Xiao Li Zhu

Re-entrant relaxor behavior of Ba5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics

Ba5RTi3Nb7O30 (R = La, Nd, Sm) tungsten bronze ceramics were prepared, and the dielectric and ferroelectric properties were investigated over a broad temperature range. The relaxor nature was determined for all compositions in their permittivity curves, and a second anomaly of the dielectric loss (tan δ) was observed around 250 K in Ba5NdTi3Nb7O30 and around 275 K in Ba5SmTi3Nb7O30. Both the maximum and remanent polarization tended to decrease and vanish at low temperatures in the ferroelectric phase for all compositions, which was referred to as the low temperature re-entrant relaxor behavior. The remanent polarization increased with decreasing temperature first and then reached the maximum value at the re-entrant temperature (Tr). For Ba5RTi3Nb7O30 (R = La, Nd, Sm), Tr decreased with the radius of R3+ cations and the applied field amplitude.

Relaxor ferroelectric characteristics of Ba5LaTi3Nb7O30 tungsten bronze ceramics

Ba5LaTi3Nb7O30 tungsten-bronze ferroelectric ceramics were synthesized and characterized. The tetragonal tungsten bronze structure in space group P4/mbm was confirmed, and a broad permittivity peak with strong frequency dispersion was observed around 250 K where the peak points well fitted the Vogel-Fulcher relationship [H. Vogel, Phys. Zeit. 22, 645 (1921); G. S. Fulcher, J. Am. Ceram. Soc. 8, 339 (1925)]. The temperature dependence of the ferroelectric hysteresis indicated the paraelectric to ferroelectric phase transition in the temperature range between 153 and 273 K. The high temperature permittivity curve deviated from the Curie-Weiss law in quite a narrow temperature region above Tmax, which reflected the weak correlations between the polar nanoregions. The Curie-Weiss constant (C) was 1.218 × 105 K, which was consistent with that for the displacive type ferroelectric. No DSC peak was detected over the temperature region investigated here. Moreover, the Curie-Weiss constant below Tmax (C′) was just...

Effects of Ca-substitution on structural, dielectric, and ferroelectric properties of Ba5SmTi3Nb7O30 tungsten bronze ceramics

Effects of Ca-substitution on the crystal structure and dielectric properties were investigated for Ba5SmTi3Nb7O30 tungsten bronze ceramics. The tetragonal tungsten bronze structure in space group P4bm was determined in Ba4CaSmTi3Nb7O30 ceramics, which was the same for Ba5SmTi3Nb7O30. The c-axis off center displacement in both B1 and B2-sites for Ba5SmTi3Nb7O30 was slightly larger than that in BaCaSmTi3Nb7O30, while the distortion of Ti/Nb(2)O6 octahedra in Ba4CaSmTi3Nb7O30 was obviously suppressed compared with that in Ba5SmTi3Nb7O30. Though there were also two dielectric anomalies observed in Ba4CaSmTi3Nb7O30, the relaxor ferroelectric nature was changed to the normal ferroelectric one, and both the endothermal peak in differential scanning calorimetry curve and the significant thermal hysteresis of the phase transition confirmed the first order ferroelectric transition. These results indicated that the ferroelectric nature of Ba4CaSmTi3Nb7O30 was more close to that in Ba4Sm2Ti4Nb6O30, and it could be c...

Dielectric relaxations, ultrasonic attenuation, and their structure dependence in Sr4(LaxNd1-x)2Ti4Nb6O30 tungsten bronze ceramics

The dielectric anomalies and their structure dependence were evaluated and discussed in Sr 4 (La x Nd 1-x ) 2 Ti 4 Nb 6 O 30 ceramics, together with the analysis of ultrasonic velocity shift and attenuation spectra in the low-temperature range. The room-temperature structure was confirmed as the tetragonal in space group P4bm for all compositions. One diffuse ferroelectric peak and three relaxor ferroelectric peaks corresponding to the commensurate/incommensurate modulation of oxygen octahedra, polar clusters of A-site ion ordering, and B-site ion ordering, respectively, were observed in the composition with x = 0.25. With decreasing the radius difference between A1- and A2-ions (increasing x), the dielectric relaxations, especially the one originating from the polar clusters of A-site ion ordering, tended to increase significantly and overlap the diffuse ferroelectric peak, which was completely overlapped for x ⩾ 0.75. This process just reflected the increased disordering degree of both A- and B-site ions, and the analysis of ultrasonic attenuation strongly supported the above conclusions on dielectric relaxations and their structural origins. The ultrasonic attenuation peak at approximately 100 K corresponded to the freezing process of the dielectric relaxations, and the fluctuation with composition of the ultrasonic attenuation peaks between 150 and 260 K suggested the possible structure variation.

Dielectric relaxation and ultrasonic attenuation of Sr4La2Ti4Nb6O30 tungsten bronze ceramics

Dielectric relaxation and ultrasonic attenuation spectra of Sr4La2Ti4Nb6O30 tungsten bronze ceramics were investigated over a broad temperature range. A broad permittivity peak with strong frequency dispersion was observed around 200K, which followed well the Vogel-Fulcher relationship, and is attributed to relaxor ferroelectric behavior with spin glass nature. The ultrasonic investigation provided further evidence on this thermally activated process, and both the dielectric relaxation and the freezing of polar clusters were clearly observed in the ultrasonic velocity shift and ultrasonic attenuation spectra.

