Orientation effects on the bandgap and dispersion behavior of 0.91Pb（Zn_（1/3）Nb_（2/3））O_3-0.09PbTiO_3 single crystals

0.91Pb(Zn_1/3Nb_2/3)O₃--0.09PbTiO₃ (PZN--9%PT) single crystals with different orientations are investigated by using a spectroscopic ellipsometer, and the refractive indices and the extinction coefficients are obtained. The Sellmeier dispersion equations for the refractive indices are obtained by the least square fitting, which can be used to calculate the refractive indices in a low absorption wavelength range. Average Sellmeier oscillator parameters E_o, $\lambda$_o, S_o, and E_d are calculated by fitting with the single-term oscillator equation, which are related directly to the electronic energy band structure. The optical energy bandgaps are obtained from the absorption coefficient spectra. Our results show that the optical properties of [001] and [111] poled crystals are very similar, but quite different from those of the [011] poled crystal.     

Nanoscale domain structures in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 (91PZN-9PT) single crystals studied by piezoresponse forcemicroscopy

High-resolution domain studies have been performed in 91PZN-9PT single crystals by piezoresponse force microscopy. Nanometer- and micron-sized strip-shaped 180º and parallel 90º domains were observed in the rhombohedral (R) and tetragonal (T) phases. Domain patterns with the typical sizes 5–200 nm were observed on the (001)_cub surface of unpoled sample. The existence of nanodomains in the sample is tentatively attributed to the relaxor nature of PZN-PT where small polar clusters may form under zero-field cooling conditions. The domain observation confirms the structure of the morphotropic 91PZN-9PT crystals, which is composed of both the R and T (or monoclinic) orientations states. The outstanding piezoelectric properties may result from the cooperative response of the microscale/nanoscale domain structure.     

Soft phonon anomalies in the relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))0.92Ti0.08O3

Neutron inelastic scattering measurements of the polar transverse optic phonon mode dispersion in the cubic relaxor Pb(Zn(1/3)Nb(2/3))0.92Ti0.08O3 at 500 K reveal anomalous behavior in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q = 0.2 A(-1) measured from the zone center. We speculate that this behavior is the result of nanometer-sized polar regions in the crystal.     

弛豫铁电单晶0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3的电光性能研究

以波长为632.8 nm的He-Ne激光作为光源,研究了三方相弛豫铁电单晶0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3在室温下的折射率和线性电光性质.通过测量晶体的布儒斯特角,确定了单晶的两个主轴折射率:no=2.466,ne=2.488.利用改进的马赫-泽德尔干涉法测量了单晶的线性电光系数:r33=71 pm/V,r13=4 pm/V.计算得到有效电光系数rc=67 pm/V及半波电压Vπ=610 V.单晶优异的电光性能使其在激光调制与偏转领域有着巨大的应用潜力.     

铁电单晶0.62Pb(Mg1/3Nb2/3)O3-0.38PbTiO3折射率的研究

用最小偏向角法在20℃下精确测量了0.62Pb(Mg1/3Nb2/3)O3-0.38PbTiO3(0.62PMN-0.38PT)单晶的折射率,给出了该温度下折射率色散的Sellmeier方程.研究了能带结构与折射率的关系,计算了样品的Sellmeier光学系数:对no,E0=5.50eV,λ0=0.226μm,S0=1.004×1014m-2,Ed=28.10eV;对ne,E0=5.57eV,λ0=0.223μm,S0=1.017×1014m-2,Ed=28.10eV.ABO3型钙钛矿材料中,BO6八面体基元决定了晶体的能带结构,对折射率产生重要影响.     

0.7Pb(Mg1/3 Nb2/3)O3-0.3PbTiO3单晶/环氧树脂1-3型复合材料的压电性能研究

模拟了0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3(PMN-0.3PT)单晶1-3型压电复合材料的性能与单品体积分数的关系,得出性能最优时压电相的体积分数为64%,在这一体积分数下,采用切割-填充法,并使用了不同类型的环氧树脂填充制备复合材料.系统地研究了聚合物相对复合材料性能的影响,研究表明,减小聚合物相的刚度系数c和密度ρ有利于提高复合材料的性能,且聚合物相与压电相的结合强度对性能的影响非常明显,制备的PMN-0.3 PT单晶1-3型复合材料的厚度伸缩机电耦合系数k1高达90.1%,压电系数d33大于1000 pC/N,机械品质因数Qm为10.39,声阻抗Z也大大降低,性能明显优于传统的Pb(Zrx,Ti1-x)O3(PZT)陶瓷及其1-3复合材料,在压电换能器和传感器中显示出广阔的前景.     

