Z.-G. Ye

Ferroelectric to relaxor crossover and dielectric phase diagram in the BaTiO3-BaSnO3 system

The (1−x)BaTiO3–xBaSnO3 (0⩽x⩽0.30) perovskite solid solution ceramics were prepared by solid state reaction and studied by dielectric spectroscopy. The complex dielectric permittivity was measured as a function of frequency (0.1Hz–100kHz) in the temperature (T) range of 123–573K. The transition from the high-temperature paraelectric state where the dielectric constant obeys the Curie-Weiss law to the ergodic cluster state is found to occur at the same temperature of 485K in all the compositions of x⩾0.04 and at lower temperatures in those with a smaller x. For 0⩽x⩽xc=0.19, the temperature of the dielectric peak Tm, corresponding to the diffuse transition from the ergodic polar cluster state to the ferroelectric state, decreases with increasing x and does not depend on frequency. The diffuseness of the peak gradually increases. For x>xc, the permittivity exhibits relaxor behavior with the frequency-dependent Tm satisfying the Vogel-Fulcher law. The temperature variation of the permittivity on the high-temp...

Relaxor behavior in the solid solution between dielectric Ba(Mg1∕3Nb2∕3)O3 and ferroelectric PbTiO3

Relaxor behavior has been found in the solid solution between complex perovskite Ba(Mg1∕3Nb2∕3)O3 (BMN) and ferroelectric PbTiO3 (PT), 0.4BMN-0.6PT, prepared by solid state reactions. A strong dispersion of the maximum dielectric permittivity (e′) appears around the temperature of Tm, which shifts towards higher temperatures with increasing frequency. The variation of Tm with frequency follows the Vogel-Fulcher relationship. The variation of 1∕e′ with temperature above Tm deviates from the Curie-Weiss law but satisfies a Lorentz-type function. The dielectric hysteresis loops are displayed at several temperatures which become slimmer at T>Tm. The remnant polarization gradually decreases with increasing temperature and persists up to above Tm.

Field-induced shift of morphotropic phase boundary and effect of overpoling in (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 piezocrystals

The domain structure and phase symmetry of the (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 piezoelectric crystals with a composition close to the morphotropic phase boundary (MPB) between the rhombohedral R3m and monoclinic Pm phases (x≅0.3) have been studied by polarized light microscopy. It is found that poling by an electric field parallel to the ⟨001⟩ pseudocubic direction shifts the MPB toward the lower x side. This shift is the origin of the overpoling effect which degrades the performance of the materials, e.g., the piezoelectric coefficient (d33) and dielectric constant decrease with increasing poling field. In optimally poled crystals, d33 can reach as high as 3100pC∕N.

Enhanced ordered structure and relaxor behaviour of 0.98Pb(Mg1/3Nb2/3)O3–0.02La(Mg2/3Nb1/3)O3 single crystals

High quality Pb(Mg1/3Nb2/3)O3 (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seeded solution growth method. At a doping level as low as 2 at.% of La, the 1:1 B-site cation (chemical) order was significantly enhanced as revealed by the presence of superlattice peaks in the x-ray diffraction pattern and comparatively large (~100 nm) chemically ordered regions (CORs) in the transmission electron microscopy dark field images. The average chemical order parameter was calculated (from the intensity of superlattice peaks) to be S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumably rhombohedral) phase. The effect of La doping on the dielectric relaxation was not substantial. Though the magnitude of the dielectric constant peak in PLMN was reduced by half as compared to pure PMN, the peak temperature and diffuseness as well as the shape of the dielectric relaxation spectra remained almost unchanged. The analysis of the temperature dependence of the characteristic time for the main relaxation process in terms of the Vogel–Fulcher law revealed the freezing of dipole dynamics at temperature Tf = 192 K, which was only slightly lower than the Tf = 213 K in PMN. To reconcile the structural and dielectric data we suggest that the rhombohedral phase forms inside CORs while the chemically disordered matrix remains cubic and is populated by dynamic polar nanoregions that give rise to the characteristic dielectric response.

