in the pure SrTiO 3 , seems to play the most important role. From the soft-mode behaviour and its splitting, it appears that in the studied BaTiO 3 films all the phase transitions, as in single crystals, seem to be present, but smeared with phase coexistence regions down to 10 K. In the quasi-epitaxial BST-0.9 film a smeared ferroelectric transition appears near 150 K.

Grain-size effect in BaTiO3 ceramics: study by far infrared spectroscopy

The article offers comparative study of two nanocrystalline (50 and 100 nm averaged grain size) and two coarse-grain (1.2 and 10 µm averaged grain size) ceramics by means of Fourier transform infrared spectroscopy in a broad temperature range (10–570 K). Far infrared reflectivity spectra were fitted with the factorized model of the dielectric function and the evaluated dielectric function was compared with the results of low-frequency dielectric measurements. It appears that the stiffening and weakening of the overdamped soft mode is the only reason for the reduced dielectric permittivity of nanocrystalline ceramics in the paraelectric phase, but a strong grain-size dependence of the dielectric constant in the ferroelectric phase is not connected with the changes in lattice dynamics. All single-crystal symmetry changes were detected in all of the samples, but no phase-transition discontinuities was seen in nanocrystalline ceramics. A coexistence of more than one phase is suggested. Strong dependence of the Curie-Weiss temperature on the grain size, earlier revealed by others, is confirmed.

Broadband Dielectric Spectroscopy of Ba(Zr,Ti)O3: Dynamics of Relaxors and Diffuse Ferroelectrics

Broadband dielectric spectroscopy from Hz up to the infrared (IR) range and temperature interval 10-300 K was carried out for xBaZrO3-(1-x)BaTiO3 (BZT-x, x = 0.6, 0.7, 0.8) solid solution ceramics and compared with similar studies for x = 0, 0.2, 0.4, 1 ceramics published recently (Phys. Rev. B 86, 014106 (2012)). Rather complex IR spectra without appreciable mode softening are ascribed to Last-Slater transverse optic (TO) phonon eigenvector mixing and possible two-mode mixed crystal behavior. Fitting of the complete spectral range requires a relaxation in the 100 GHz range for all the samples. Below 1 GHz another relaxation appears, which is thermally activated and obeys the same Arrhenius behavior for all the relaxor BZT samples. The frequently reported Vogel-Fulcher behavior in BZT relaxors is shown to be an artifact of the evaluation from the permittivity or loss vs. temperature dependences instead of its evaluation from loss vs. frequency maxima. The relaxation is assigned to local hopping of the off-centered Ti4+ ions in the frozen BTO clusters, whose size is rather small and cannot grow on cooling. Therefore BZT is to be considered as a dipolar glass rather than relaxor ferroelectric.