Heat capacity of the 0.7PbNi1/3Nb2/3O3-0.3PbTiO3 ferroelectric ceramics

The temperature dependence of the heat capacity of the 0.7PbNi_1/3Nb_2/3O₃-0.3PbTiO₃ compound has been studied in the temperature range 120?C800 K. The temperature dependence of the heat capacity C _p has two diffuse anomalies in the temperature ranges 250?C450 K and 450?C650 K and a ?? anomaly at temperatures T ?? 225 K. The results are discussed with inclusion of the dielectric and structural data.     

弛豫型铁电单晶0.67Pb(Mg_(1/3)Nb_(2/3))O_3-0.33PbTiO_3的拉曼散射研究

本文采用拉曼散射技术在温度从374到-196℃的范围内,研究了弛豫型铁电单晶0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3(PMNT)。观察了在不同温度下拉曼光谱的变化,经分析这些变化反映了该晶体经历了两个相变:第一个相变温度发生在120℃,从顺电立方相到铁电四方相的相变,780 cm-1处模式在VH偏振下的改变标志了这一相变;第二个相变温度发生在34℃,是从铁电四方相到铁电三方相的相变,软模的出现代表了这一相变。     

Field-induced piezoelectric response in Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

The electric field induced strain, piezoelectric and electromechanical response in relaxor based single crystal piezoelectrics (1−x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMNT100x) were investigated as a function of dc bias and temperature along the crystallographic [001] orientation. With increasing temperature and dc bias, monoclinic and rhombohedral phases were found to coexist in the crystals, exhibiting large strain hysteresis, while above a threshold temperature and dc bias, the induced monoclinic and tetragonal phases were found to show very low strain hysteresis. The electric field level for the monoclinic to tetragonal phase transformation was found to exponentially decay with temperature. The mechanical quality factor Q for the tetragonal phase exhibits higher values when compared to rhombohedral and monoclinic phases, displaying improved domain stability. The domain stability of tetragonal crystals as a function of sample thickness was also studied, it was found that a higher dc bias was needed for thinner tetragonal crystals in order to stabilize the single domain configuration and reduce the strain hysteresis.     

Dielectric response of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 in the nonergodic state after a DC electric field is turned off

The Pb(Mg_1/3Nb_2/3)O₃ (PMN) relaxor system is used as an example to analyze the temperature dependences of the low-frequency dielectric permitivity (?′(T)) measured during zero-field heating (ZFH) from T = 10 K to T = 300 K after using different field cooling (FC) conditions. No changes in the temperature dependences of the permittivity have been detected during the transition from a nonergodic relaxor state (NERS) into an ergodic relaxor state (ERS) (at T _f ≈ 216 K). However, the difference Δ?′(T) between the curves corresponding to different field cooling conditions in the same electric field has different shapes and different values below and above T → (T _f + 9 K)^? (for E _dc = 1.52 kV/cm). The reduced permittivities ?′_r(T, f) recorded under different cooling conditions are shown to change their behavior when passing through T = T _f + 9 K. In NERS, these curves diverge: the stronger the field (0 ≤ E _dc ≤ 3 kV/cm), the larger the divergence. In ERS, however, the ?′_r(T, f) curves coincide under different cooling conditions irrespective of the field. The character of the changes in Δ?′(T) and ?′_r (T, f) during the NERS-ERS transition is frequency-independent. The difference in the behavior of the dielectric response during ZFH after cooling in different (ZFC, FC) modes (even in a weak field), for both transition through T _f and cooling down to T = 10 K, indicates different NERSs forming under these conditions. The contribution to ?′(T) from slowly relaxing regions (ω ～ 0.1 mHz), whose polarization is reoriented after the field is turned off, is responsible for the fact that, during the NERS-ERS transition, the ?′_r(T, f) curves coincide at a temperature that is higher than T = T _f.     

