DIELECTRIC RELAXATION BEHAVIOR OF CERAMICS IN THE Pb(Zn1/3Nb2/3)O3-BaTiO3-PbTiO3 SYSTEM

The dielectric properties of ceramics in Pb(Zn_1/3Nb_2/3)O₃-BaTiO₃-PbTiO₃ system were characterized using dielectric-temperature spectra. A spontaneous (zero field) relaxor-normal ferroelectric tran sition was observed for tetragonal rich compositions. A significant hysteresis effect accompanied by this transition, similar to first-order phase transition of normal ferroelectrics. This behavior was different from that of other relaxors, in which such transitions occurred only under a biased dc field. This observation was explained in terms of a thermally driven transformation from an ensemble of polar microregions to normal long-range ferroelectric state (micro-macro domain transition), which was attributed to the internal field resulting from the tetragonal strain.     

Study of new rare earth family Pb1.6K1.2R0.2Nb5O15 (R=La, Nd, Sm, Eu and Gd) of tetragonal tungsten bronze-type ferroelectrics

A new ferroelectric rare earth family Pb_1.6K_1.2R_0.2Nb₅O₁₅ with R=La, Nd, Sm, Eu and Gd (PKRN) of tetragonal tungsten bronze type ferroelectrics was synthesized. The ferroelectric transition with nonuniform distribution of critical temperature over a ceramic sample was found from dielectric measurements. According to X-ray diffraction measurements, the ferroelectric phase has an orthorhombic symmetry. The transition temperature was shown to decrease weakly with increasing radius of the rare earth ion R.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

The p–T phase diagram of KNbO3 by a dielectric constant measurement

A dielectric constant measurement was carried out on perovskite-type ferroelectrics KNbO₃ over a wide range of temperature under high pressure. The temperature- and pressure-dependence of the dielectric constant clarified that all temperatures of the transitions from the ferroelectric rhombohedral to orthorhombic, to tetragonal and then to the paraelectric cubic phase, decrease with increasing pressure. These results indicate that the orthorhombic–tetragonal transition takes place at 8.5 GPa and the tetragonal–cubic transition at 11 GPa, at room temperature.     

钛酸锶钡材料在外加电场作用下的拉曼光谱研究

采用传统的固相反应烧结方法制 备Ba_xSr_1-xTiO₃（0.40≤ x ≤0.70）陶瓷,借助于Raman散射光谱,研究了陶瓷样品在不同原位电场作用下Raman振动模式的变化,观察到居里温度附近显著的电场诱导的四方–立方相之间的转变. 结果表明A₁（TO₃）和E（TO₄）两种振动模式与晶体的结构存在密切的联系,这两种模式源于O-Ti-O沿晶格中c轴的方向和ab面内的振动. A₁（TO₃）/E（TO₄）之间Raman峰的相对强度比,随外加场强的增加明显升高,顺电相逐渐转变为铁电相,晶格的畸变越来越明显,其宏观性能上表现为介电常数的降低,可调率的增加. 同时对居里温度附近电场诱导的结构相变对顺电相下介电非线性的贡献进行了探讨. 关键词： 钛酸锶钡 Raman散射光谱 结构相变     

顺磁性La2/3Sr1/3MnO3层对Bi0.8Ba0.2FeO3薄膜多铁性能的影响

界面效应在提升异质结构材料的多铁性能方面有着重要的作用. 本文采用脉冲激光沉积技术在SrTiO₃(STO)基片上制备了Bi_0.8Ba_0.2FeO₃(BBFO)/La_2/3Sr_1/3MnO₃(LSMO)异质结. X-射线衍射图谱表明异质结呈现单相外延生长, 利用高分辨透射电镜进一步证实了BBFO为四方相结构. X-射线光电子能谱证实异质结中只存在Fe³⁺ 离子, 没有产生价态的变化, 揭示了异质结铁电和铁磁性的增强与BBFO/LSMO的界面有关. 同时, 测试了磁电阻(MR)和磁介电(MD), 当磁场强度为0.8 T, 温度为70 K时, MR约为-42.2%, MD约为21.2%. 并且发现在180 K时出现磁相的转变. 实验结果揭示出异质界面效应在提升材料的多铁性和磁电耦合效应方面具有超常的优点, 是加快多铁材料实际应用的有效途径.     

