J. Dec

Diffuse phase transition in BaTi1−xSnxO3 ceramics: An intermediate state between ferroelectric and relaxor behavior

Dielectric relaxation and polar structures of BaTi1−xSnxO3 ceramics, x=0.10–0.20, are investigated by means of dielectric spectroscopy and piezoresponse force microscopy. A transition regime between “normal” ferroelectric and relaxor behaviors is encountered. In the compositions with x=0.10, a complex domain pattern confirming the ferroelectric state is observed. Strong dielectric relaxation around Tm is attributed to domain wall motion. On the other hand, the dielectric spectra in the sample with x=0.20 are very similar to those observed in relaxor ferroelectrics. Analysis of the relaxation spectra at the intermediate concentration, x=0.15, reveals both domain wall response and an additional contribution related to mesoscale polar structures. The appearance of relaxor behavior in BaTi1−xSnxO3 is discussed within the framework of the random field model.

Crossover from ferroelectric to relaxor behavior in BaTi1− x Sn x O3 solid solutions

Dielectric relaxation of BaTi1− x Sn x O3 ceramics is investigated by means of dielectric spectroscopy. The gradual crossover from ferroelectric to relaxor behavior is characterized by vanishing of the contribution due to domain walls and appearance of relaxation related to reorientation of polar nanosized regions. Typical behavior of relaxors is observed only in ceramics with x = 0.30, while the compositions with 0.175 ≤ x ≤ 0.25 show coexistence of both ferroelectric and relaxor features. The relaxor properties are supposed to be due to both weak random fields and disorder inherent in pure BaTiO3.

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.

Dynamic phase transitions in ferroic systems with pinned domain walls

The creep and relaxation of domain walls under an ac electric field that are observed in an ideal model system, periodically poled superionic KTiOPO4 (KTP), appear to occur in different regimes that are separated by a dynamic phase transition at a well-defined frequency, f m = 0.003 Hz, at temperature T = 233 K. The power-law dispersion of the creep susceptibility, χ ∝ 1 + (iωτ)− β (with β ≈ 0.4), and the large nonlinearity encountered for f<f m is contrasted with the Cole–Cole-type relaxational dispersion, χ ∝ [1 + (iωτ)1− α −1 (with α ≈ 0.3), for f > f m. Similar creep-to-relaxation transitions are observed at low frequencies in other ferroic systems with weak disorder: the multidomained uniaxial relaxor Sr0.69Ba0.31Nb2O6 (SBN), the quantum ferroelectric domain state of SrTi18O3 (STO18) and the superferromagnetic nanoparticle system [(Co80Fe20(1.4 nm)/Al2O3(3 nm)]10, which appear to belong to the same dynamical universality class.

Computer-controlled susceptometer for investigating the linear and nonlinear dielectric response

A fully automatized alternating current (ac) susceptometer is constructed for simultaneous measurements of the phase resolved complex linear and complex nonlinear ac susceptibilities of lossy and dispersive dielectric materials. This relatively simple setup allows measurements over a wide range of experimental variables, such as ac amplitudes up to 40 V, frequencies from 10(-2) to 10(3) Hz, and temperatures from 100 to 600 K utilizing only current/voltage and analog/digital converters and a personal computer. In contrast with the commonly used analysis of the charge accumulated on a standard capacitor in series with the sample our method is based on the analysis of the current flowing directly through the sample. Absence of any capacitive voltage dividers in the measurement circuit eliminates uncontrolled phase shifts. This is why the instrument provides high quality, nonlinear susceptibility data and in particular appears as a very convenient tool for discrimination between continuous and discontinuous phase transitions when determining the sign of the real part of the third order dielectric susceptibility.

Ferroelectric Domains in SrxBa1 − xNb2O6 Single Crystals (0.4 ≤ × ≤ 0.75)

The crossover of single crystalline Sr x Ba 1 − x Nb 2 O 6 (SBN) from ferroelectric to relaxor behavior when increasing the Sr 2+ content from x = 0.4 to x = 0.75 is investigated by dielectric low-frequency susceptometry and scanning piezo-force microscopy. Increasing quenched random electric fields are conjectured to be responsible for the observed increasing dielectric polydispersivity and the growing fractality of the polar 180° domains. A simple statistical model taking into account the varying occupation of the different A-sites with vacancies, Sr 2+ and Ba 2+ ions, and their influence on the average RF strength is proposed to describe the transformation to the relaxor behavior with increasing x.

Study on surface and domain structures of PbTiO3 crystals by atomic force microscopy

The surface morphology of flux-grown PbTiO3 crystals is examined by atomic force microscopy (AFM) at room temperature. Surface undulations due to a and c domains are observed on as-grown and heated crystals. The surface bending angle at 90° a-c domain walls is measured to be (3.58°±0.05°) in good agreement with the theoretical value, 3.6°. Footprints of ancient domains are found to be overlapped by surface undulations of the actual domain after polishing and heating process. Reciprocal 180° domains embedded in a and c domains are observed by both AFM and by polarizing optical microscopy on etched crystals. Details of the etched pattern are explored. Contrary to abrupt changes of height at 180° walls in c domains, only very small grooves are detected at 180° walls in a domains.

Evolution of the Polar Structure in Relaxor Ferroelectrics Close to the Curie Temperature Studied by Piezoresponse Force Microscopy

We present results of piezoresponse force microscopy studies on “uniaxial” relaxors Sr x Ba1− x Nb2O6 (SBN) and on “cubic” relaxors Pb[Mg1/3Nb2/3]1− x Ti x O3 (PMN-PT). The chosen compositions exhibit a spontaneous transition from the ergodic relaxor into the ferroelectric state. In both materials static regions of non-zero piezoresponse were found above the corresponding transition temperature. We attribute them to large polar nanosized regions (PNRs), which are “frozen” on the experimental time scale. The temperature evolution of the observed structures was investigated. It has been found that in PMN-PT these quasi-static PNRs are larger and exist in a broader temperature range than those in SBN.

Scaling behaviour of strontium titanate

The dielectric susceptibility, = ´ - i´´, of nominally pure strontium titanate is measured between 4 and 70 K as a function of an external bias field. Static scaling analysis of the susceptibility reveals a very good scaling behaviour with a quasi-critical exponent a = /( - 1) = 2.00 ± 0.02 extracted consistently from two different experiments.

Crossover from ferroelectric to relaxor and cluster glass in BaTi1−xZrxO3 (x = 0.25–0.35) studied by non-linear permittivity

The electric susceptibilities χ1, χ2, and χ3 of BaTi1−xZrxO3 ceramics with 0.25 ≤ x ≤ 0.35 are measured at 90 ≤ T ≤ 350 K and 37 ≤ f ≤ 106 Hz. Crossover from ferroelectric (x = 0.25) to relaxor behavior (x = 0.35) via coexistence of both (x = 0.30) is indicated by increasing polydispersivity and suppression of χ2 intensity and negative χ3 tails. The relaxor properties are due to weak random fields acting on dipolar Ti4+ clusters in non-polar Zr4+ environment. Frustrated interaction between blocked polar nanoregions yields a cluster glass ground state for x = 0.35.