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: I. Strain analysis and a static order parameter

The structural evolution of Pb(Mg(1/3)Nb(2/3))O(3) (PMN) has been reviewed in terms of characteristic temperatures, length scales and timescales, with a view to considering the overall relaxor behaviour from the perspectives of strain and elasticity. A conventional analysis of lattice parameter data in terms of spontaneous strain and strain/order parameter coupling shows that even though a normal phase transition does not occur the relaxor ordering process is accompanied by a significant volume strain which follows the pattern of a static order parameter evolving according to that expected for a tricritical phase transition with T(c) ≈ 350 K. This matches the evolution of the intensity of the elastic central peak in neutron scattering spectra, and reflects the development of static (or quasistatic) polar nanoregions (PNRs) as if by a mean-field phase transition. Use of a Landau free energy expansion, which includes Γ4(-) order parameter components to describe ferroelectric contributions and an R1(+) order parameter to describe cation ordering together with their formal coupling with strain, then allows the pattern of elastic softening expected for a cubic → rhombohedral phase transition to be anticipated. The extent to which observed softening differs from this static mean-field pattern serves to highlight the additional roles of local heterogeneity and relaxation dynamics in determining the relaxor properties of PMN.     

Effect of electric field on the smeared phase transition in crystals PbIn1/2Nb1/2O3-28PbTiO3 and PbIn1/2Nb1/2O3-37PbTiO3

The effect of a direct-current electric field (0 < E < 3 kV/cm) on the smearing of the phase transition and the Curie-Weiss exponent (γ) for two PbIn_1/2Nb_1/2O₃-28PbTiO₃ and PbIn_1/2Nb_1/2O₃-37PbTiO₃ crystals lying at different distances from the morphotropic phase boundary has been investigated. The universal Curie-Weiss law has been used to approximate the temperature dependences of the dielectric permittivity. It has been shown that the more remote is the crystal from the morphotropic phase boundary, the larger is the exponent γ and, consequently, the more smeared is the phase transition (in the zero field, γ = 1.67 and 1.49 for PbIn_1/2Nb_1/2O₃-28PbTiO₃ and PbIn_1/2Nb_1/2O₃-37PbTiO₃, respectively). It has been found that a weak electric field (no greater than 2–3 kV/cm) in the case of the more smeared phase transition almost does not affect the Curie-Weiss exponent, whereas for the PbIn_1/2Nb_1/2O₃-37PbTiO₃ crystal, this exponent decreases with increasing electric field strength and approaches γ = 1, which is characteristic of the conventional ferroelectric.     

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: II. Strain-order parameter coupling and dynamic softening mechanisms

Elastic and anelastic behaviour of single crystal and ceramic samples of Pb(Mg(1/3)Nb(2/3))O(3) has been investigated at frequencies of ~0.1-1.2 MHz through the temperature interval 10-800 K by resonant ultrasound spectroscopy (RUS). Comparison with data from the literature shows that softening of the shear modulus between the Burns temperature and the freezing interval is independent of frequency. The softening is attributed to coupling between acoustic modes and the relaxation mode(s) responsible for central peaks in Raman and neutron scattering spectra below the Burns temperature, and can be described with Vogel-Fulcher parameters. Shear elastic compliance and dielectric permittivity show similar patterns of temperature dependence through the freezing interval, demonstrating strong coupling between ferroelectric polarization and strain such that the response to applied stress is more or less the same as the response to an applied electric field, with a frequency dependence consistent with Vogel-Fulcher-like freezing in both cases. Differences in detail show, however, that shearing induces flipping between different twin orientations, in comparison with the influence of an electric field, which induces 180° flipping: the activation energy barrier for the former appears to be higher than for the latter. Below the freezing interval, the anelastic loss also has a similar pattern of evolution to the dielectric loss, signifying again that essentially the same mechanism is involved in the freezing process. Overall softening at low temperatures is attributed to the contributions of strain relaxations due to coupling with the local ferroelectric order parameter and of coupling between acoustic modes and continuing relaxational modes of the polar nanostructure. Dissipation is attributed to movement of boundaries between PNRs or between correlated clusters of PNRs. Overall, strain coupling is fundamental to the development of the characteristic strain, dielectric and elastic properties of relaxors.     

Specific-heat anomaly caused by ferroelectric nanoregions in Pb(Mg(1/3)Nb(2/3))O3 and Pb(Mg(1/3)Ta(2/3))O3 relaxors

The specific heat of typical relaxors, Pb(Mg(1/3)Nb(2/3))O3 (PMN) and Pb(Mg(1/3)Ta(2/3))O3 (PMT), was measured by adiabatic and relaxation methods between 2 and 420 K. A broad anomaly was found in the specific heat curve over the wide temperature range between 150 and 500 K for PMN, and between 50 and 400 K for PMT, which provides evidence for the formation of ferroelectric nanoregions (FNR) in the paraelectric matrix. The entropy of the anomaly was estimated as 3.3 J K(-1) mol(-1) and 2.9 J K(-1) mol(-1) for PMN and PMT, respectively, which implies an order-disorder-type mechanism for the formation of FNR.     