Bismuth Aluminate BiAlO 3 : A New Lead-free High-T C Piezo-/ferroelectric

Ferroelectric materials have applications in non-volatile random access memory devices and micro electromechanical systems. For such applications, the materials must remain ferroelectric up to high temperatures. The best materials that currently exist for these applications are lead-containing compounds like Pb(Zr_1-xTi_x)O₃. Owing to the toxicity of lead, there is a demand for lead-free high-temperature ferroelectrics. Bismuth aluminate has been predicted to be one such material [1]. In this work, BiA1O₃ is synthesized using a high-pressure high-temperature technique at 6 GPa and 1000degC. The diffraction experiments show that BiAlO₃ crystallizes in a rhombohedral unit cell that is elongated along the c-axis (R3c; Z = 6; a = 5.37546(5) A and c = 13.3933(1) A). The characterization of the dielectric, ferroelectric, and piezoelectric properties of the ceramic BiAlO₃ demonstrate that it is indeed a lead-free ferroelectric with a Curie temperature Tc > 520degC, a piezoelectric coefficient d33 = 28 pC/N and a room-temperature remnant polarization P_r = 9.5 muC/cm². P_r increases with temperature, reaching 26.7 muC/cm² at 225degC. The dielectric, ferroelectric and piezoelectric properties of BiAlO₃ are comparable to those of BiFeO₃ (BFO) and SrBi₂Ta₂O₉ (SBT), making it a promising new high-Tc lead-free piezo-and ferroelectric for memory and transducer applications.

Temperature dependence of the capacitance of a ferroelectric material

We present an alternate version of the undergraduate laboratory experiment developed by Dixon [Am. J. Phys. 75, 1038–1045 (2007)] that is suitable for second-year students. We study the temperature variation of the capacitance of a ferroelectric ceramic derived from barium titanate, the Ba(Ti0.9Sn0.1)O3 solid solution. The ratio of tin to titanium is chosen to provide a convenient Curie temperature near 50°C. Using careful temperature control and real-time capacitance measurements, we track the time evolution of the capacitance in response to temperature changes at 5Hz for runs that last up to a day. At temperatures well above the Curie temperature, TC, the capacitance relaxation is well-described by a single exponential decay. Near TC, the relaxation is linear in the logarithm of time over more than three decades. For T>TC, the permittivity deviates from the Curie–Weiss law and follows another phenomenological form commonly used to describe relaxor perovskite-ceramic capacitors.

Growth and characterization of relaxor ferroelectric Pb (Fe2/3W1/3 )O3-PbTiO3 single crystals

Single crystals of the complex perovskite solid solution (1-x)Pb(Fe2/3W1/3)O 3 —xPbTiO3 [(1−x)PFW—xPT], with x = 0, 0.07, 0.13, 0.27 and 0.75, have been synthesized by the high temperature solution growth using PbO as flux, and characterized by X-ray diffraction and dielectric measurements. With the increasing amount of PT, the crystal structure at room temperature changes from a pseudo-cubic (x = 0, 0.07 and 0.13) to a tetragonal phase (x ≥ 0.27). The dielectric properties of the crystals exhibit a typical relaxor behaviour for x = 0 to 0.27 with frequency-dependent peak temperature (T m ) satisfying the Vogel-Fulcher law, while the crystals with higher PT-content undergo a first-order transition at the ferroelectric Curie temperature T C .

Growth and characterization of relaxor ferroelectric (1-x)Pb(Sc/sub 1/2/Nb/sub 1/2/)O/sub 3/ - xPbTiO/sub 3/ single crystals

Growth and characterization of the single crystals of complex perovskite (1-x)Pb(Sc/sub 1/2/Nb/sub 1/2/)O/sub 3/-xPbTiO/sub 3/ [PSNT] with composition near the MPB (x=0.425) are reported in this paper. The PSNT crystals were grown by high temperature solution method using the (PbO + /spl delta/B/sub 2/O/sub 3/) system as flux. By analyzing the defects and morphology of the as-grown crystals, we have optimized the growth parameters and the flux system, leading to a significant improvement in the quality of crystal grown. Domain observation by polarized light microscopy reveals that the structure of the PSNT57.5/42.5 crystal has very complex symmetry at room temperature. The PSNT crystals have been characterized by means of the X-ray, dielectric and ferroelectric measurements.