Evolution of structure, dielectric properties, and re-entrant relaxor behavior in Ba5LaxSm1−xTi3Nb7O30 (x = 0.1, 0.25, 0.5) tungsten bronze ceramics

The effects of A site occupation on the structure, dielectric, and re-entrant relaxor behaviors were studied for Ba5LaxSm1−xTi3Nb7O30 (x = 0.1, 0.25, 0.5) tungsten bronze ceramics. The tetragonal tungsten bronze structure in space group P4bm was indicated for all compositions. The dielectric and ferroelectric properties were investigated over a broad temperature range. For all compositions, broad permittivity peaks with strong frequency dispersion were observed, where the peak points well fitted the Vogel-Fulcher relationship. The temperature dependency of ferroelectric hysteresis loops indicated the re-entrant relaxor behavior for all compositions: both the maximum and remanent polarization tended to decrease and vanish at low temperatures. In Ba5LaxSm1−xTi3Nb7O30, the re-entrant temperature (Tr) decreased with La-substitution, and the reduced remanent polarization at Tr (PTr) was determined. It seemed that the A1 site cation substitution exerted direct effects on the long-range ferroelectric order, and ...

Incommensurate and commensurate modulations of Ba5RTi3Nb7O30 (R = La, Nd) tungsten bronzes and the ferroelectric domain structures

Incommensurate and commensurate structural modulations of Ba5RTi3Nb7O30 (R = La, Nd) tungsten bronze ceramics were investigated by using a cooling holder equipped transmission electron microscopy in the temperature range from 100 K to 363 K. The incommensurate modulation was observed in both Ba5LaTi3Nb7O30 and Ba5NdTi3Nb7O30 at room temperature, while there was a transition from incommensurate tilted structure to commensurate superstructure for Ba5NdTi3Nb7O30 with decreasing temperature. The incommensurate and commensurate modulations were determined by the A-site occupancy of Ba and R cations. The A-site disorder resulted in larger incommensurability parameter δ and the diffusion of the satellite reflection spots. The effect of A-site disorder on the coupling between long-range dipolar order and the commensurate modulation was also discussed. The obvious ferroelectric 180° domains with spike-like shape parallel to c axis were observed for Ba5NdTi3Nb7O30, while no macro ferroelectric domain was determined...

Crystal structure, ferroelectricity and polar order in a Ba4R2Zr4Nb6O30 (R = La, Nd, Sm) tetragonal tungsten bronze new system

It has become very important to search for Pb-free ferroelectric new materials because of increasing environmental concerns, and the tungsten bronze family might provide great potential for this challenge due to the various possibilities of structural tailoring. In the present work, a zirconate niobate ferroelectric new system Ba4R2Zr4Nb6O30 (R = La, Nd, Sm) with a filled tetragonal tungsten bronze structure has been synthesized and investigated. The ferroelectricity and polar order are characterized together with the crystal structure. The present materials show a crossover from relaxor to normal ferroelectric features upon increasing the A-site ionic radii difference (Δr). The relaxor behavior in Ba4La2Zr4Nb6O30 is associated with incommensurate modulation, while commensurate structural modulation dominates the normal ferroelectric transition in Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30. Compared with titanate niobate tungsten bronzes, the present materials exhibit a stronger relaxor nature. The ferroelectric transition in Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30 shows first order features with an unusually large thermal hysteresis (>100 K), and is followed by a low temperature dielectric relaxation. Micro polar regions are detected for Ba4La2Zr4Nb6O30 under piezoresponse force microscopy (PFM), while the coexistence of macro ferroelectric domains and micro polar regions is observed in the low temperature relaxation region for Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30, and the micro polar regions can grow into the macro domains under a high electric field. An abnormal decrease is observed at lower temperatures in the curve of remanent polarization vs. temperature for Ba4Nd2Zr4Nb6O30 and Ba4Sm2Zr4Nb6O30, which is attributed to the disruption of the ferroelectric polar order by the micro polar regions appearing at the same temperature region. The present new system is expected to be a promising candidate for Pb-free ferroelectric materials.

Relaxor nature in lead-free Sr5LaTi3Nb7O30 tetragonal tungsten bronze ceramics

Lead-free tetragonal tungsten bronze Sr5LaTi3Nb7O30 ceramics were prepared and the correlation of the relaxor nature and crystal structure was studied using dielectric spectroscopy and powder X-ray diffraction. Three dielectric relaxations were observed below the deviation temperature TD ∼ 330 K. Relaxation I and II followed the Vogel-Fulcher law with the freezing temperatures of 189 K and ∼90 K. Low temperature relaxation III, which was first observed in filled tungsten bronze, followed well the Arrhenius law. Dielectric response becomes static below 50 K. Polarization-field (P-E) hysteresis loops were evaluated from 183 K to 298 K. Pr value of 0.41μC/cm2 was observed at 183 K. Deviation of lattice parameter c from the linear contraction and increasing of tetragonality (c/a ratio) were observed below TD, reflecting the structure change during the formation of polar nanoregions and the following freezing process. Opposite tendency was observed below 100 K for all the lattice parameters, corresponding to r...

Ferroelectric phase transition and low-temperature dielectric relaxations in Sr4(La1−xSmx)2Ti4Nb6O30 ceramics

Ferroelectric transition and the low-temperature dielectric relaxations were investigated in Sr4(La1−xSmx)2Ti4Nb6O30 ceramics. The dielectric nature of the present ceramics was governed by the radius difference between A1- and A2-site ions. Low-temperature dielectric relaxations were indicated for all the compositions, and the diffuse ferroelectric peak appeared gradually with increasing x (increasing ion radius difference between A1- and A2-sites), and it became the normal ferroelectric one for x = 1. There was no peak on the differential scanning calorimetry (DSC) curves for the compositions with x = 0, 0.25, 0.5, and 0.75, while an obvious endothermic peak was observed for x = 1. Based on the analysis of the dielectric data and the DSC results, the ferroelectric transition for x = 1 was determined as a first order phase transition, and the second order phase transition was suggested for other compositions.