等静压下0.75Pb（Mg1/3Nb2/3）O3-0.25PbTiO3陶瓷的介电性能研究

研究了等静压对0.75Pb（Mg_1/3Nb_2/3）O₃-0.25PbTiO₃（PMN-25PT）陶瓷介电温谱的影响,PMN-25PT剩余极化随等静压变化和等静压压致相变.结果表明,随着压力增加,PMN-25PT的介电峰值温度T_m降低,/+{dT_m}/-{dP}≈-4℃/kbar,极化弛豫增强;剩余极化随压力增加连续减小;介电常数对压力的依赖关系与对温度场的依赖相似,压力诱导PMN-25PT发生弛豫铁电—顺电相变,相变为宽化的渐变过程,频率色散和极化弛豫更加强烈和普遍. 关键词： 铌镁酸铅-钛酸铅 等静压 介电弛豫 压致相变     

Effects of strontium on the characteristics of Pb(Zr1/2Ti1/2)O3--Pb(Zn1/3Nb2/3)O3 ceramics

The dielectric and piezoelectric properties of pyrochlore-free lead zirconate titanate-lead zinc niobate ceramics were investigated systematically as a function of Sr doping. The powders of Pb_{(1? x )}Sr _x [0.7(Zr_{1 / 2}Ti_{1 / 2})–0.3(Zn_{1 / 3}Nb_{2 / 3})]O₃, where x?=?0–0.06 were prepared using the columbite-(wolframite) precursor method. The ceramic materials were characterized using X-ray diffraction, dielectric spectra, hysteresis and electromechanical measurements. The phase-pure perovskite phase of Sr-doped PZN--PZT ceramics was obtained over a wide compositional range. The results showed that the optimized electrical properties were also achieved at composition x?=?0.0, which were K _P?=?0.69, d ₃₃?=?670?pC?N^?1, P _r?=?31.9?µC?cm^?2 and ε_rmax?=?18600. Maximum dielectric constant values of the systems decreased rapidly with increasing Sr concentration. Moreover, with increasing Sr concentration dielectric constant versus temperature curves become gradually broader. The diffuseness parameter increased significantly with Sr doping. Furthermore, Sr doping has been shown to produce a linear reduction in the transition temperature (T _m)?=?294.1–12.7x°C with concentration (x). Sr shifts the transition temperature of this system at a rate of 12.7°C?mol^?1%.     

Phase transition at a nanometer scale detected by acoustic emission within the cubic phase Pb(Zn1/3Nb2/3)O3-xPbTiO3 relaxor ferroelectrics

Pb(Zn(1/3)Nb(2/3))O3-xPbTiO(3) (x=4.5%-12%) relaxor ferroelectric crystals have been studied by means of acoustic emission (AE) in the 400-540 K temperature range. An anomalous AE activity independent of the ground state relaxor/morphotropic/ferroelectric crossover has been revealed at around 500 K, and it is associated with the "waterfall" feature related to the existence of polar nanoregions (PNRs). The 500 K AE anomaly is attributed to local martensitelike cubic-to-tetragonal ferroelectric transitions within the PNRs imbedded in a nonpolar (cubic) matrix.     

Inelastic neutron scattering study of the relaxor ferroelectric PbMg1/3Nb2/3O3 at high temperatures

Results of inelastic neutron scattering experiments between 300 K and 900 K on the relaxor ferroelectric PbMg _1/3 Nb _2/3 O₃ are presented. Within a mode-coupling analysis the data are consistent with the observation of a strongly damped quasi-optic excitation. It is demonstrated that below K a dynamical crossover takes place manifesting itself by a narrow central peak. This crossover is accompanied by the appearance of strong damping of the transverse acoustic phonons, with the damping constant proportional to q⁴. Different physical models of the crossover are discussed. Received 8 February 1999     

Polarization switching by stretched exponential functions in 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3

Polarization reversal and domain dynamics have been investigated in 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ using a method of current transients. Investigations were performed as a function of applied electric field. The kinetics of the transients have been modelled by a stretched-exponential-type function.