Composition Dependence of Electrocaloric Effect in (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 Single Crystals

Composition dependence of electrocaloric effect is investigated in (1-x)Pb(Mg_1/3Nb_2/3)O_3-xPbTiO₃ single crystals by using an eighth-order Landau—Devonshire theory. The applied electric field along [001] direction reduces the ferroelectric-ferroelectric phase transition temperatures, but increases the Curie temperatures. The electrocaloric coefficients of tetragonal phase are much larger than that of rhombohedral and monoclinic phase. A negative electrocaloric effect is observed near the M_C-T phase transition in 0.69 Pb(Mg_1/3Nb_2/3)O₃-0.31 PbTiO₃ single crystal. The application of a strong enough electric field results in a high adiabatic temperature change over a broad range of temperature. Therefore, it would be useful to construct a solid state cooling cycle over a broad temperature range for practical applications.     

四方相多铁BiMnO_3电控磁性的理论研究

钙钛矿结构BiMnO_3作为同时具有铁电性与铁磁性的多铁材料,在人工神经网络方面可以作为一种潜在的人工突触材料,从而设计出新型多铁人工突触器件.本文使用第一性原理计算的方法,分别研究了四方相BiMnO_3在xy面内施加0.18%与4%应力条件下的铁电情况,以及Mn原子磁矩随着铁电极化强度变化的曲线.结果表明,在四方相多铁BiMnO_3中,Mn原子磁矩会随着极化强度的增强而增大,表示其铁磁性可以在一定程度上由其铁电极化来进行调控,并且应力越大,其磁矩变化范围就越大.这一结果使得多铁BiMnO_3在人工突触器件设计方面拥有潜在的应用价值,多铁性使其在作为人工突触器件材料中具有更多可调控的自由度,从而可用于模拟多突触连接.这可为将来构造类脑芯片打下一定的理论基础.     

The crystallographic symmetries of single BaPb1−xBixO3 crystals grown from BaCO3-PbO2-Bi2O3 solutions

Single crystals of Perovskite-type oxide BaPb_1−xBi_xO₃ are grown from BaCO₃-PbO₂-Bi₂O₃ solutions which are weighed in two kinds of mixing ratios: X/2 mol % BaCO₃ − (100−X) mol % PbO₂ − X/2 mol % Bi₂O₃ and (10+X/2) mol % BaCO₃ − (90−X) mol % PbO₂ − X/2 mol % Bi₂O₃ These room temperature crystal structures are examined by using an X-ray powder diffraction method. The crystals grown from X/2−(100−X)−X/2 mol % solutions are orthorhombic at room temperature, while the structures are tetragonal with crystals grown from relatively Ba rich and Pb poor ( (10+X/2)−(90−X)−X/2 mol % ) solutions. This result indicates that the difference in the mixing ratio of the initial materials brings about a drastic structural change. The orthorhombic and the tetragonal crystals of x0.25 exhibit superconducting transition at 10K and 12K, respectively. The transition temperature in the latter is 2K higher than the former. In the light of this result, some difference between orthorhombic and tetragonal crystals is considered to influence superconductivity in this system.     

铌镁酸铅-钛酸铅铁电阴极电子发射特性

研究了铌镁酸铅-钛酸铅铁电材料的铁电、介电性能对阴极发射阈值电压的影响, 以及铁电阴极发射电流与激励脉冲电压和抽取电压之间的关系, 并分析了其发射机理. 结果表明, 室温介电常数高、极化强度变化量大的弛豫铁电体0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃具有较小的发射阈值电压; 铁电阴极电子发射与快极化反转和等离子体的形成有关; 由极化反转所致电子发射的自发射电流随激励脉冲电压的增大呈幂律增长关系, 其发射电流开始于激励脉冲电压的下降沿; 在抽取电压较大时, 发射电流随抽取电压的增大呈线性增长关系, 说明大电流主要取决于抽取电压; 其发射电流开始于激励脉冲电压的上升沿, 与“三介点”处的场增强效应和等离子体的形成有关; 当抽取电压为2500 V 时, 得到的发射电流幅值为210 A, 相应的电流密度为447 A/cm².     

无铅四方相钙钛矿短周期超晶格压电效应机理研究

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.     

BaTiO_3/La_(0.67)Sr_(0.33)MnO_(3-δ)复合薄膜的磁致电极化和磁介电特性研究

采用脉冲激光沉积法制备了BaTiO_3(BTO)与缺氧的铁磁绝缘态La_(0.67)Sr_(0.33)MnO_(3-δ)(LSMO)构成的磁电复合薄膜,研究了20—300K温度区间内磁场对电极化特性和介电特性的影响.研究发现,施加磁场使得电滞回线的剩余极化强度和矫顽场均增大,其变化率峰值分别为111.9%和89.6%,峰值温度分别为40K和60K.异质结具有显著的磁介电效应,在测量温度区间内,磁场使得介电常数增大,介电损耗减小.在0.8T场强下,介电常数的最大磁致变化率出现在60K,达到了300%,而介电损耗也在此温度实现了最大变化,减小为零场时的50.9%.该磁电复合薄膜的磁致电极化和磁介电特性的极值均出现在LSMO层的磁电阻峰值温度附近,这说明磁场对电滞回线和介电参数的调制应该源自电荷相关的耦合作用.其可能的机理是磁场使得锰氧化物中的Mn离子局域磁矩趋于有序排列,并通过自旋-轨道耦合以及界面效应间接影响了BTO的电极化特性.研究结果对于多铁器件的开发和应用具有重要意义.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Phase transition and superconductivity of (Ba,Sr,La)2YCu3Oz (z>6.9)

A series of (Ba,Sr,La)₂YCu₃O_z (z>6.9) samples is studied to elucidate the relationship between structure, cation composition and oxygen content. The concentration of La plays an important role in the orthorhombic to tetragonal phase transition. The tetragonal structure is stabilized by increasing oxygen content and decreasing average ionic radius of cations on the Ba site. The phase transition is explained in terms of the random distribution of CuO₅ pyramids in the BaO–CuO_x slab. Superconductivity does not depend significantly on the phase transition.     

Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr_0.8Ba_1.2CuO_3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La₂CuO₄-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO₂ planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr_0.8Ba_1.2CuO_3+δ and also in the isostructural superconductor Sr₂CuO_3+δ are of an identical nature.     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

应力作用下EuTiO3铁电薄膜电热效应的唯象理论研究

基于Laudau-Devonshire的热动力学模型, 计算了EuTiO₃铁电薄膜材料的电热效应. 结果显示在外加应力的调控下, 电极化、电热系数以及绝热温差都会随之变化. 外加垂直于表面的张应力加大, 薄膜的相变温度升高, 绝热温差增加, 最大绝热温差所对应的工作温度向高温区移动. 对于二维平面失配应变u_m =-0.005的薄膜, 当外加张应力σ₃ = 5 GPa时, 其最大电热系数为1.75×10^-3 C/m²·K, 电场变化200 MV/m 时室温下绝热温差ΔT 的最大值可达到14 K 以上, 绝热温差ΔT ≥13 K 的工作温区超过120 K, 表明可以通过调控外部应力来获取室温时较大的绝热温差. 此结果预示着铁电EuTiO₃ 薄膜在室温固态制冷方面可能具有较好的应用前景.     

Lithium insertion to ReO3-type metastable phase in the Nb2O5–WO3 system

Lithium insertion to distorted ReO₃-type metastable solid solution Nb_xW_1−xO_3−x/2 (0≤x<0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y (in Li_yNb_xW_1−xO_3−x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y<0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li_0.1Nb_xW_1−xO_3−x/2 (0≤x≤0.23) was found to increase with niobium content when x≤0.10, and to saturate at 4×10⁻⁹ cm²/s.     

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

The effects of Fe-substitution of YBa₂Cu₃O_y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa₂(Cu_{1 − x}Fe_x)₃O_y with different dopant concentration (0 x 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T_c, is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x=0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A_g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x.     

Tb掺杂双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7的磁熵变和电输运性质

研究了Tb掺杂对双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7磁熵变和电输运性质的影响.样品采用传统固相反应法制备,两样品的名义组分可以表示为(La_(1-x)Tb_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.025),磁场为7 T时的最大磁熵变?S_M分别为-4.60 J/(kg·K)和-4.18 J/(kg·K).比较后发现,Tb元素的掺杂使得最大磁熵变值减小,但同时增大了相对制冷温区.电性测量结果表明,x=0.025的样品在高温区的导电机制可以用小极化子模型解释,与母体三维变程跳跃模型不同;当温度降低至三维长程铁磁有序温度(T_c~(3D))附近时,掺杂样品发生金属绝缘相变;掺杂后样品在T_c~(3D)附近,磁电阻取得极大值(约为56%),表明是本征磁电阻效应.